Analog IO Signal Conditioning and Calibration on B&R Automation

Analog input and output signals are the most failure-prone signals in any B&R installation. Noise, grounding problems, aging sensors, and drifting calibration all produce symptoms that are difficult to distinguish from software bugs. This document covers how B&R analog IO cards handle 0-10V, 4-20mA, thermocouple, and RTD signals; how to perform calibration; how noise and grounding issues manifest as reading errors; and how to systematically determine whether a problem originates at the sensor, the cabling, or the IO card itself. Cross-references: io-card-hardware.md for IO card hardware internals, grounding-emc.md for grounding and EMC troubleshooting, and memory-map.md for direct memory access to analog channels.

Comprehensive Technical Reference

Table of Contents

1. Analog Input Signal Types and Module Overview
2. Signal Conditioning Circuits on B&R Analog Modules
3. Calibration Procedures for Analog Inputs and Outputs
4. Offset/Gain Adjustment Methods
5. Noise and Grounding Issues as Analog Reading Errors
6. Distinguishing Sensor Faults from IO Card Faults
7. Resolution and Accuracy Specifications
8. Sampling Rate and Aliasing Considerations
9. Cold Junction Compensation for Thermocouple Inputs
10. 3-Wire and 4-Wire RTD Measurement
11. Current Loop (4-20 mA) Diagnostics
12. Filter Settings and Response Time Trade-offs
13. Grounding Best Practices for Analog Signals
14. Calibration Certification and Traceability

1. Analog Input Signal Types and Module Overview

1.1 Voltage Inputs (0-10V, +/-10V)

B&R analog input modules support voltage measurement through differential input configurations. The front-end circuit presents a high input impedance (typically 20 MOhm on modules like the X20AI4622) to minimize loading on the signal source.

Supported modules and ranges:

LSB resolution for voltage (13-bit modules):

• +/-10V range: 1 LSB = 2.441 mV (INT format 0x8001 to 0x7FFF)

LSB resolution for voltage (16-bit modules):

• +/-10V range: 1 LSB = 0.305 mV (finer granularity from higher resolution)

The conversion procedure on standard modules uses Successive Approximation Register (SAR) ADCs with conversion times as low as 400 us for all inputs (X20AI4622). Galvanically isolated variants (e.g., X20AI2237, X20AI2437) provide per-channel isolation with their own sensor power supply, eliminating channel-to-channel coupling.

Input protection: All B&R analog voltage inputs include protection against wiring with supply voltage, tolerating up to +/-30V on the input without damage.

1.2 Current Inputs (4-20 mA, 0-20 mA)

Current measurement on B&R modules is performed using an internal precision shunt resistor. The input circuit automatically switches between voltage and current measurement modes based on the terminal used and the software configuration (an integrated switch inside the module activates the appropriate signal path).

Supported current ranges:

LSB resolution for current (13-bit modules):

• 0-20 mA range: 1 LSB = 4.883 uA
• 4-20 mA range: INT -8192 to +32767 (value 0 corresponds to 4 mA)

Input protection: Current inputs tolerate up to +/-50 mA without damage.

Galvanically isolated current modules (X20AI2437, X20AI2438) provide single-channel galvanic isolation with an independent sensor power supply, making them ideal for applications requiring isolation between measurement loops. The X20AI2438 additionally supports the HART protocol, enabling bidirectional digital communication superimposed on the 4-20 mA analog signal.

1.3 Thermocouple Inputs

B&R offers dedicated thermocouple input modules that support types J, K, N, S, B, and R:

Measurement ranges per thermocouple type:

The X20AT6402 supports dual resolution: 0.1C (INT, range +/-32767) and 0.01C (DINT, range +/-2,147,483,647). Raw value measurement without linearization is also available for custom sensor types.

Conversion procedure: Thermocouple modules use sigma-delta converters (as opposed to SAR on voltage/current modules). Conversion times depend on the number of enabled channels and the filter setting:

• Function Model 0, n channels: (n + 1) * (2 * Filter time + 200 us)
• Function Model 1, n channels: n * (2 * Filter time + 200 us)
• Example: 6 channels with 50 Hz filter = 281.4 ms total conversion time

1.4 RTD Inputs (Pt100, Pt1000)

B&R provides both dedicated RTD modules and universal multi-signal modules:

Measurement method: RTD modules use a constant current source (250 uA +/-1.25% on X20AT2222) with a precision reference resistor (4530 Ohm +/-0.1%) to measure resistance. The sigma-delta ADC converts the voltage drop across the RTD element. Internal linearization follows IEC/EN 60751.

Measurement range: -200 to 850C for both Pt100 and Pt1000. The resistance measurement range is 0.1 to 4500 Ohm (gain=1) or 0.05 to 2250 Ohm (gain=2).

Key RTD specifications (X20AT2222):

• Max. error at 25C: Gain 0.037%, Offset 0.0015%
• Gain drift: 0.004%/C
• Offset drift: 0.00015%/C
• Non-linearity: <0.0010%
• Crosstalk between channels: <-93 dB
• Common-mode rejection: DC >95 dB, 50 Hz >80 dB

1.5 Universal/Multi-Signal Modules

The X20AI8039 is the most versatile analog input module, offering 8 configurable channels where each channel can independently be set to:

• +/-10V voltage
• 0-20 mA / 4-20 mA current
• Pt100 / Pt1000 (2- or 3-wire)
• ICTD measurement (integrated circuit temperature diode)

This makes it ideal for mixed-signal applications where temperature and process signals coexist on the same I/O node.

1.6 Strain Gauge / Load Cell Inputs

For precision measurement applications, B&R offers:

• X20AI1744 (1 channel) and X20AIA744 (2 channels): Full-bridge strain gauge inputs, 24-bit resolution, 5 kHz input filter
• X20AIB744 (4 channels): Full-bridge strain gauge inputs, 24-bit resolution, 2.5 kHz input filter

These support both 4-wire and 6-wire load cell connections, with internal compensation for lead wire resistance.

1.7 Specialty Modules

• X20AP3111/3121/3131/3161/3171: Energy metering modules with 3 analog voltage inputs (up to 480 VAC) and 4 analog current inputs (20 mA / 1 A / 5 A / Rogowski), calculating RMS values, active/reactive/apparent power and energy.
• X20CM4800X: Vibration measurement module with configurable sampling rate of 200 to 50,000 samples/second and 4 input channels.
• X20RT8201/8381/8401: reACTION Technology modules with 500 kHz sampling frequency, 13-bit resolution analog inputs for ultra-fast control loops.
• X20SA4430: Safe analog current input module (2x2 type A), 0.5 to 25 mA, individually galvanically isolated channels for safety-related applications.

2. Signal Conditioning Circuits on B&R Analog Modules

2.1 Input Stage Architecture

B&R analog input modules follow a consistent front-end architecture:

Terminal Block (field wiring)
    |
    v
Input Protection (PTC, clamping diodes)
    |
    v
Multiplexer / Analog Switch (channel selection)
    |
    v
Signal Conditioning
    |-- Voltage mode: Buffer amplifier (high-Z input, 20 MOhm)
    |-- Current mode: Precision shunt resistor + instrumentation amplifier
    |-- TC mode: Low-drift amplifier with high CMRR
    |-- RTD mode: Constant current source + differential amplifier
    |
    v
Anti-Aliasing Filter (3rd-order low-pass, 500 Hz - 1 kHz)
    |
    v
ADC (SAR for V/I, Sigma-Delta for TC/RTD)
    |
    v
Digital Filter (configurable first-order or third-order)
    |
    v
Linearization / Calibration Coefficients
    |
    v
Output Register (INT/DINT format)

2.2 Voltage Signal Conditioning

For voltage inputs, the conditioning chain consists of:

1. High-impedance buffer amplifier (20 MOhm input impedance) prevents loading the signal source
2. Third-order low-pass anti-aliasing filter with 1 kHz cutoff frequency (on X20AI4622)
3. SAR ADC performs the digitization
4. Digital output filter provides additional configurable smoothing (see Section 12)

The input circuit includes PTC thermistors and protection circuitry to guard against miswiring (connection to supply voltage up to +/-30V).

2.3 Current Signal Conditioning

Current measurement uses an internal shunt resistor topology:

AI+ (current in) --> [PTC] --> [Shunt Resistor < 400 Ohm] --> [GND reference]
                         |
                    [Amp to ADC]

The switch between voltage and current measurement is implemented via an integrated analog switch inside the module, automatically activated based on:

1. Which terminal is used (U terminals for voltage, I terminals for current)
2. The channel type configuration register setting

2.4 Thermocouple Signal Conditioning

Thermocouple modules use a significantly different architecture:

TC+ / TC- terminals
    |
    v
Multiplexer (channels 1-6)
    |
    v
Low-noise instrumentation amplifier (high CMRR >70 dB)
    |
    v
Sigma-Delta ADC (16-bit)
    |
    v
Digital filter (first-order low-pass, 500 Hz cutoff)
    |
    v
Linearization (polynomial approximation per TC type to EN 60584)
    |
    v
Cold Junction Compensation (internal PT1000 sensor)
    |
    v
Temperature output (C)

Key features of the TC conditioning chain:

• Common-mode range: +/-15V (accommodates the high common-mode voltages that thermocouples may pick up)
• Permissible input signal: Max +/-5V
• Common-mode rejection: DC >70 dB, 50 Hz >70 dB (critical for rejecting ground-loop noise in TC wiring)

2.5 RTD Signal Conditioning

RTD measurement on the X20AT2222 uses constant-current excitation:

RTD+ terminal
    |
    v
[Constant Current Source: 250 uA]
    |
    v
[RTD Sensor element]
    |
    v
[3-wire compensation network]
    |
    v
[Reference Resistor: 4530 Ohm]
    |
    v
[Sigma-Delta ADC]
    |
    v
[IEC/EN 60751 Linearization]
    |
    v
Temperature output

The 3-wire measurement compensates for lead wire resistance by measuring the voltage drop on the sense leads and subtracting the lead resistance from the total measured resistance.

2.6 Galvanic Isolation

B&R offers modules with different isolation levels:

Standard X20 analog modules isolate all channels from the bus (500 Veff) but not from each other. For applications requiring channel-to-channel isolation, the galvanically isolated variants (X20AI2237 for voltage, X20AI2437/2438 for current) provide this with independent sensor power supplies per channel.

3. Calibration Procedures for Analog Inputs and Outputs

3.1 Factory Calibration

B&R analog modules are factory-calibrated at the B&R facility in Eggelsberg, Austria. The calibration is performed using traceable reference standards, and the calibration coefficients are stored in the module’s internal firmware.

Key points about B&R factory calibration:

• Each module is individually calibrated; calibration data is stored in the module’s internal memory
• No user-accessible calibration trim pots exist on the modules (calibration is digital)
• Modules ship with a Certificate of Conformance confirming they meet published specifications
• Factory calibration covers gain error, offset error, and linearity within the published specifications

3.2 Verification Procedure for Analog Inputs

To verify the accuracy of a B&R analog input module in the field:

Equipment required:

• Calibrated multi-function process calibrator (e.g., Fluke 789, Beamex MC6)
• Calibrated precision multimeter (for cross-checking)
• Test leads and appropriate terminations

Procedure for voltage inputs (e.g., X20AI4632):

1. Disconnect field wiring from the channel to be tested
2. Configure the channel for the appropriate voltage range via Automation Studio
3. Set the process calibrator to output 0.000V
4. Apply the signal to the AI+/AI- terminals
5. Read the value in Automation Studio (mapp View, online values in PLC program, or via the oscilloscope function on X20AI4632)
6. Record the reading and compare to the expected value (should be near 0)
7. Repeat at 25%, 50%, 75%, and 100% of the range (+/-2.5V, +/-5.0V, +/-7.5V, +/-10V)
8. Calculate the error at each point as: Error (%) = [(Reading - Applied) / Full Scale] * 100

Acceptance criteria (X20AI4622, 13-bit, at 25C):

• Gain error: < 0.08% of measured value
• Offset error: < 0.015% of 20V range
• Non-linearity: < 0.025% of 20V range

Procedure for current inputs (e.g., X20AI4622):

1. Configure the channel for 4-20 mA mode
2. Apply 4.000 mA from the calibrator in series with the input
3. The reading should be near 0 (4 mA = 0 in 4-20 mA scaled mode)
4. Step through 8 mA, 12 mA, 16 mA, and 20 mA
5. Record readings and calculate error at each point

Acceptance criteria for 4-20 mA (X20AI4622):

• Gain error: < 0.1% of measured value
• Offset error: < 0.0375% of 20 mA range

3.3 Verification Procedure for Analog Outputs

Procedure for X20AO4622 (4 channels, +/-10V or 0-20 mA):

1. Disconnect field wiring from the channel to be tested
2. Connect the calibrated multimeter across AO+/AO-
3. Configure the channel for the appropriate range
4. Write known values to the output register in Automation Studio
5. Measure the actual output voltage/current
6. Compare measured vs. commanded values

Acceptance criteria (X20AO4622 at 25C):

• Voltage gain error: < 0.08% of measured value
• Voltage offset error: < 0.05% of full range
• Current gain error: < 0.09% of measured value
• Current offset error: < 0.05% of full range
• Non-linearity: < 0.007% (voltage), < 0.005% (current)

3.4 Verification Procedure for Temperature Inputs

Thermocouple module (X20AT6402):

1. Disconnect the field thermocouple
2. Connect a thermocouple calibrator (cold-junction compensated) to the TC+/TC- terminals
3. Set the sensor type in the module configuration (e.g., Type K)
4. Set the calibrator to 0.0C and observe the module reading
5. Step through temperature points (e.g., 100C, 250C, 500C, 1000C)
6. Verify readings are within specifications

Acceptance criteria (X20AT6402 at 25C):

• Gain error: +/-0.06%
• Type K offset error: +/-0.05%
• Non-linearity: +/-0.001%

RTD module (X20AT2222):

1. Disconnect the field RTD
2. Connect a precision decade resistance box (or RTD calibrator)
3. Configure the channel for Pt100 (IEC 60751)
4. Apply resistance values corresponding to known temperatures (e.g., 100 Ohm = 0C, 138.5 Ohm = 100C, 214.6 Ohm = 300C)
5. Compare the module reading to the expected temperature

Acceptance criteria (X20AT2222 at 25C):

• Gain error: 0.037% of current resistance value
• Offset error: 0.0015% of full resistance range
• Non-linearity: <0.0010%

3.5 Software-Based Calibration Correction

Since B&R modules do not have user-accessible trim adjustments, calibration correction (if needed beyond factory accuracy) must be performed in software:

Two-point calibration in Automation Studio (ST language):

FUNCTION AnalogCalibrate : LREAL
VAR_INPUT
    RawValue : INT;
    CalLow : INT;    (* Raw reading at zero/low calibration point *)
    CalHigh : INT;   (* Raw reading at span/high calibration point *)
    ScaleLow : LREAL; (* Engineering units at low point *)
    ScaleHigh : LREAL; (* Engineering units at high point *)
END_VAR

AnalogCalibrate := ScaleLow + (TO_LREAL(RawValue - CalLow) / TO_LREAL(CalHigh - CalLow)) * (ScaleHigh - ScaleLow);

mapp components: B&R’s mapp Control technology provides built-in scaling and linearization function blocks that can incorporate calibration offsets directly.

3.6 Recalibration and Repair

B&R modules that drift beyond specification should be returned to B&R for recalibration or repair through the B&R Support Portal or Material Return Portal. B&R offers repair services at their facilities with recalibration using traceable standards.

4. Offset/Gain Adjustment Methods

4.1 Understanding Offset and Gain Errors

In ADC systems, two fundamental errors affect measurement accuracy:

• Offset error: A constant shift in the reading, present even when the input is zero. On B&R modules, offset is specified at 25C (e.g., X20AI4622 voltage offset: 0.015% of 20V range = 3 mV).

• Gain error: The slope of the transfer function deviates from the ideal. A reading at full scale will be proportionally incorrect (e.g., X20AI4622 voltage gain error: 0.08% of measured value).

• Linearity error: The transfer function is not perfectly straight between the calibrated endpoints. B&R specifies this separately (e.g., <0.025% for voltage on the X20AI4622).

4.2 Temperature Drift

Both offset and gain errors drift with temperature. B&R specifies maximum drift coefficients:

Practical impact: For a X20AI4622 measuring +/-10V, operating in a 30C range (e.g., 10C to 40C ambient), the worst-case gain drift is:

• 0.006%/C * 30C * 20V range = 0.036V = 36 mV

This is larger than the 1 LSB resolution (2.441 mV), demonstrating why temperature control in the cabinet matters for precision applications.

4.3 Software Offset/Gain Compensation

Since B&R modules have no hardware trim adjustments, compensation is done in software:

Method 1: Two-Point Calibration (offset + gain correction)

Apply two known reference signals and compute correction factors:

Apply ZERO reference -> Read RawZero
Apply SPAN reference -> Read RawSpan

CorrectedValue = (RawValue - RawZero) * (SpanReference / (RawSpan - RawZero)) + ZeroReference

Method 2: Single-Point Offset Correction

For applications where gain error is acceptable but offset drift needs correction:

Apply ZERO reference -> Read RawZero
CorrectedValue = RawValue - RawZero

Method 3: mapp Scaling Function Blocks

Use B&R’s mapp Control components (AsCal, AsScale) which provide integrated scaling with offset and gain parameters configurable through Automation Studio.

4.4 Using the Oscilloscope Function for Calibration

The X20AI4632 and X20AI4636 modules feature built-in oscilloscope functions that allow real-time visualization of analog signals directly in Automation Studio. This is useful during calibration to:

• Observe signal stability and noise levels
• Verify filter performance
• Detect oscillations or interference
• Capture transients that may affect calibration accuracy

4.5 Oversampling for Improved Resolution (X20AI4636)

The X20AI4636 supports oversampling functions, where multiple ADC conversions are averaged to effectively increase resolution and reduce noise:

• Oversampling ratio is configurable
• Effective resolution improvement follows: N_additional_bits = log2(oversampling_ratio) / 2
• 4x oversampling yields approximately 1 additional bit of resolution
• Trade-off: increased conversion time

5. Noise and Grounding Issues as Analog Reading Errors

5.1 How Noise Manifests in Analog Readings

Noise on B&R analog inputs appears in several characteristic patterns:

5.2 Noise Sources in B&R X20 Systems

External noise sources:

• Variable frequency drives (VFDs) and motor inverters
• Switching power supplies
• Relay and contactor switching
• Radio frequency interference (RFI) from wireless devices
• Welding equipment
• Power line harmonics

Internal noise sources:

• X2X Link communication (between bus modules)
• Digital I/O switching on adjacent modules
• Power supply ripple on the internal I/O supply
• ADC quantization noise

5.3 Crosstalk Between Channels

B&R specifies crosstalk between channels:

• X20AI4622 (V/I): <-70 dB
• X20AT6402 (TC): <-70 dB
• X20AT2222 (RTD): <-93 dB

At -70 dB crosstalk, a 10V signal on one channel can induce up to approximately 3.16 mV on an adjacent channel (which is about 1 LSB on a 13-bit module). For high-precision applications, this is significant and requires careful signal routing and filtering.

5.4 Common-Mode Rejection

Common-mode noise (noise that appears equally on both input terminals) is rejected by the differential input architecture:

70 dB CMRR means that 1V of common-mode noise is reduced to approximately 316 uV at the ADC input. For the RTD module (X20AT2222) with 95 dB CMRR at DC, 1V of common-mode noise is reduced to approximately 18 uV.

5.5 Ground Loops

Ground loops occur when there are multiple ground paths between the sensor and the I/O module, creating a current loop that introduces offset errors and noise:

Sensor Ground --> [Sensor Cable] --> AI- terminal
     |                                        |
     +------ [Earth Ground Path] -------> [Module Ground]

The resulting loop current flows through the cable shield or signal ground and creates voltage drops that appear as measurement error. A 10 mA ground loop current through a 1 Ohm ground path creates 10 mV of error, which exceeds the 0.015% offset specification of the X20AI4622.

Solutions:

• Single-point shield grounding (see Section 13)
• Use galvanically isolated modules (X20AI2237, X20AI2437) which break the ground loop path
• Isolation amplifiers or signal conditioners at the sensor
• Star-point grounding in the panel

6. Distinguishing Sensor Faults from IO Card Faults

6.1 Systematic Fault Isolation Approach

When an analog reading is incorrect, follow this systematic approach to isolate the fault:

Step 1: Check module diagnostics

B&R analog modules provide real-time diagnostics via:

• LED indicators: Per-channel green LEDs (solid = OK, blinking = overflow/underflow/open), module status LEDs (green = RUN, red = error)
• Status registers: StatusInput01 and StatusInput02 registers report per-channel status bits

Status bit interpretation for X20AI4622:

For thermocouple modules (X20AT6402), additional status bits indicate:

• Open circuit (wire break detection)
• Range overshoot/undershoot
• General fault

Step 2: Swap channels

Move the sensor to a different channel on the same module:

• If the error follows the sensor –> sensor or wiring fault
• If the error stays on the original channel –> IO card fault

Step 3: Apply known reference signal

Disconnect the sensor and apply a precision reference:

• Voltage: Use a calibrated voltage source
• Current: Use a calibrated current source (4-20 mA calibrator)
• Thermocouple: Use a TC simulator with cold junction compensation
• RTD: Use a precision decade resistance box

If the module reads the reference correctly, the fault is in the sensor or field wiring.

Step 4: Cross-check with another module

If a spare analog channel or module is available, connect the sensor to it:

• Confirms whether the original module has degraded

6.2 Common Sensor Faults and Symptoms

6.3 Common IO Card Faults and Symptoms

6.4 Using B&R Diagnostic Tools

B&R provides several diagnostic tools:

• Automation Studio Diagnostics: Real-time I/O value monitoring, status register inspection
• Oscilloscope function (X20AI4632): Visualize raw analog waveforms to detect noise, oscillations
• mapp AlarmX: Configure alarms based on analog status bits (open circuit, limit violations)
• mapp View: Create HMI screens showing real-time status with color-coded health indicators
• Web browser diagnostics: Many B&R controllers expose diagnostic web pages for remote monitoring

7. Resolution and Accuracy Specifications

7.1 Understanding Resolution vs. Accuracy

Resolution is the smallest change in the analog input that can be represented in the digital output. It is determined by the ADC bit depth.

Accuracy is the combined effect of all errors (gain, offset, non-linearity, noise, drift) and describes how close the reading is to the true value. Accuracy is always worse than resolution.

7.2 Resolution by Module Type

7.3 Accuracy Summary

Voltage measurement accuracy (25C):

Current measurement accuracy (25C):

Thermocouple accuracy (25C, X20AT6402):

RTD accuracy (25C, X20AT2222):

7.4 Total Error Budget

When calculating total measurement uncertainty, combine all error sources:

1. Module errors (gain, offset, linearity) – from datasheet
2. Temperature drift errors – calculated from drift coefficients and operating temperature range
3. Sensor errors – from sensor datasheet (e.g., Class A Pt100: +/-0.15C + 0.002*|t|)
4. Lead wire errors – for RTD, uncompensated lead resistance
5. Noise and resolution – typically 1-2 LSB
6. Calibration reference uncertainty – uncertainty of the calibration standard

Example total error calculation for Pt100 at 300C on X20AT2222:

• Module gain: 0.037% of 212.05 Ohm = 0.078 Ohm = 0.2C
• Module offset: 0.0015% of range = negligible
• Sensor (Class A): +/-0.15C + 0.002*300 = +/-0.75C
• Module non-linearity: <0.001% = negligible
• Total (RSS): sqrt(0.2^2 + 0.75^2) = +/-0.78C

7.5 Accuracy Over Temperature

The derating curves in B&R datasheets specify how accuracy degrades at elevated temperatures. For example, the X20AO4622 shows reduced permissible load at temperatures above 50C (horizontal mounting) due to increased power dissipation in the output stage.

For the X20AI4622 operating between 0C and 55C:

• Worst-case gain drift from 25C reference: 0.006%/C * 30C = 0.18%
• Worst-case offset drift from 25C reference: 0.002%/C * 30C = 0.06%

8. Sampling Rate and Aliasing Considerations

8.1 Sampling Architecture

B&R analog input modules use two different sampling approaches:

Standard modules (X20AI4622, X20AI4632):

• SAR ADC with fixed conversion time per channel
• X20AI4622: 400 us conversion for all 4 inputs (channels sampled sequentially, 200 us offset between channels)
• Conversion is asynchronous to the network cycle
• Update rate is determined by bus cycle time (minimum 100 us without filter, 500 us with filter)
• Minimum I/O update: 300 us (no filter), 1 ms (with filter)

Thermocouple/RTD modules (X20AT6402, X20AT2222):

• Sigma-delta ADC with configurable filter time
• All enabled channels are converted during each conversion cycle
• Conversion time depends on filter setting and number of channels
• Example: X20AT6402 with 50 Hz filter, 6 channels = 281.4 ms total update

8.2 High-Speed Modules

8.3 Anti-Aliasing Filters

All B&R analog input modules include hardware anti-aliasing filters before the ADC:

The hardware filter attenuates frequencies above the cutoff, preventing aliasing. For the standard 13-bit modules, the 1 kHz third-order filter provides approximately -18 dB/octave rolloff, which combined with the effective sampling rate (minimum 2.5 kHz for 400 us conversion), ensures adequate anti-aliasing for most industrial process signals.

8.4 Nyquist Considerations

For standard process control (voltage/current, 400 us conversion):

• Effective sampling rate: ~2,500 samples/s per module (all 4 channels within 400 us)
• Nyquist frequency: ~1,250 Hz
• With 1 kHz hardware filter: signals up to ~500 Hz are accurately captured
• Sufficient for virtually all process control applications

For thermocouple measurement (50 Hz filter):

• Effective sampling rate: 1 channel = 80.4 ms = ~12.4 samples/s
• Nyquist frequency: ~6.2 Hz
• Temperature signals typically change at <1 Hz, so this is adequate

For vibration monitoring (X20CM4800X):

• Configurable 200-50,000 samples/s
• Anti-aliasing must be considered: with 50,000 samples/s, Nyquist = 25,000 Hz
• External anti-aliasing filters may be needed if measuring signals above 10 kHz
• Standard practice: use at least 2.56x oversampling relative to the highest frequency of interest

8.5 Practical Aliasing Mitigation

1. Use the built-in digital filter to reject high-frequency noise (see Section 12)
2. Ensure analog signals are bandwidth-limited before reaching the module (shielded cable acts as a low-pass filter for long runs)
3. For the X20CM4800X vibration module: configure the sampling rate at least 2.56x the maximum frequency of interest
4. Apply input ramp limiting (X20AI4622) to suppress transient spikes that could alias

9. Cold Junction Compensation for Thermocouple Inputs

9.1 Principle of Cold Junction Compensation

Thermocouples generate a voltage proportional to the temperature difference between the measurement junction (hot) and the reference junction (cold), where the thermocouple wires connect to the copper terminals of the IO module. To determine the absolute temperature at the measurement point, the temperature of the reference junction (the terminal block) must be known and added to the measured temperature difference.

B&R thermocouple modules (X20AT2402, X20AT6402) implement cold junction compensation (CJC) using an integrated PT1000 sensor mounted on the terminal block. The module measures the terminal temperature and applies the correction internally.

9.2 Internal Cold Junction Compensation

How it works on X20AT6402:

1. The internal PT1000 sensor measures the terminal block temperature
2. This value is available in the CompensationTemperature register (-250 to +850, representing -25.0 to +85.0C)
3. During each conversion cycle, the module:
   • Converts all enabled TC channels
   • Measures the terminal temperature (one extra conversion in Function Model 0)
   • Applies the polynomial correction for the TC type (per EN 60584)
   • Adds the cold junction temperature correction
   • Outputs the linearized temperature value

Conversion time impact:

• Function Model 0 (internal CJC): (n+1) conversions per cycle (n TC channels + 1 terminal temperature)
• Function Model 1 (external CJC): n conversions per cycle (terminal temperature measurement skipped)

9.3 Cold Junction Compensation Accuracy

The CJC precision depends on thermal conditions:

Factors affecting CJC accuracy:

• Self-heating: Other modules on the same X2X Link dissipating heat onto the TC module
• Air currents: Fans or forced ventilation create temperature gradients across the terminal block
• Ambient temperature fluctuations: Rapid changes in cabinet temperature cause lag in the CJC sensor
• Module power dissipation: The X20AT6402 itself consumes 0.91 W (internal I/O)

9.4 Configuring Ambient Conditions

The X20AT6402 provides an “Ambient Conditions” configuration register to adapt the internal CJC thermal model:

How to configure: Check the power consumption of the modules immediately adjacent to the X20AT6402 on the X2X Link (from their datasheets). Use the higher value for the setting.

9.5 External Cold Junction Compensation

For improved accuracy or for long thermocouple cable runs, B&R supports external cold junction compensation:

When to use external CJC:

1. Large distances between the controller and measurement point (copper extension wires from external junction to module)
2. Adjacent high-power modules (>1 W) are connected to the same X2X Link
3. Fluctuating ambient conditions (drafts, temperature cycling)
4. No other modules connected near the TC module

Implementation (using X20AT6402 + X20AT4222):

Measurement Point
    |
    v
[Thermocouple]
    |
    v
External Cold Junction (copper wire extension)
    |
    +--> [X20AT4222 RTD module] measures cold junction temperature
    |         |
    |         v
    |    Register: ExternalCompensationTemperature
    |
    v
[X20AT6402 TC module] uses ExternalCompensationTemperature
    |
    v
Corrected temperature output

Configuration:

1. Select Function Model 1 on the X20AT6402 (External Cold Junction Temperature)
2. Measure the cold junction temperature with a separate RTD module (e.g., X20AT4222 with a Pt100 sensor)
3. Write the cold junction temperature to the ExternalCompensationTemperature register of the X20AT6402
4. The X20AT6402 will use this value for all channel corrections

9.6 Recommended Wiring for Thermocouples

B&R recommends connecting the minus input of the thermocouple to the minus input of the power supply module (e.g., X20PS2100) to prevent measurement errors from ripple voltage effects:

[Power Supply X20PS2100]
    |-
    |
    v
[X20AT6402]
    TC1- --> Connected to PS minus input
    TC2- --> Connected to PS minus input
    TC3-
    ...

10. 3-Wire and 4-Wire RTD Measurement

10.1 RTD Wiring Configurations

2-Wire Connection: The simplest but least accurate method. Both the current source and voltage measurement share the same lead wires, so lead wire resistance adds directly to the measured RTD resistance.

Module                    RTD
I+ -------- Lead Wire A --------+
                               RTD Element
I- -------- Lead Wire B --------+

Error = 2 * R_lead. For 10m of 0.5 Ohm/m cable: error = 10 Ohm = ~26C for Pt100.

3-Wire Connection (B&R X20AT2222, X20AI8039): The most common industrial configuration. Two wires carry the excitation current, and the third wire provides a sense connection. If all three wires have equal resistance, the lead resistance cancels out.

Module                    RTD
I+ -------- Lead Wire A --------+
                               RTD Element
Sense ----- Lead Wire B --------+
                               RTD Element (continued)
I- -------- Lead Wire C --------+

The module measures:

• V1 = V across (RTD + 2 * R_lead) [using I+ and I-]
• V2 = V across (RTD + R_lead) [using Sense and I-]
• RTD_resistance = V2 - (V1 - V2) = 2*V2 - V1 (assuming equal lead resistances)

Residual error = R_lead_A - R_lead_C (difference in lead wire resistance). For matched wires: negligible.

4-Wire Connection: The most accurate method, using separate pairs for current excitation and voltage sensing. No lead resistance error.

Module                    RTD
I+ -------- Lead Wire A --------+
                               RTD Element
I- -------- Lead Wire B --------+

V+ -------- Lead Wire C --------+
                               RTD Element (continued)
V- -------- Lead Wire D --------+

Error = 0 (lead resistance fully eliminated). This is used in laboratory and calibration applications.

10.2 B&R Module RTD Wiring Support

Note: B&R X20 RTD modules do not natively support 4-wire RTD connections. For 4-wire measurement, a dedicated 4-wire RTD transmitter or signal conditioner can be used to convert to a 4-20 mA signal, which is then connected to a B&R current input module (e.g., X20AI2437).

10.3 RTD Measurement Parameters (X20AT2222)

10.4 Conversion Time for RTD

With 2 channels and 50 Hz filter, the update rate is 12.5 Hz, which is more than sufficient for temperature process control.

10.5 Self-Heating Considerations

The 250 uA excitation current causes self-heating in the RTD element. For a Pt100 sensor:

• At 100C: R = 138.5 Ohm
• Power dissipation: I^2 * R = (250e-6)^2 * 138.5 = 8.66 uW

This is negligible for most industrial Pt100 sensors in typical mounting configurations (sensor element to sheath thermal resistance is much larger than this would heat).

For Pt1000 sensors (10x the resistance), self-heating is 10x higher (~86.6 uW), still negligible.

11. Current Loop (4-20 mA) Diagnostics

11.1 Current Loop Fundamentals on B&R Modules

The 4-20 mA current loop provides inherent diagnostic capabilities because:

• 4 mA = live zero (scale minimum): any reading below 4 mA indicates a problem
• 0 mA = broken wire or no power: definitive open circuit
• >20 mA = over-range: sensor fault or incorrect calibration

B&R modules leverage this by monitoring the input against configurable limits.

11.2 Open Circuit Detection

B&R analog modules detect open circuits differently depending on the signal type:

Voltage mode (X20AI4622):

• Open circuit detection is supported via the status register
• When the input is disconnected, the high-impedance input floats to an undefined voltage
• The module detects this and sets the status bit to “Open circuit”
• The analog value is frozen at +32767 (0x7FFF)
• Per-channel green LED turns off (no green = no signal)

Current mode (X20AI4622):

• Open circuit detection is based on limit monitoring
• When the current loop is broken, the current drops to 0 mA
• For 4-20 mA configuration: values <4 mA can be detected by setting the lower limit to 0 mA (corresponding to INT value -8192)
• The status bit shows “Lower limit value undershot”

B&R recommended configuration for 4-20 mA open circuit detection:

1. Set the channel type to “4 to 20 mA current signal”
2. Set the lower limit value register to -8192 (corresponding to 0 mA)
3. Monitor the StatusInput register for the “Lower limit value undershot” bit

11.3 Short Circuit Detection

On analog inputs:

• A short circuit on a voltage input results in 0V reading
• A short circuit on a current input may cause the current to exceed 20 mA (if the transmitter is not current-limited)
• B&R modules tolerate up to +/-50 mA on current inputs without damage (input protection)
• The upper limit monitoring detects readings above 20 mA

On analog outputs (X20AO4622):

• Outputs are short-circuit proof with current limiting at +/-40 mA
• This protects both the module and the field wiring
• A short circuit on a current output will not damage the module

11.4 Diagnostic Status Values

When monitoring detects a fault, B&R modules freeze the analog value at defined error codes:

For thermocouple modules (X20AT6402):

11.5 Implementing Current Loop Diagnostics in Software

Example ST implementation for 4-20 mA channel monitoring:

FUNCTION CheckCurrentLoop : BOOL
VAR_INPUT
    RawValue : INT;
    Status : USINT;
    Channel : USINT;
END_VAR
VAR
    ChStatus : USINT;
    Shift : USINT;
BEGIN
    Shift := Channel * 2;
    ChStatus := SHR(Status, Shift) AND 16#03;

    CASE ChStatus OF
        0: (* No error - normal operation *)
            CheckCurrentLoop := FALSE;
        1: (* Lower limit undershot - possible open circuit *)
            CheckCurrentLoop := TRUE;
        2: (* Upper limit exceeded - possible short or over-range *)
            CheckCurrentLoop := TRUE;
        3: (* Open circuit - for voltage mode *)
            CheckCurrentLoop := TRUE;
    END_CASE;
END_FUNCTION

11.6 NAMUR NE43 Recommendation

For advanced current loop diagnostics, the NAMUR NE43 standard defines failure modes:

To implement NAMUR NE43 on a B&R module:

• Set the lower limit to detect <4 mA (or <3.6 mA for the first threshold)
• Use software to check if the raw value is between 3.6 mA and 4 mA (warning zone)
• The X20AI2438 (HART-capable) provides additional digital diagnostic data from smart transmitters

12. Filter Settings and Response Time Trade-offs

12.1 B&R Module Filter Architecture

B&R analog modules implement a two-stage filtering approach:

1. Hardware anti-aliasing filter: Fixed 3rd-order (voltage/current modules) or 1st-order (temperature modules) low-pass filter at the ADC input
2. Digital filter: Configurable first-order IIR filter or input ramp limiter applied after ADC conversion

12.2 Digital Filter on Voltage/Current Modules (X20AI4622)

The X20AI4622 provides a configurable digital filter with two components:

Component 1: Input Ramp Limiting (applied first)

Suppresses transient spikes by limiting the maximum change per cycle:

Component 2: Averaging Filter (applied after ramp limiting)

First-order IIR filter with configurable time constant:

Filter formula:

Value_New = Value_Old - (Value_Old / Filter_Level) + (Input_Value / Filter_Level)

This is equivalent to: Value_New = Value_Old * (1 - 1/N) + Input * (1/N), where N is the filter level.

Important constraint: The filter function is disabled for cycle times <500 us. The minimum cycle time with filtering is 500 us.

12.3 Filter Settings on Temperature Modules (X20AT6402, X20AT2222)

Temperature modules offer selectable hardware-level filter time constants:

Default setting: 50 Hz (20 ms) – this is tuned to reject 50 Hz power-line interference, which is the most common noise source in temperature measurements. For 60 Hz regions, use setting 4 (60 Hz, 16.7 ms).

12.4 Response Time Trade-offs

Key trade-off: Higher filter levels (longer time constants) provide better noise rejection but introduce latency. In closed-loop control systems, excessive filtering can cause instability or poor controller performance. The filter time constant should be significantly shorter than the process time constant.

12.5 Using Input Ramp Limiting for Spike Rejection

Input ramp limiting is particularly effective for suppressing:

• Contact bounce from relay switching
• Inverter switching noise
• Electromagnetic pulses from nearby equipment

Example configuration for a noisy 4-20 mA pressure signal:

• Input ramp limiting = 4 (limit 2047 LSB)
• Filter level = 16 (16 bus cycles)

This combination will:

1. Block any single-cycle spike >2047 LSB (approx. 10 mA instantaneous)
2. Smooth remaining fluctuations over ~16 cycles

12.6 Minimum Cycle Time Impact

13. Grounding Best Practices for Analog Signals

13.1 B&R Shield Grounding System

B&R provides a dedicated cable shield grounding clamp (model X20AC0SG1) that latches to the X20 terminal block and connects to the bus module’s ground connection using a cable lug. This provides a proper EMC ground path for cable shields.

Shield grounding clamp specifications:

• Model: X20AC0SG1 (package of 10 pcs: X20AC0SG1.0010, package of 100: X20AC0SG1.0100)
• Mounts to X20 terminal block
• Accepts cable shields from 3 mm to 8 mm diameter (depending on variant)
• Connects to bus module ground via cable lug

13.2 Shield Wiring Rules for B&R X20

Rule 1: Use shielded cables for all analog signals

The X20 system user’s manual mandates shielded cables for analog input modules. The ground connection for the shield is made on the terminal block shield connections provided on each module.

Rule 2: Single-point shield grounding (default)

Ground the cable shield at one end only – typically at the B&R I/O module (panel end). This prevents ground loops:

Field                      Panel
[Sensor] -- [Shielded Cable] -- [X20 Module]
              |                      |
           Floating              Shield clamp
                                  to bus module ground

Rule 3: Both-end grounding for high-frequency noise (with caution)

For RF interference above 10 MHz, grounding the shield at both ends provides better shielding. However, this creates a ground loop for low-frequency noise. If both-end grounding is required:

• Use galvanically isolated modules (X20AI2237, X20AI2437) to break the ground loop
• Or use a shield isolation barrier

Rule 4: Separate analog and digital cable routing

Route analog signal cables separately from:

• Motor power cables
• VFD output cables
• Digital I/O cables with high switching rates
• Power supply cables

Maintain at least 200 mm separation, or use grounded metallic cable trays as segregation.

Rule 5: Star-point grounding in the panel

All ground connections in the control panel should converge to a single star-point ground bar, which is then connected to the protective earth (PE) at one point. This prevents ground loop currents between different equipment grounds.

13.3 Grounding for Thermocouple Signals

Thermocouple wiring is particularly susceptible to ground loops because the TC junction itself may be grounded (welded to a metal pipe or vessel). B&R recommendations:

1. Connect the TC minus terminal to the power supply minus (X20PS2100) to prevent ripple coupling
2. Use shielded twisted-pair thermocouple extension wire
3. Ground the shield at the module end only
4. If the TC measurement junction is grounded, ensure the module’s common-mode range is not exceeded (check that the ground potential difference between the TC location and the panel is within +/-15V)

13.4 Grounding for RTD Signals

RTD wiring is less susceptible to noise than thermocouples due to the low impedance of the sensor. However:

• Use shielded cables for runs >10m
• Ground the shield at one end only
• Ensure all three wires in a 3-wire connection have equal length and routing

13.5 Grounding for 4-20 mA Current Loops

Current loops are inherently immune to ground-loop-induced voltage noise (since the signal is a current, not a voltage). However:

• The transmitter (sensor) and receiver (B&R module) may have different ground references
• Use shielded twisted-pair cable
• Ground the shield at the panel end (B&R module)
• If using 2-wire transmitters, ensure the loop power supply can drive the total loop resistance

13.6 B&R Coated Modules (X20c) for Harsh Environments

B&R offers coated X20c modules with a protective conformal coating for the electronics. The coating protects against:

• Condensation (certified per BMW GS 95011-4)
• Corrosive gases (certified per EN 60068-2-60, method 4, 21 days exposure)

Coated modules are electrically and functionally identical to standard X20 modules. They require the same grounding practices but offer additional reliability in harsh environments.

13.7 EMC Installation Guide Reference

B&R publishes a dedicated Installation / EMC Guide (document MAEMV) that covers:

• Panel layout and cable routing
• Shield termination methods
• Ground bar installation
• Filter placement
• EMC testing and verification

This document should be consulted for new installations and when troubleshooting EMC-related analog signal issues.

14. Calibration Certification and Traceability

14.1 B&R Factory Calibration and Certificates

B&R analog modules are calibrated during manufacturing with reference standards traceable to national and international standards (SI units). The calibration process includes:

1. Gain calibration: Applying known reference signals at multiple points across the measurement range
2. Offset calibration: Verifying the zero/offset point
3. Linearity verification: Checking linearity across the full range
4. Temperature testing: Verifying specifications over the operating temperature range
5. EMC testing: Ensuring accuracy is maintained under electromagnetic stress

Documentation supplied with each module:

• Certificate of Conformance (CoC): Confirms the module meets all published specifications
• Test report: Available upon request, containing measured performance data
• Traceability chain: Calibration standards used are traceable to national metrology institutes (e.g., PTB in Germany, BEV in Austria)

14.2 ISO/IEC 17025 Calibration

For applications requiring formal calibration certification (pharmaceutical, food & beverage, aerospace, nuclear):

Option 1: Third-party calibration laboratory

• Send the B&R module to an ISO/IEC 17025 accredited calibration laboratory
• The lab will verify all analog specifications using their traceable standards
• A calibration certificate is issued with:
  • Measurement results (as-found and as-left data)
  • Uncertainty of measurement for each point
  • Traceability statement
  • Environmental conditions during calibration
  • Accreditation body and certificate number
  • Validity period and recommended recalibration interval

Option 2: B&R factory recalibration

• Contact B&R Support Portal or Material Return Portal
• B&R can recalibrate modules at their facility in Eggelsberg, Austria
• Module firmware is updated with new calibration coefficients if needed

Option 3: In-house calibration (if accredited)

• If your facility has an ISO 17025 accredited metrology lab
• Perform verification per the procedures in Section 3 of this document
• Issue calibration certificates under your lab’s accreditation scope

14.3 Calibration Intervals

Recommended recalibration intervals for B&R analog modules:

14.4 Traceability Chain

International Bureau of Weights and Measures (BIPM)
    |
    v
National Metrology Institute (e.g., NIST, PTB, BEV)
    |
    v
Calibration Laboratory (ISO 17025 accredited)
    |
    v
Reference Standards (multifunction calibrator, decade box, etc.)
    |
    v
B&R Analog Module Under Test
    |
    v
Process Measurement (connected to field sensor)

14.5 Calibration Record Keeping

Maintain calibration records including:

• Module serial number and B&R ID code
• Calibration date and next due date
• Calibrating organization (with accreditation number)
• As-found data (measurements before any adjustment)
• As-left data (measurements after calibration)
• Uncertainty of measurement at each test point
• Environmental conditions during calibration
• Reference standards used (with their calibration dates)
• Technician name and approval signature

14.6 Uncertainty Budget for Analog Verification

When performing calibration verification, the total measurement uncertainty includes:

The calibration uncertainty should be significantly smaller than the module’s specification (typically at least a 4:1 TUR – Test Uncertainty Ratio).

Appendix A: B&R Analog Module Quick Reference

Voltage/Current Input Modules

Voltage/Current Output Modules

Temperature Input Modules

Appendix B: Common Error Codes and Troubleshooting

Appendix C: Register Quick Reference

X20AI4622 Key Registers

X20AT6402 Key Registers

Cross-References

• io-card-hardware.md – analog IO card signal conditioning and ADC/DAC paths
• grounding-emc.md – grounding and EMC for analog signal integrity
• system-variables.md – runtime IO status monitoring
• physical-layer-sniffing.md – analog signal quality measurement
• memory-map.md – analog IO memory address mapping
• encoder-diagnostics.md – analog encoder signal analysis
• safe-io-diagnostics.md – safe analog input diagnostics
• alarm-logging.md – analog limit alarm configuration
• online-changes.md – runtime calibration parameter adjustment
• diagnostic-workstation.md – oscilloscope requirements for analog signals

Document compiled from B&R Automation official datasheets, X20 system user’s manuals, and installation/EMC guides. All specifications are subject to change – refer to the latest B&R documentation at br-automation.com for current values.

Sources: X20(c)AI4622 Data Sheet V3.40, X20(c)AT6402 Data Sheet V3.22, X20AT2402 Data Sheet V3.09, X20AT2222 Product Page, X20(c)AO4622 Data Sheet V3.25, B&R X20 System User’s Manual, B&R Installation/EMC Guide.

Key Findings

1. Analog card resolution ranges from 12-bit to 16-bit depending on module (e.g., X20AI4622 = 13-bit incl. sign, X20AI4632 = 16-bit, X20AI2322 = 12-bit). Higher resolution is critical for precision measurement but requires better signal conditioning. See Appendix A for the full module reference table.
2. Distinguishing sensor faults from IO card faults requires a systematic approach: check with a known reference signal, swap channels on the same module, and inspect the analog LED diagnostic patterns.
3. Noise and grounding issues are the most common cause of analog reading errors — always check shield termination and grounding before suspecting the IO card or sensor. See grounding-emc.md.
4. B&R analog input modules use software-configurable ranges (0-10V, +/-10V, 0/4-20mA) — an incorrect range setting on an undocumented machine can cause readings to appear wrong when the hardware is fine.
5. B&R analog modules are factory-calibrated with digital calibration coefficients stored in the module’s internal memory. When replacing a module, the new module has its own factory calibration — you verify it, you don’t re-program it. Document your verification results before and after any analog module replacement.
6. Thermocouple cold junction compensation is built into B&R temperature modules — incorrect thermocouple type selection (J/K/S/R/T/B/N) produces readings that are wrong by a predictable offset, not random noise.