SDAA150 December   2025 ADS124S06 , ADS124S08

 

  1.   1
  2.   Abstract
  3. 1Circuit Design and Test System
    1. 1.1 Design Overview
    2. 1.2 Overview of EMC Test Board for RTD Measurements
      1. 1.2.1 Input Configurations and ADC Settings
        1. 1.2.1.1 Configuration and settings for a 3-wire RTD measurement with a low-side reference
        2. 1.2.1.2 Configuration and settings for a 3-wire RTD measurement with a high-side reference
      2. 1.2.2 Temperature Error - RTD Measurement
        1. 1.2.2.1 Calculating RTD temperature from RTD resistance
        2. 1.2.2.2 Calculating the temperature error from RTD measurements
        3. 1.2.2.3 Experimental setup and results
    3. 1.3 Overview of EMC Test Board for TC Measurements
      1. 1.3.1 Input Configurations and ADC Settings
        1. 1.3.1.1 Input Configurations
        2. 1.3.1.2 Thermocouple Characteristics and ADC Settings
      2. 1.3.2 Temperature Error - TC Measurement
        1. 1.3.2.1 Calculating temperature from TC thermoelectric voltage
        2. 1.3.2.2 Calculating the temperature error from TC measurements
        3. 1.3.2.3 Experimental setup and results
    4. 1.4 Circuit Design Considerations for EMC compliance
      1. 1.4.1 Analog Input Protection
      2. 1.4.2 Anti-aliasing Filter
      3. 1.4.3 High-voltage Capacitors on Every Input Connector Pin
      4. 1.4.4 High-voltage Capacitors and Resistors for Discharging Path
      5. 1.4.5 Series Resistors on Digital Signals
      6. 1.4.6 Digital Isolation
      7. 1.4.7 Power Supply and Protection
    5. 1.5 PCB Layout Consideration for EMC compliance
      1. 1.5.1 PCB Layer Stack-up and Ground Plane
      2. 1.5.2 Avoiding a Long Return Path
      3. 1.5.3 Avoiding 90-degree Bends in PCB Traces
      4. 1.5.4 Using Guard Ring to Isolate Interference Signals
      5. 1.5.5 Decoupling Capacitors
      6. 1.5.6 Differential Signal Routing
      7. 1.5.7 Stitching Vias
      8. 1.5.8 Layout for Isolation Barrier
      9. 1.5.9 Component Placement
    6. 1.6 Test System
  4. 2Test Details and Results
    1. 2.1 Standards and Test Criteria
    2. 2.2 Electrostatic Discharge (ESD)
    3. 2.3 Radiated Immunity (RI)
    4. 2.4 Electrical Fast Transients (EFT)
    5. 2.5 Surge Immunity (SI)
    6. 2.6 Conducted Immunity (CI)
  5. 3Schematic, PCB Layout and Bill of Materials
    1. 3.1 Schematic - RTD EMC Test Board
    2. 3.2 Schematic - TC EMC Test Board
    3. 3.3 PCB Layout - RTD EMC Test Board (4-Layer)
    4. 3.4 PCB Layout - RTD EMC Test Board (2-Layer)
    5. 3.5 PCB Layout - TC EMC Test Board (4-Layer)
    6. 3.6 PCB Layout - TC EMC Test Board (2-Layer)
    7. 3.7 Bill of Materials - RTD EMC Test Board
    8. 3.8 Bill of Materials - TC EMC Test Board
  6. 4Summary
  7. 5References

1.1 Design Overview

Many industrial systems require precise temperature measurement, including temperature transmitters, temperature controllers, and temperature sensing input modules in programmable logic controllers (PLCs). Many industrial applications require high accuracy RTD and TC measurements that are also protected against electromagnetic interference (EMI), overvoltages, and other interference signals. This application note discusses two different types of EMC test boards that are designed for temperature measurement systems, one that supports all RTD wiring configurations, and another that can measure TCs using different bias techniques. Each type of circuit has a 4-layer and 2-layer circuit board. Table 1-1 shows the function and features supported by each circuit board:

Table 1-1 Circuit Board Design Summary
RTDPCB Board LayersSupport PT100/PT1000?Support 2/3/4-wire RTD?Support low and high-side Reference?Support 1 or 2 IDACs?
Board 14YesYesYesYes
Board 22YesYesYesYes
TCPCB Board LayersPullup and pulldown resistor biasing?Resistor biasing to the negative lead?VBIAS for sensor biasing?REFOUT biasing with a pullup resistor?
Board 34YesYesYesYes
Board 42YesYesYesYes

Each board in Table 1-1 uses the ADS124S08, a precision, 24-bit, delta-sigma (ΔΣ) analog-to-digital converter (ADC) offering low power consumption and many integrated features to reduce system cost and component count in applications measuring small-signal sensors. This ADC features a low-noise, programmable gain amplifier (PGA) to amplify low-level signals for resistive bridge, RTD or thermocouple applications. Additionally, two programmable current sources (IDACs) allow for easy and accurate current excitation in RTD applications. Finally, an input multiplexer supports up to 12 input signals that can be connected to the ADS124S08 in any combination for design flexibility.

All circuit boards in Table 1-1 also include external isolated power supplies and a digital isolator. The digital isolator provides galvanic isolation between the ADC serial peripheral interface (SPI) and the precision host interface (PHI) controller card that monitors conversion data from the ADS124S08. The PCB is designed to satisfy the IEC 61000-4-x standards for systems operating in a harsh electromagnetic environment.

Table 1-1 shows that all EMC test boards include the same circuit and characteristics other than 4-layer and 2-layer PCB design. RTD circuit board 1 and board 2 have the same schematic but use a different number of layers. TC circuit board 3 and board 4 have the same schematic but use a different number of layers. See Section 3 for the details.

For simplicity, the rest of this document uses two distinct design examples to illustrate the behavior and operation of a temperature measurement system:

  • RTD temperature measurement: Circuit board 1 using a 3-wire RTD with a low-side and high-side voltage reference configuration.

  • TC temperature measurement: Circuit board 3 with VBIAS and REFOUT biasing configuration.

If applicable, each section details any differences between circuit board 1 or circuit board 3 and the other circuit boards described in Table 1-1.

Figure 1-1 shows the ADS124S08 RTD EMC test board 1.

RTD EMC Test Board with ADS124S08 (4-Layer)Figure 1-1 RTD EMC Test Board with ADS124S08 (4-Layer).

Figure 1-2 shows the ADS124S08 TC EMC test board 3.

TC EMC Test Board with ADS124S08 (4-Layer)Figure 1-2 TC EMC Test Board with ADS124S08 (4-Layer).