TIDUFC1 November   2025

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 ADS127L21B
      2. 2.3.2 REF81
      3. 2.3.3 REF54
      4. 2.3.4 RES21A
      5. 2.3.5 THP210
      6. 2.3.6 OPA828
  9. 3System Design Theory
    1. 3.1 Range Selection
    2. 3.2 Linearity and Low-Noise Signal Chain
    3. 3.3 Calibration
    4. 3.4 Additional System Design Considerations
  10. 4Hardware, Software, Testing Requirements, and Test Results
    1. 4.1 Hardware Description
      1. 4.1.1 PCB Interface
      2. 4.1.2 Input Multiplexer
      3. 4.1.3 Gain Multiplexer
      4. 4.1.4 Power Supplies
      5. 4.1.5 Clock Tree
    2. 4.2 Software Requirements
    3. 4.3 Test Setup
    4. 4.4 Test Results
      1. 4.4.1 Integral Nonlinearity Measurements
      2. 4.4.2 Noise Simulation
      3. 4.4.3 Noise Measurements
      4. 4.4.4 Conclusion
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
    2. 5.2 Tools
    3. 5.3 Documentation Support
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Author

3.4 Additional System Design Considerations

The TPS7A39 is a dual-output LDO which generates the positive and negative power rails for THP210 and ADS127L21B. The TPS7A39 outputs are set to ±2.75V to keep the THP210 input common-mode in range.

This design consumes 335mA at start-up and approximately 75mA during normal operation. In power-sensitive applications, several strategies can minimize the power consumption. REF81 provides 80% of the power consumption. The REF81 heater draws approximately 60mA during normal operation. This design uses the +15V rail to power the REF81 heater; however, increasing the heater power supply between the HEATP and HEATM pins reduces the REF81 current consumption. See the REF81 Temperature Controlled Precision Voltage Reference with 0.05ppm/°C Temperature Drift and < 1ppm Stability datasheet for more details. Additionally, the PCB includes mounting holes around the REF81 for an external enclosure. Adding an enclosure allows for better heater regulation and less overall output voltage drift.

Any small resistance on the ground plane causes an unpredictable voltage drop in the measurement path because the REF81 current draw is so high. To avoid errors from this current drop, all components in the signal measurement path are referenced to a star-ground plane (GND_SENSE). The components referenced to this star-ground plane have lower current. A resistance on the star-ground plane causes a lower voltage drop than an equivalent resistance on the REF81 ground plane, resulting in minimized errors. The star-ground and REF81 ground planes are connected at one point, R12. See Schematic and PCB With the GND_SENSE Plane.

TIDA-010970 Schematic and PCB With the GND_SENSE Plane TIDA-010970 Schematic and PCB With the GND_SENSE Plane Figure 3-5 Schematic and PCB With the GND_SENSE Plane

REF54 is chosen as the ADC reference due to the low 1/f noise and low temperature drift. There is no need for additional reference circuitry to provide an accurate and stable ADC reference because REF54 has a buffered output. Using a REF81 as the ADC reference is possible; however, REF54 is smaller and consumes less current than REF81. Additionally, REF54 can be placed closer to the ADC without heating concerns.