TIDUF45 May   2024

 

  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 ADS127L21
      2. 2.3.2 PGA855
      3. 2.3.3 REF70
  9. 3System Design Theory
  10. 4Hardware, Software, Testing Requirements, and Test Results
    1. 4.1 Hardware Description
      1. 4.1.1 Board Interface
      2. 4.1.2 Power Supplies
      3. 4.1.3 Clocking Tree
    2. 4.2 Software Requirements
    3. 4.3 Test Setup
    4. 4.4 Test Results
      1. 4.4.1 DC Accuracy Tests
      2. 4.4.2 Gain and Offset Temperature Drift
      3. 4.4.3 Nonlinearity
      4. 4.4.4 SNR and Noise Performance
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
    2. 5.2 Software
    3. 5.3 Documentation Support
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Author

3 System Design Theory

The front-end is a key building block to a digital multimeter, and many data acquisition systems as well. Digital multimeters measure across a wide selection of ranges. A switching matrix with individual gain resistors for every range can take up a lot of space, especially in a multichannel data acquisition system.

The PGA855 gain settings allow for a wide input range and are controlled with three digital signals, allowing for easier switching between gains. DMMs have several different input ranges, for example 100mV, 1V, 10V, and 100V, so the design is simplified when multiple input ranges can be handled with one input stage. The PGA855 gain is controlled with 3 digital lines, making switching between gain stages relatively easy when using a microcontroller or a processor.

The amplifier must be able to accurately scale signals to the full-scale range of the data converter, without sacrificing resolution on the data converter. Accuracy and precision are key to any DMM design. A meter's reading needs to be close to the true value and repeatable. Most initial DC errors can be calibrated relatively easily, however nonlinearities, drift, and noise must be low to enable accuracy and precision.

Additional input scaling for a high-voltage input, and input protection can be added onto this design to fully complete the analog front-end signal chain.

TIDA-010945 PGA and ADC Input FiltersFigure 3-1 PGA and ADC Input Filters

The R-C-R differential low-pass filter at the input of the PGA, as shown in Figure 3-1 helps reduce EMI/RFI high frequency extrinsic noise. This filter can be customized per the bandwidth and application requirements. Using the 10-to-1 ratio for differential capacitor CIN_DIFF versus common-mode capacitors CIN_CM offers good differential and common-mode noise rejection. This ratio also tends to be less sensitive to the tolerance variation and mismatch of the filter capacitors.

The feedback capacitor, CFB, is in parallel with the PGA855 output-stage internal 5kΩ feedback resistors (see Figure 2-1) to implement additional noise filtering. The internal resistors have ± 15% absolute resistance variation, and this variation must be taken in to account when implementing noise filtering. On this board, CFB is set to 25pF, providing a typical f–3dB corner frequency of 1MHz. The estimated minimum f–3dB corner frequency for this circuit ranges from approximately 904kHz to 1.119MHz when accounting for the feedback-resistor variation. The filter at the ADS127Lx1 inputs works as a charge reservoir to filter the sampled input of the ADC. The charge reservoir reduces the instantaneous charge demand of the amplifier, maintaining low distortion and low gain error that can otherwise degrade because of incomplete amplifier settling. The ADC input precharge buffers significantly reduce the input charge that raises the ADC input impedance to decrease gain error.