SBAA531 November   2021 ADS8860 , ADS8862 , ADS8881 , ADS9110 , ADS9224R

 

  1.   Trademarks
  2. 1Introduction
  3. 2 Internal Topology of SAR ADC Model
    1. 2.1  Sample and Hold
    2. 2.2  Sample and Hold Timing
    3. 2.3  Reference Transients
    4. 2.4  Bandwidth Modeling
    5. 2.5  Noise Modeling
    6. 2.6  Reference Droop and Reference Noise Errors
    7. 2.7  Gain, Offset, and Input Leakage Modeling
    8. 2.8  Differential input behavior
    9. 2.9  ESD Protection Diodes and Parasitic Capacitance
    10. 2.10 Summary of Parameters
    11. 2.11 Summary of Model Pins
  4. 3Downloading and Using PSpice® Example Projects From Web
    1. 3.1 Selecting the Amplifier and Optimizing the RC Circuit
    2. 3.2 Worst-Case Settling by Adjusting the Reset Capacitor
    3. 3.3 Verification of Reference Droop
    4. 3.4 System Noise Verification
    5. 3.5 Gain, Offset, and Input Leakage Verification
  5. 4Summary

2.6 Reference Droop and Reference Noise Errors

Some of the noise applied to the reference input of a SAR ADC is in the digitized output signal. The reference noise detects the bandwidth restrictions from a filter formed by the reference capacitor CREF and input switch resistance RSH. The reference capacitor value is typically one-fourth the sample and hold capacitor. In this example CSH is 55 pF so the reference capacitor is 55 pF / 4 = 13.75 pF.

Reference noise is also impacted by the input signal level. For a 0-V input signal, no reference noise appears at the output and for a full-scale signal, all the reference noise appears at the output. Figure 2-8 illustrates the noise scaling inside the model.

Another aspect of the data converter voltage reference input is that droop in the ideal reference voltage causes a gain error on the ADC. For example, if the ideal reference voltage is 5 V and the actual reference droops to 4.99 V this introduces a 0.2% error in the measured output signal (100 × (4.99 V – 5 V) / 5 V = 0.2%). The reference circuit shown in Figure 2-8 also models this behavior. Note that the input called Vref_ideal is used to set the ideal reference value. This input floats to the VREF parameter if it is not connected. If a different Vref_ideal is desired, the pin can be driven as needed.

GUID-20211025-SS0I-3RRL-XQQW-FSXK7ZJ01VCP-low.gif Figure 2-8 Reference Noise Scaling Model