SDAA114 September   2025 AMC0311D , AMC0311D-Q1 , AMC0311R , AMC0311R-Q1 , AMC0311S , AMC0311S-Q1 , AMC0330D , AMC0330D-Q1 , AMC0330R , AMC0330R-Q1 , AMC0330S , AMC0330S-Q1 , AMC0336 , AMC0380D , AMC0380D-Q1 , AMC0381D , AMC0381D-Q1 , AMC0381R , AMC0381R-Q1 , AMC1211-Q1 , AMC1311 , AMC1311-Q1 , AMC1350 , AMC1350-Q1 , AMC1351 , AMC1351-Q1 , AMC1411 , AMC1411-Q1 , AMC3311 , AMC3311-Q1 , AMC3330 , AMC3330-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Signal Chain
    1. 2.1 Inside the Multiplexed SAR ADC
    2. 2.2 Driving ADC With an Amplifier
  6. 3Experimental Results
    1. 3.1 DC Characteristics
    2. 3.2 AC Characteristics
  7. 4Summary
  8. 5References

2.2 Driving ADC With an Amplifier

Figure 2-1 shows the scenario when an operational amplifier, or output of a single-ended isolation amplifier, drives the analog input of the ADC. The analog amplifier exhibits a transient on the output as the sample and hold switch SW connects the sample and hold capacitor CSH to the output.

Operational Amplifier Driving the Multiplexed ADC Input Figure 2-3 Operational Amplifier Driving the Multiplexed ADC Input

During the sample and hold process, the amplifier must swiftly recharge the sample and hold capacitor CSH to the desired level VOUT. However, this process is not fully deterministic. Most (but not all) ADCs do not reset the sample and hold capacitor between each conversion. This means that the capacitor holds residual voltage (charge) VSH as a result of the previous channel conversion. For this reason, there are three scenarios that can occur:

  1. VSH > VOUT­ results in overshoot as the external amplifier has to discharge CSH to lower voltage
  2. VSH = VOUT­ does not cause any transient as voltage are same
  3. VSH < VOUT­ results in undershoot as the external amplifier has to charge CSH to higher voltage

Adding a charge-bucket filter (Rb, Cb) partly helps controlling the output transient. However, the filter cannot improve settling speed of the amplifier. This only allows tuning the compromise between the settling time and the overshoot/undershoot magnitude.