4 Summary

Table 4-1 lists summary of the measurements. The spurious-free dynamic range (SFDR) is more relevant in the communication industry, but this is also a practical indicator that can be clearly seen in the frequency domain. The difference in performance between the TLV9001- and OPA365-based difference amplifier comes from the timing specification of the test board. The TLV9001 operational amplifier is a valid choice in systems that can work with longer sampling times, allowing the difference amplifier to fully settle before the sample and hold switch (SW) opens. However, this slows down the conversion rate, as other channels have to wait for the slowest one. Alternatively, split slow and fast channels between two ADCs if present in the MCU.

Systems that require faster ADC sampling benefit from higher-bandwidth operational amplifiers, such as the OPA365 or TLV365. The most economical choice is a single-ended output isolated amplifier, such as the AMC0330S or AMC0330R, in cases where the distance between the isolation amplifier and the ADC is short.

A list of recommendations when designing and validating an isolated signal chain interfacing with a SAR ADC is provided below:

• Use the single-ended output isolated amplifier (AMC0330R, AMC0330S) when the common-mode noise is not a concern
• Always use a charge bucket filter in front of the ADC input
• Measure the transient response of the amplifier during sampling for different conditions on the previously sampled signal
• Investigate the sampled signal in both the time and frequency domains (using FFT)
• Work closely with firmware implementers to make sure that the sampling order and timing do not change during development
• Use operational amplifiers that are capable of settling the input within the sampling time of the ADC (e.g. OPA365, TLV365)
• Keep in mind that operational amplifier's AC characteristics can affect the DC accuracy
• Simulate the sampling in a tool like pSpice to optimize the performance and charge-bucket filter
• Retest the behavior when changing the operational amplifier even if the family name remains same

Table 4-2 lists good-performing component values for the system with TMS320F28P650 MCU and the ADC running in single-ended mode with 16-bit resolution.