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ADS8688A: Large error exist in DC signal acquire, when connect the 2nd order filter of ADS8688A EVM before ADC input

Part Number: ADS8688A
Other Parts Discussed in Thread: OPA2209

Dear team,

I am testing our ADS8688A EVM and my input signal is a DC voltage from 0V to 5V.

Use the input range of 0~5.12V,

connect the OPA2209 filter between input DC signal and ADC input pin. then the ADC output error is extremely larger: 2V DC Vin, ADC output 1.698V, as shown in following picture.

Here is some test results during debug.

The voltage in A0+ of J25 is 2V DC, but voltage in J8 pin is 1.69V, so the ADC input pin voltage is 1.69V.

So I think this error between 2V and 1.69V ADC output is come from OPA2209 circuit, but this is only a 2nd order LPF.

Can you give me some help on this issue? Thank you!

Best regards