ADC3422: Question for schematic of TSW3422EVM TIDA-00799

Hi Xiaoqiang,

1) Why D1 circuit needed as U1 has similar structure?

This is additional protection so that the ESD diodes in the OPA656 do not sustain damage in case of over/under voltage condition.

2) What's purpose of R22? 

This can be used to reduce current through the ESD diode incase of over/under voltage condition.

3) What's the purpose of R20+L4+R25+R21?

The is a low pass (AntiAliasing) filter to reduce the noise and high order harmonics before the ADC samples the signal.

Question for page 2 and 3 

1) What's the main difference of application for channel A, B, C, D?

CHA and CHB appear to be the same configuration, and can be used for DC coupled fully differential input to fully differential output.

CHC is a DC coupled, single ended input to DC coupled fully differential output.

CHD is AC coupled single ended or fully differential input to the transformers, and fully differential output.

2) Should channel D only for AC measurement

Yes.

Best Regards,

Dan

Hello Dan,

Thanks for quick reply!

One more question regarding to low pass filter of R20+L4+R25+R21, how to decide the component value vs target sample rate?

Hi Xiaoqiang,

Since the ADC3244 has an unbuffered analog input, the low pass filter cutoff frequency (fc) should be at or below the sample rate. Doing this will help to attenuate the glitch/kickback energy from the switching of the ADC sample/hold capacitors.

Of course, you can also make the low pass filter with a lower fc depending on your input signal requirements.

Best Regards,

Dan