How to calculate if I need to buffer the input to my ADC?

Question

I am putting the output of a current sensing IC (ACS70331) into an ADC (LTC2373). I am that a buffer on the input to an ADC is often required if the output of the source is high impedance. However, I am not sure how to find the cutoff where that becomes necessary for the ADC and I do not see the impedance on the current sensor data sheet.

I have some guesses that you could look at the source current (0.4mA) of the current sensor and calculate the dV/dt of the input capacitance of the ADC input. I am looking for confirmation if that is the proper approach for this type of problem.enter image description here

• Andy aka: Generally the correct approach yes.
• MaxwellEE: @Andyaka How would I know what is "fast enough" for the dV/dt? The ADC samples at 1MS/s, if I am expecting a 2V signal should I be able to hit that point in a single sample? So 2V/us?
• Andy aka: The devil is in the detail for sure so try using a simulator.
• Andy aka: I am looking for confirmation if that is the proper approach for this type of problem. - my comment is not a full answer because that is not what you asked for.

Answer 1

The datasheet for the ACS70331 says it can drive 100pF and maintain stability. But in order to maintain the 1Mhz bandwidth, the capacitive load can be no greater than 50pF. The sampling capacitor for the LTC2373 is 75pF.

You can run it without a voltage follower and be fine. But since you want to maintain the 1Mhz bandwidth, you will need to use a voltage follower.