Impedance Calculation

Have a read of this

see if it helps

regards

You probably shouldn't be doing any such thing as "impedane matching". The SAR should have a high input impedance. That will not only prevent it loading down the LPF and changing its response, but also prevent the LPF from messing up the SAR.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

You need to be careful about the impedance you drive the ADC with. The sample and hold needs to be able to quickly charge / discharge to the next sample value. In this sense quickly is relative to the sample rate and the operational parameters of your converter. Some converters have a setting for the track time before they will isolate the sample cap from the driving circuit. For best results, you really need to understand the characteristics of your converter.

In the absence of other advice, I would suggest driving the circuit with a low impedance buffer or amplifier circuit rather than attempting to match its impedance. You will want at an impedance at least 10X greater at the input to the converter than the driving circuit to account for loading.