Active probe

Intro

While working on the capacitive sensor for the Sit.Up project I stumbled about a common problem: connecting an oscilloscope or the Analog Discovery to the capacitance measuring pin disturbs the whole measurement. This happens because the load presented by the Analog Discovery or your typical oscilloscope: about 1MOhm || 24pF. Most importantly, the 1MΩ was discharging the sense capacitor. To make it work, I needed something with much smaller capacitance and much much larger resistance.

How

The solution is rather simple: build a simple active probe. Since I was not interested in going beyond +/-5V signals, a simple CMOS rail to rail opamp was sufficient. My parts collection complied and provided the MCP6H92 which has a much better input configuration  10TOhm || 6pF, not perfect, but a great improvement.

Schematic

active probe

Prototype

DSC_4778

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