As you can see below, I am breadboarding the new peak detector changes before committing to a new PC board. One of the changes involves using a MAX5387 digital potentiometer to control the square wave amplitude and EC amplifier gain. The new design uses the Raspberry PI to generate the square waves instead of an op amp square wave oscillator. These updates allow you to have square waves with software programmable frequency and amplitude. The op amp that was used as a square wave oscillator is instead used as a comparator to convert the 3.3V GPIO square wave into a full scale 5V square wave. Half of the MAX5387 is used to attenuate the 5V square wave before driving the EC gain stage input. The other half is used to control the EC amplifier gain.
For very high conductivity solutions, you need a small amplitude square wave, because the small EC resistance causes a large amplifier gain. For maximum accuracy, you want to make sure that the amplified square wave at the output of the EC amplifier does not get too close to the supply rails, so you minimize the square wave amplitude and EC amplifier gain to keep the output within range. The EC amplifier gain depends on the feedback resistor and the EC resistance seen by the probe in the solution.
After wiring up the test circuit and figuring out how to get programmable frequency square waves out of the RPI, I started testing the digital pot and ran into the anomalous glitch shown below.
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