Chris pointed out to me that the VGND buffer (op amp) – takes every incoming change – including noise – drive low impedance as hard as it can.  Any noise will affect that.  Will this be a problem in the measurement of pH?  Given the below – I don’t think so.  However I do not have the context to know if I am correct.  So I leave this post as something to be evolved as I learn more about noise on the power wires.

 I wanted to quantify  how much noise on the power wires affects the pH measurements.  Here are measurements for the power coming from my test regulated power source.  The first one is run without the Arduino also plugged in.  The second image was taken with the Arduino plugged in.

V+ WallWart

V+ WallWart + Arduino plugged in

 Both Vpp are the same = 160mV.  I was expecting a higher Vpp when the Arduino was plugged in because the power source is coming over USB from my computer.  I would think a measurable amount of noise would be introduced.

I made similar readings on VGND relative to AGND.


VGND + Arduino plugged in

“zoomed in” a bit
In this case, the Vpp were pretty much the same at 24mV.  I then ran the test code that turned one of the pumps on and off.  The Vpp only changed when the pump was turned on or off, going to 36mV when switching to on or switching to off.