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Interference when sensing pH and EC

Hi,

I'm developing a measurement circuit for hydroponics which has a pH electrode and an EC electrode too. Both electrodes are in the same tank and when the EC electrode is turned on, the pH measurement is highly affected. For pH measurement I am using the LMP91200 Integrated Circuit, and the circuit is well implemented because when I only sense pH, the measurement is fine. Anyone had a problem like this, or anyone knows a how fully isolate pH from the rest of the design? It seems that the problem is related with ground loops, because I only can eliminate it by removing all the power connections of EC electrode (VDD and GND).

regards,

Lucas.