I am a chem eng. but little background in electronics.
I'd like to interface a pH probe with an AD620A, the original PCB that came with pH probe was damaged by a current leak in the solution.
I made the following circuit in spice and on breadboard.
A pH probe has high impedance (>10Mohm), the AD620A seems OK to me for this application with its low bias current. Is that correct?
The pH probe ranges from -200mV to +200mV, little less range from pH4 to 10.
+Vs = +8.46V
-Vs = -8.33V
+IN = positive pH probe
-IN = ground pH probe, connected to GND and Vref pin of AD620A.
Output voltage should be centered on 0V as seen on screenshot, but in reality I read the following:
pH10: output = 0.82V (should be near -2.0V)
pH7: output = 2.51V (should be near 0V)
pH4: output = 4.92V (should be near +2.5V)
(R²=0.99 for linear fit, readings are stable)
In this configuration I could feed my ADC, would DC offset increase or decrease in time or with different power sources? Linearity is good enough for my application.
The output never goes negative. I checked my negative voltage -Vs directly at the AD620 pin too. I tried putting 1M and 3M resistors on inputs with ground, but it seems it's not high enough or it's not the good direction.
What should I do to restore DC offset to zero? Should I apply negative voltage on Vref?
The DC offset is roughly +2.5V with the 4.4K gain.