AD620 pH probe has a positive voltage offset!

Question

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:

gain 4.4K
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.

gain 3K
pH10: 0.93
pH7: 3.47
pH4: 6.87

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.

Answer 1

Actually the 1nA bias current of the AD620 is pretty high for this application. The probe can be 100-1000M\$\Omega\$ output resistance. With your gain of 12.2 the output-referred offset with a 200M probe resistance would be about what you are seeing with a 1nA bias current.

Normally we would use a low input bias current amplifier for this application, pA or tens of fA not nA. The bias current will not be stable with temperature and I believe the source resistance will vary with probe conditions so you really don't want to just blindly try to null it out.

Select a more appropriate op-amp (hint: you do not need or want an instrumentation amplifier in this application).

Answer 2

If you wish to reduce Input bias current, change AD620 1nA to AD8220 3pA @ 25'C

1nA * 1Megohm * 3k gain = 3V offset and significant loading with 1M affects gain error or 1% sensitivity on 100M probe

3pA * 100 Megohm * 3k gain = 0.9V which can be offset with much better sensitivity.

Although your numbers inside need an explanation. (non-std)

Your probe response should be;

REF

PH  mV      Gain
--  ---     10x with 100M input imepdance. ESD protected
4   177 mV  1.77V
7   0       0
10  -177 mV -1.77V

This may require a better solution than what you asked for.

Edit 11:06am

Just read the spec on the sensor; The resistance of Sensitive Membrane: \$≦200*10^6Ω\$

This requires pristine cleanliness on board to achieve such high impedance even if INA is 1pA and will be very ESD sensitive so series R's are needed to limit ESD current.

This was the schematic for the boards no longer in supply-chain. They will probably have made changes to improve it.

Answer 3

I managed to obtain decent results using a LMC662CN opAmp (2fA input bias). It has an unity gain, so output will go from -200 to +200 mV or little less for pH4-10 range.

I used +Vs=8.46V and -Vs=-8.33V (LT1054 chip with proper caps). My reference is the circuit ground. Some 104 cap(0.1uF) on +Vs and -Vs near the opAmp.

Now my readings are: pH7 = 0 mV ph4 = 187 mV pH10 = -144 mV (should be less but I will buy a new buffer 10 solution to check) edit: more likely my pH10 is pH9.2 solution, that would give perfect fit.

I don't plan to add a second stage amplification as my 16 bit ADC can work on +/-512mV range. (Although I think the AD620A would be fine to make a 2nd stage amplification).

Edit: use a RC lowpass before the ADC.