High Impedance Sensors Interfacing

[natmag]

Hi,

I am trying to interface a high impedance pH electrode using an LMC6462 op-amp. The LMC6462 op-amp is a high impedance op-amp with an input bias current of 150fA. The input pin of this op-amp is surrounded with a guard ring such that the op-amp is guarded from any leakages and supposedly the pH probe potential is measured. The circuitry is cleaned thoroughly to ensure that ESD does not effect the input stage of the opamp as well. I am having trouble in getting the necessary measurement readings. On connecting the LMC6462 op-amp to the sensor the output of the buffer saturates negatively instead of giving the required 177mV reading. It is as if the op-amp has a lower input impedance than the sensor (500M) and thus the potential measurement cannot be read. This result occurred even when the op-amp was changed.
How can one please avoid this from happening and guard the LMC6462 input stage? Please help ...how can I measure from such a high impedance sensor?

Thanks

 

[FvM]

The OP is basically fine for a pH sensor. I guess something else is wrong. Are you sure that the pH electrode is O.K.?

Show your circuit.

 

[natmag]

Yes the pH sensor works fine when tested with an off-the shelf pH meter.

This is my circuit's schematic



I am buffering both the reference and glass electrode as I need to measure both using an ADC in my MCU.

 

[chuckey]

The guard ring does go to the O/P of the amp?
Frank

 

[natmag]

Yes and to be more exact it looks like this

Thanks

 

[FvM]

Short W2 and omit U3. There's no need for a differential buffer, neither can it work, both inputs will be just floating beyond the amplifiers's specified common mode range. Or do you have an electrode connecting the liquid to circuit ground?

 

[natmag]

I have an electrode connecting the liquid to circuit ground, which is offset by 1.024V such that the op-amp only sees positive voltages.

 

[chuckey]

I see no CX and R1 on your circuit, also R1 is not guarded.
Frank

 

[natmag]

Why is R1 not being guarded..the high impedance node is connected to the input of the amplifier which is guarded

 

[FvM]

CX and R1 aren't shown in your schematic. What are they good for?

 

[natmag]

I included Cx so as to try to eliminate the parasitic capacitance of the Coax cables and also serve as protection from ESD for the op-amp input.

R1 is being used to obtain the membrane resistance of the pH probe, although for now it has been omitted.