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I expect that low input bias current (Ib) of the TLC4502 is important in the pH meter application. Its Ib is specified as 1 pA typical, 60 pA maximum at 25°C. The full temperature range Ib is 500 pA maximum.
Of the three precision op amps I suggested the OPA2325 has the lowest Ib. It is rated as +/-0.2 pA typical, +/-10 pA maximum at 25°C, and 500 pA maximum, over TA = –40°C to +85°C.
Regards, Thomas
Precision Amplifiers Applications Engineering
The OPA320 offers similar dc performances to the OPA325, but with higher bandwidth and slew rate. You probably don't need that in a pH application. The OPA316 is a very good, general purpose op amp so the voltage offset and voltage offset drift will be higher than that achieved with a more precision op amp. Its 25°C Ib is ±5 pA typical, and ±15 pA maximum.
I am not finding a specific pH meter reference design, but TI does have an AFE product that is intended for pH measurement applications. The product isn't from the Precision Amplifiers group that I am part of so I don't know a great deal about it. You can find its datasheet here: