Operational amplifier for PH measurements

Thread Starter

ecka

Joined Feb 19, 2021
7
Hello, i need to select relatively cheap operational amplifier for PH measurement circuit (2-3 Euros buying in low quantities). Circuit measures PH level with 0.01pH level of precision. Because PH sensor produces approximately 68mV per pH unit so 0.01pH difference gives 0.68mV in the pH sensor output.
ph circuit schematics.jpg
I have used AD8617 opamp previously, but now it is discontinued. Now i am searching for replacement. The PCBs are already manufactured so i need direct replacement (pins must fit). I am trying to figure out which amplifier IC parameters are most important. So we have:
Offset voltage (mV);
Noise: (nV/√Hz);
Bandwidth;
Slew rate (V/us);
No phase reversal (not sure what it means??)
Low input bias current (pA);
Unity-gain stable.

I think the most important parameter is input bias current - i am searching with 1pA or less. How about other parameters mentioned above (or not mentioned)?
Found cheap Microchip opamps MCP6007 and MCP6477, both costs about 0.60 Euro. Also have found AD8646 - 3.45 Euro, AD8692 - 2.76Eur.

What you think?
 

bloguetronica

Joined Apr 27, 2007
1,522
The phase reversal only would happen with some amps, it the input voltages are going far from the specified limits. In your application, this is not an issue (it would be an issue when using an op-amp as a comparator). Anyway, for your application, you'll probably need a precision op-amp that has a very low offset. As for the bandwidth and slew rate, I'm not imagining that your application requires anything over a few MHz. You'll probably need rail-to-rail.

I would suggest the OPA2705, or even the superior OPA2703, for your application. The latter is more precise. Both are unity gain stable, which is required for your application.

About the MCP6007 and the MCP6477, both seem to be suitable for your application. They are unity gain stable and have low offset voltage, even better than the OPA2705 (but still worse than the OPA2703). The AD8646 seems to be very similar to the original.
 

Ian0

Joined Aug 7, 2020
5,903
My favourite precision op amps at the moment are Microchip’s MCP60xx series. They are available in singles, pairs and quads, and that gives you the last digit (1, 2 or 4) and different speeds: third digit=5 for a slow one, 6 for a medium one, 7 for a fast one, and 2 (why 2?) for a very fast one.
They all have extremely high input impedance, rail to rail inputs and outputs, work at 3.3V, have standard pinouts, and have offsets of <150uV.
Faster ones are quieter.
 
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