differential opamp

[Asjad]

Dear All,

I hope all is well.

I have a quick question:

I wish to interface an PH meter to some electronics using this probe:



Do I need to use a opamp in differntial mode??

Just insert one of the wires to ground and the other to a
PICS ADC??

Thank you in advance!!

 

[Phasor]

I wouldn't recommend connecting the probe directly to a microprocessor.

You need to use opamps with VERY high input resistances, eg TL071, TL081 (typically 10^12 ohms input resistance). Your run-of-the-mill 741 just won't make the grade.

You'll need at least 3 opamps - Buffer each side of the probe with a voltage follower, then put the third one in differential mode, using the output of the followers. While you're about it, give your differential amp a gain of about 10, maybe more, maybe less, depending on the input requirements of your ADC. THEN take the output to your microprocessor.

 

[mechie]

Instrumentation amplifier for pH measurement?

Hi, my 2 penneth worth ...

I know that a full instrumentation amp set-up as described by Phasor is the ideal but I would have thought that for 'playing' with the concept of pH a 'simple', single op-amp would suffice (still with FET input stages; Phasor's TL071 or similar).
If the op-amp input impedance is a bit low then it will load the voltage down a bit so you will get less than the ideal 59mV per pH but you cannot rely on getting this signal anyway - you will need to calibrate the system, both for 'slope' (mV/pH - effectively a gain control) and for 'isopotential' (!) (the pH value that produces 0mV - approximately 7pH - this is effectively a zero shift).

I think the biggest hurdle here is likely to be obtaining the pH probe pair.
IF you want to go buy a probe then consider a "combination probe", both measuring and reference junctions in a single body - usually cheaper than seperate probes and probably more than adequate for your experiments.
A single screened cable will provide both connections from a combination probe, centre core is measuring junction, screen is ref. junction.