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I want to interface a pH probe. The Op-Amp I would like to use is INA128.

My ADC's range is 0 to 5 volts. pH sensor gives a voltage output that is ranged from -0.5 to 0.5 volts, for 0 pH to 14 pH respectively.

Perhaps, I need to give an offset of about 2 volts to Op-Amp and set its gain to 2. If I do that, will negative voltages be substracted from 2 volts?

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3 Answers

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As far as i know, you need opamps with super low input bias current to interface PH probe. This one has it at 10 nA, that's rather too big. I was able to build one by using opamp with input bias current in femto-amperes range.

You are right about offsetting voltage.

I'd go by finding an opamp which has PH probe interfacing as typical application and implementing the given circuit.

AD549 is a good candidate

LMC6001 -- i've based my design on this

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so, why i can't do it with ina128? – iaggggg Jun 18 '12 at 12:23
As i've said, it's input bias current is too big - it will load the PH probe too much and you will get inaccurate readings. PH probes need very high input impedance opamps to buffer them. You can use high input impedance opamp to buffer it and another opamp to scale/shift the voltage to a level you need. Note, that you will need a split supply for buffer opamp and use noninverting configuration. – miceuz Jun 18 '12 at 12:42
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Like miceuz says, what sets a pH sensor apart is its high impedance, which calls for a FET opamp or instrumentation amplifier for minimum error due to input bias current. Next to the parts miceuz already mentioned I would add:

LMC6041: the lowest cost opamp listed at Digikey with input bias current less than 10 fA (2 fA typical).

INA116: the only InAmp listed by Digikey with an input bias current less than 10 fA (3 fA typical, 2 mV offset maximum).

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Thank you all.. – iaggggg Jun 19 '12 at 8:16
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I used an AD620, this is not an opamp but a full instrumentation amplifier. Found it on Ebay ;)...

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Perhaps a little more detail, or a schematic, would have helped viewers, rather than the bare statement about the InAmp. – Anindo Ghosh Oct 31 '12 at 15:55

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