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### Topic: Circuit (Op Amps) for standard BNC pH probe ... (Read 4604 times)previous topic - next topic

#### simkard

##### Sep 17, 2011, 02:21 pm
Hi,

I'm actually looking for a fast&cheap circuit to interface a standard pH probe.
Details about the pH probes :

• pH 7 = 0v @ 25°C

• 1 pH level is = 0.05916v

• min probe value (pH = 0) = -0.41412v

• max probe value (pH = 14) = 0.41412v

I need the circuit output to be directly connected to an analog pin of an Arduino (or whatever ADC // µC), so levels should be between "0v and +5v"

I know that the pH probes has a few Tohms (//Tera-Ohms) of resistance and that the first stage Op Amp on which it is directly connected needs to has, idealy, a few fA (femto-Amperes // 10^-15 Amperes).

I found some ICs that could do the trick but i need someone to explain some things to me :

• CA3140 (max : 10>50pA)

• CA3140A (max : 10>40pA)

• LMC6001AI (25fA)

It could be really fine to realise this circuit and sharing with other people that needs the same.

I faced problems with the circuit explained inside the datasheets because they uses non-standard values resistances.

Thanks for the ones who will help me/us !

PS : i already have the pH probes, bought from eBay for some \$ (cheap)

#### dc42

#1
##### Sep 17, 2011, 04:19 pm
What resolution are you hoping to achieve? Can you point us to a datasheet for the probe? Is it sensitive to the potential of the liquid it is measuring? (If so, you'll need a differential amplifier, or preferably an external ADC with differential inputs).
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### retrolefty

#2
##### Sep 17, 2011, 04:46 pm
ph measurements are very demanding of the input electronice needed to interface with the probe. One might consider buying a purpose based interface board rather then try and homebrew.

http://atlas-scientific.com/Details.cfm?ProdID=32&category=

Lefty

#### dc42

#3
##### Sep 17, 2011, 06:33 pm
It looks fairly straightforward to me assuming one side of the ph probe can be grounded, which I guess is the case since the probes normally use bnc connectors. You need 2 op amps (1 with low input current) and 4 close tolerance resistors. Also 2 capacitors, 2 diodes and a resistor to generate a negative supply for the op amp from a pwm pin. I'd post a schematic, but i'm sending this from a mobile. You'll need to calibrate it at one point, e.g. short the probe and calibrate to ph7.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### EmilyJane

#4
##### Sep 17, 2011, 07:03 pm
I've had good luck in similar applications with this part: