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Topic: Circuit (Op Amps) for standard BNC pH probe ... (Read 2917 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:
When you are breadboarding with it, if you follow the example in the data sheet and bend the IC input leads out away from the breadboard and just solder directly to them you will prevent any problems with leakage current.

It will run from +- 5V, but is happier at +-12 to +-15. I know it costs \$10 but in the long run, you'll be hard pressed to duplicate its performance with op-amps.

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