Pimp my pH input

Can you post the circuit somewhere and the specs on the probe?

Tom

A pH probe set is a kind of electro-chemical cell, with one fairly low resistance connection to the solution (the reference probe that makes a connection to the solution with minimal voltage drop) and one very high resistance connection that includes the voltage offset produced by the cell. The hydrogen ion (naked protons that are much smaller than any atom) concentration of the solution determines the voltage produced by the cell, as they diffuse through the glass bulb of the measurement probe and produce a net voltage across the glass membrane.

You must pass nearly zero current through either of these connections to get an undistorted measure of the voltage difference produced by the protons. It sounds like you have a very high resistance load (very limited current through) the measurement probe, but how about the reference probe. If the solution has other ground paths, you may be passing current through it, and getting some voltage drop through it. Is the test solution in a glass container (and electrically isolated from any current path that includes the reference probe) or is it grounded through other means than just the reference probe?

See for example

Mr.pH --

Hi Tom,

Here is the circuit.

1M ___ .-----------|___|---------. | | | | | 5.77V | | | | | | | | | | | | pH probe | |\\| | .-----. '-----------|-\\ | | G | | >----------o------------- |_-_-_|--------------------|+/ | | | |/| TL084 '--o--' --- | | --- | | | | | 0V 0V |

-3.88V

(created by AACircuit v1.28.6 beta 04/19/05

Are the DC supplies batteries or AC derived supplies? If the latter, you have to include the AC paths through the inter winding capacitances back to the line and from there to ground and from there, back to the zero volt node.

Have you checked to see if there is any AC hum on the opamp output?

Thankyou that is a good page, I have read it before. My circuit is very similar except it is unity gain. The only reason I have the resistor in the feedback at all is that I read on this page:

(search for "ph") that if the resistor has the same impedance as the probe, the voltage across it due to the opamp leakage current will match and thus cancel the voltage error due to the same leakage through the probe.

I forgot to mention to Tom the specs of the probe, but no I don't know what they are. I am trying to make it work with typical probes, which from what I've heard are round bulb types and have a impedance of about 400Mohms.

Fox one! Fox one! wrote: (snip)

The probe resistance is not known and very high, compared to

1 meg. You need an opamp that has a bias current so low that it produces an insignificant drop across the 100 meg or so probe resistance. Then you can eliminate the feedback resistor. I would go for an opamp with sub pico ampere bias current. perhaps the LMC6001 with 25 fempto amps of bias current.

My understanding of the probes is that the junction and the reference are all in the one package with only two terminals. In my circuit one terminal is connected to ground and the other directly to the non-inverting input of a TL084, so that should be zero current shouldn't it?

It does respond to ph changes, I should have mentioned this: In a pH 7 buffer the output is 75mV (expecting 0mV) In a pH 4 buffer the output is 195mV (expecting 180mV, as output should change roughly 60mV for each pH)

The test solutions are in disposable plastic cups with no other wires in them.

.

That is just comp[act [packaging. There are still two distance probes that contact the solution, one through a direct electrical connection through a fluid bridge, and one through a membrane.

I see. That simplifies things from a DC standpoint. Is the cup sitting on a metal sink or a non conductive surface? In a separate post, I have questions for you about the possible hum being injected through the supply.

I've noticed that if I ground the ph input the opamp gives 1.7mV!! Maybe the probe is the problem?

Good point I just thought since the space is there I would put in the largest value I have in my parts pile. I tried bypassing it (I am beginning to get superstitious now) and it made no difference (of course I knew it wouldn't).

Ah yes I have seen that part but this is a project for a hobby community on the net, so I am trying to use parts you can get from the local electronics store in case they ever have to repair it. My local store stocks these which seem like they are the right type (FET input):

CA3130 5pA bias current CA3140 10pA TL084 30pA (what I am using) TL074 65pA

By my calculations, assuming the probe has an impedance of around 675Mohm (based on 'standard glass', hemispherical shape on this page:

then a worst case

30pA leakage current would cause a voltage drop of 20mV. That is surprisingly large but still doesn't explain where 75mV is coming from.

I've tried a DC supply and an AC supply (my PCB supports both) but both give the same results. In normal operation I'd expect it to be used with an AC supply, and for it to be installed in an earthed metal case, with circuit ground connected to the case by the screw mounting points. i.e. circuit ground should aways be mains earth

I've probed it with my CRO and no I can't see any mains frequency on the output at all.

Thanks for your help.

They are sitting on top of my computer. I tried putting a book between them and lifting the cup upwards but that made no difference.

Then I am fresh out of leads to follow. Sorry. Perhaps your probe set is just contaminated or the reference probe plug clogged and is producing the 75 mV offset, no matter what amplifier is used to buffer it. I have seen probe sets go bad this way. You might try leaving the probe in the pH

7 buffer solution for a few days and see if the offset drifts in either direction. No need to keep the amplifier on all the time, though it shouldn't hurt.

messagenews:0s7ei.16532$ snipped-for-privacy@news-server.bigpond.net.au...

Or your opamp has a lot higher bias current than it is supposed to have. Replace the probe with a 10 meg resistor and see how high the output goes.

Sheesh that was the problem! I don't have a 10meg resistor but I replaced the TL084 and voila, it works. I could swear I already tried this, god knows I changed everything else... Thanks a lot for your help.

:-)

Live long and prosper.