Impedance matching with pH probe

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Impedance matching with pH probe

Postby ScottHorn » April 4th, 2017, 10:48 am


First off, Hello everyone, this is my first post to the forum. I'm a long time Hack A Day reader.

I need some help, and figured this might be a good place to ask. Basically, I have a pH probe that I'm trying to interface with an A/D converter. The A/D converter has an input impedance of 1 M Ohm. This particular pH probe has a VERY high output impedance due to its peculiar geometry. Probably in the Giga Ohm range. Previously I've used an off the shelf unity gain amplifier for impedance matching, with good results, however the amplifier is a bit pricey at $150 a pop, considering that I need quite a few of these, they only last about 2 years, and the battery is not user replaceable.

I found a company that was willing to build me something similar in small quantity, but the build quality was quite poor (most failed within a year), and they chew through batteries like crazy.

This is not a situation where I'm going to be able to get boards made, use an off the shelf op amp IC designed for this purpose, and whip them up myself. Surely there is something commercially available I can use to match the impedance in this setup that isn't going to break the bank, especially considering I need about 10 of them. I've seen pre-populated evaluation boards for IC's commonly used for this purpose, but I'm limited to qty 1 for those.

tl;dr I need to match a multi Giga Ohm output impedance with a 1 Mega Ohm input impedance. A breakout board for an appropriate op-amp is about as "DIY" as I can go in this situation. A cheap, off the shelf product would be ideal. I'd googled the heck out of this, and I can't come up with anything. Any suggestions are greatly appreciated

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Re: Impedance matching with pH probe

Postby bandersnatch » April 5th, 2017, 12:29 am


Just as a starting point, have you already seen:
- pH meter design note ... CN0326.pdf
- pH meter design note
- Ultra-high impedance pH meter preamp
- Complete pH meter project ... -amplifier
- Stackexchange are one on the best sources for high-quality SW & HW info...

I suggest you also G$$gle for "ph probe high impedance preamplifier"

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Re: Impedance matching with pH probe

Postby ScottHorn » April 5th, 2017, 10:58 am

I've found most of those links previously, and googled all sorts of variations on the terms you suggested. I'm trying to avoid ordering pcbs and ICs etc and building it from scratch. As I said in the OP, a "kit" sort of format would be about as DIY as I'm willing to go. The reason for that is my inexperience with designing circuits, and the fact that this is for my job, and therefore I want to avoid the blame that will inevitably come if I decide to whip these up from scratch and end up wasting a bunch of money when I make a rookie mistake and they don't work. I'm aware of 2 separate off the shelf products that do this (one from Sensorex, I can't remember who makes the other one), but both lack user replaceable batteries. My ideal solution would be some sort of general purpose OP amp breakout board, allowing me to choose my own IC and passives, suited for this particular application. Ideally I would power it from a wall-wart, or at least rechargeable batteries.
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Re: Impedance matching with pH probe

Postby k-ww » April 5th, 2017, 12:01 pm

I've had to build this type of circuit for a client. The things I would stress are -

first: make sure the board is very clean after it's built - nothing like surface contamination and humidity to make your readings drift.

second: the guard ring around the input circuit really helps, but only if it is bare copper, and not with solder mask on top.

third: if you don't electrically isolate the circuit and power supply from the rest of your circuitry, [digital isolation between the ADC & the micro] you will find that a leakage path [pH probe to water to earth ground to the rest of your circuitry] will cause reading drifts like you will not believe. With dual probes [PH / ORP] you get some fun readings.

lastly: if you need ORP also, it's just the pH circuitry with the gain lower.
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