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