Hello,

I need to measure voltage on PH electrode, where output impedance is very high. So I need Op Amp with input impedance 1 TOhm or higher, that I could put in front of AD converter. Could you please recommend me suitable Op Amps for this purpose? How can I determine input impedance of Op Amp if I know only input current from datasheet? I need sampling rate only up to 10 Hz. I am not very familiar with analog technics.

Thanks

Maxxisis,

A lot depends on the accuracy required, other op amps required on the board, supply voltages available, whether you are building two or three, or several thousand per week, etc. In general, start with a CMOS or JFET op amp.

There are some good articles on the web about designing EEGs or ECGs that are similiar, in so far as they

require low currents and low noise at low frequencies.

Harry