Hello,
I am currently working on a prototype for the second generation of an electrode amplifier. It is used to amplify the signals of pH and ISE electrodes where the former ones have a very high output resistance in the 100-1000megaohm range. The input amplifier I am using is based on two matched JFET fets in a source follower configuration. The second JFET is used to get a zero DC offset. That is the first input is connected to the gate of Q1. The second JFET has its Gate-Source tied together and is connected to the source of Q1 which is also the output. The amplifiers I am using are from linear technologies (LS830,
The second task is now that I would like to measure the resistance of the electrodes. This implies that I have to connect something to the input signal which I can then measure. I though a current source would maybe be a good idea. The problems here is that the output current must be very low (1nA) to get approx 1V for 1000Megaohm and of course it should not affect the input at all;) I tried to come up with a solution based on two sense resistors with a factor of 1:10000 where the first resistor was programmed by the MCU with a constant current. An opamp then compares the two sense resistors and drives an P-Channel MOSFET which is connected to the input lines.
A small simulation showed me that there are problems with the leakage current of the MOSFET and that it reduces the dynamic performance due to its drain-source capacitance. Does anybody know a good device for this or has maybe a better idea on how to measure the resistance of an electrode?
Thanks, Christian