Circuit design for high resistance sensor measurement

[Sagi Gliksman]

Hi All,

I am a newbie in the field of electronics but have enthusiasm to learn and build.

I have a project I am involve in which needs from us to build a high-resistance sensor measurement system.

The sensor, because of the in-stability of fabrication, can have a baseline resistance between 10MOhm to 70MOhm.

The quest here is to measure the above resistance values and even expend it to 1MOhm to 70MOhm with at least 5% of accuracy

The sensor frequency is very slow, <40Hz and it is operated on a battery of ~3/5V.

My request is to build a circuit that will measure the sensor's resistance and will have a range of measurement as described above.



I did some research in the Analog search engine and I didn't find the specific terms I am looking for but this is what I came so far:



Building a wheatstone bridge with 3 legs with 100MOhm. The 4th leg is the sensor. This gives me an output voltage range of 0.3V to 1.6V (if powered by 3.3V). Both output leads goes to a differential in-Amp with high input impedance (20G). Vout due to its low current, goes to a unity-gain op-amp and than to an 12BIT ADC.



I was wondering whether I am in the right direction.



Any feedback is welcomed.



Sagi

 

[Colin Mitchell]

What sensor has an impedance of 10 meg?
You are going to get enormous problems with humidity interference.

 

[Sagi Gliksman]

The sensor is changing based on humidity changes

 

[Colin Mitchell]

Humidity sensors already have internal amplifiers and buffers and come out 1k.

 

[Sagi Gliksman]

This is a sensor we are fabricating in the lab

 

[Laplace]

Working with signal conditioning at 100MΩ is more of an art than science. A stray fingerprint on a circuit board can leave a conduction path with less resistance than the circuit components involved. Even ordinary handling of a 100MΩ resistor under less than clean-room protocol can significantly alter its value. But teflon is your friend here. Use teflon insulated cable, teflon standoffs, and use terminal guard rings placed on the PCB. Texas Instruments, Linear Technology, & Analog Devices should all have app-notes on the lore of high impedance signal handling.