Circuit design for high resistance sensor measurement

[sagi4422]

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 (AD620) 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

 

[KlausST]

Hi,

You talk about frequency....do you need an AC measurement or a DC measurement?

If you need just a DC measurement then a constant voltage (maybe 5.00V ) and a known (maybe 5.00 MOhm ) resistor forms a voltage divider. Connect the unknown resistor to 5V and to the one leg of the 5M. The other leg of the 5M to GND.
Use an non inverting Opamp stage to measure the voltage of the known resistor. Gain of 1 is Ok, it should give a low impedance signal to the ADC..

You need at least 10 bit ADC resolution to get a 5% result resolution.
A 12 bit ADC is a good selection.

Rx = ( 5MOhms x 5.00V / U_measurement ) - 5MOhms
(As long as my calculation is correct)

Hope this helps

Klaus