op amp design for sensing application with micro controller

[welove8051]

hello geeks, this is the first time i am designing a amplifier. i am using thin film sensor which has resistance in the order of 100's of mega ohm for ex 300M ohm. when it senses gas it resistance reduces to 100M ohm. i want to measure the variation of resistance in terms of voltage with micro controller. so i am using op amp for the same. what opamp will be good for it. i had planned for lm324 of lm358.
it is non inv amplifier
is my circuit correct? or is there any other method.


thank in advance

regards
kumaresh

 

[barry]

The LM324/358 is not a good choice for your application. These opamps have bias currents (250nA) that will just kill your input measurement. For example. If you've got 250nA through a 100M resistor, that will drop 25Volts!!!

You need to use a precision opamp with low bias and offset currents. I would also suggest you use a bridge circuit.

 

[welove8051]

thanks barry, how about lmp7721 op amp. do you mean this bridge circuit?

can you suggest any r values?

thank you

 

[barry]

Yes, that opamp looks like a likely candidate. Basically you want to have the same impedance at both terminals of the opamp so that offset currents will cancel. So for your situation, if you've got a sensor that varies from 100M to 300M, I'd make the resistor going to ground on the inverting input 100M, and make the upper two resistors equal. I'd also simulate the circuit; hopefully you can get a spice model for that opamp.

 

[welove8051]

thanks barry, i will try it out.

 

[m.mokhtar]

Try using fet input op amps

 

[FvM]

Yes, that opamp looks like a likely candidate. Basically you want to have the same impedance at both terminals of the opamp so that offset currents will cancel.

Offset cancellation circuits are meaningful for classical bipolar OPs, not for OPs with input current compensation and rarely for FET OPs. In case of LMP7721, it would double the maximum input offset.

The measurement problem still involves nA sensor currents, any standard JFET or CMOS OP should do.