i want to measure a 100mega ohm impedance with an current source using opamp and using adc . is there any suggestion about that and is this practical with opamp?

i want to measure a 100mega ohm impedance with an current source using opamp and using adc . is there any suggestion about that and is this practical with opamp?

Hi Azita,

Thank you for your inquiry. You have asked your question in the Help space for learning how to use EngineerZone. For product specific assistance, I am moving your question to the Operational Amplifiers area.

Thank you,

Lisa

Hi, Azita.

A good read for this application is CN0349 which you can found here - http://www.analog.com/en/circuits-from-the-lab/CN0349/vc.html

Regards,

Anna

Hi, Azita.

Yes, you can use the circuit in CN0349. Just change the conductivity cell or sensor with a higher cell constant of let's say 18/cm (if pure water has conductivity of 1uS/cm). You also have to increase the values of Rfb and Rcal so that the conductivity range should meet the nS you require.

Christo_Hrisulev is the author of the CN0349. He could help you more with your requirements.

Regards,

Anna

DEAR

**AnnaFeB**its really interesting to read the circuits notes on cn0349. there is a part i want to know more about it .there is a temperature compensation that i saw in both description and also the software. which is method or formula is used in this conductivity measurement and also what is the aim of using internal temperature sensor of ad593x in this application. best regards.azita

Hi Azita:As described in the CN0349 Circuit Note (Conductivity Sensors Section), in order to measure high impedance (small conductivities) we have to use a cell with smaller constant. For example, 1uS

and 1/cm constant will give 1Mohm impedance, if the constant is 0.01/cm, the

impedance will be 10kOhm which is 100 times smaller. All this is given in the

circuit note.Dear christo;

its really interesting to read the circuits notes on cn0349. there is a part i want to know more about it .there is a temperature compensation that i saw in both description and also the software. which is method or formula is used in this conductivity measurement and also what is the aim of using internal temperature sensor of ad593x in this application. best regards.azita

Hi Azita:

As described in the CN0349 Circuit Note (Conductivity Sensors Section), in order to measure high impedance (small conductivities) we have to use a cell with smaller constant. For example, 1uS

and 1/cm constant will give 1Mohm impedance, if the constant is 0.01/cm, the

impedance will be 10kOhm which is 100 times smaller. All this is given in the

circuit note.