Frequency calibration problem with AD 5933 for impedance measurement

[Kamran786]

Dear Sir,

I hope this message finds you well.
I am using the AD 5933 evaluation board for impedance measurement; The data sheet says it can measure the impedance up to 10M ohm. I am currently measuring the impedance of a microfluidic device, where I capture the bacteria that increase the impedance from 3M ohm to 8M ohm, which is within the range of 10M ohms. I have encountered an issue where, for instance, if the impedance measures 8M ohms and I use a 3M ohm resistor for feedback during calibration, I do not obtain the correct impedance value. However, using an 8M ohm resistor for calibration provides an accurate impedance measurement. So the problem is, every time I have to calibrate the AD 5933 if there are slight variations in the impedance value to get the correct result

I have a few questions:

1) Is it possible to use a single resistor value as a feedback Resistor for calibration to measure impedance ranging from 1 ohm to 10M ohms, thereby overcoming the challenge of frequent calibration? If so, could you please suggest some solutions to address the issue of frequent calibration?

2) I recently read a paper where the author utilized an HMC 245A switch to address the issue of frequent calibration in impedance measurement. Is this method effective? The HMC 245A switch is described as a non-reflective SP3T (Single Pole, Triple Throw) RF switch. In this paper, they used different resistors with an HMC 245A switch and it automatically calibrates AD 5933.

3) Could you recommend a method, switch, or IC that if I combine it with AD 5933 and they automatically calibrate the AD 5933 and I get the correct impedance of my device from 1 ohm to 10M ohms without needing to change the feedback resistor repeatedly?

Your assistance in this matter would be greatly appreciated.

 

[FvM]

According to datasheet, AD5933 is designed to measure impedance between 1k and 10M without external amplifiers, extendable to 100R with external circuit. How do you arrive at 1R to 10M?

If wider range above datasheet specs can be achieved with external circuit, it surely needs ranging switches and multiple calibration.

Using Mohm range Rfb doesn't sound like a good idea. Highest value in application examples is 220k.

Your use case isn't quite clear. Why do you need 1R impedance range?

--- Updated Jul 11, 2024 ---

Title "frequency calibration" is misleading. You are talking about (frequency dependant) impedance calibration. BTW, what's your frequency range of interest?

 

[Kamran786]

According to datasheet, AD5933 is designed to measure impedance between 1k and 10M without external amplifiers, extendable to 100R with external circuit. How do you arrive at 1R to 10M?

If wider range above datasheet specs can be achieved with external circuit, it surely needs ranging switches and multiple calibration.

Using Mohm range Rfb doesn't sound like a good idea. Highest value in application examples is 220k.

Your use case isn't quite clear. Why do you need 1R impedance range?

--- Updated Jul 11, 2024 ---

Title "frequency calibration" is misleading. You are talking about (frequency dependant) impedance calibration. BTW, what's your frequency range of interest?

Dear Sir,
You are correct. Actually, the title was "Frequent Calibration Problem of AD 5933 for Impedance Measurement."
for 220k ohm feedback resistor, The measurements were accurate up to the 320K resistor, but after that, It shows the incorrect values. In the datasheet, they used a 220K feedback resistor. This 220K feedback resistor gives accurate measurements up to 320K ohms, and its lower limit of accurate impedance measurement is 120K ohms. Within this range ( 120k ohm to 320k ohm) there is no need of calibration but before and after that, we should need some other feedback resistor for calibration, this is the limitation of the design.
My impedance measurement range is 4M ohm to 8M ohm, for that i have to calibrate the devices again and again for lower limit and upper limit, this thing i want to surpass ?