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  • Author Author: Zainii
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Building the impedance measurement circuit

Zainii

Dear Members,

I hope you are all doing well.

I am a PhD student and completely new to circuit design. I need a compact-sized circuit and prefer not to use an impedance analyzer due to its bulkiness. I am looking to build a circuit capable of automatically measuring the impedance of a sensor in the range of 1MΩ to 30MΩ. However, I am unsure how to approach this task, especially with the various methods available for impedance measurement. I’m not certain which method would be most suitable for automatic measurement in my case. There are different impedance measurement modules, but none seem to have the capability to measure impedance automatically within this range. My sensor’s equivalent circuit is a resistor in parallel with a capacitor.

Could someone guide me through the procedure to build the circuit design? Specifically, I would like to know which components I should purchase to begin building the circuit. If there are any tutorials or resources that you could recommend, that would be very helpful as well.

So far, I have only completed the integration of a DDS (Direct Digital Synthesis) with an Arduino to generate a signal with a frequency of 1 kHz and an amplitude of 500mV, which meets my signal requirements. However, I am uncertain about how to proceed with the remaining circuit design.

Your help in this matter would be greatly appreciated.

  • You could maybe use the DDS to sweep frequency.  Then attach a resistor in series between the DDS output and your sensor and observe on an oscilloscope. At lower frequencies the series resistor and sensor would form a voltage divider.  So you will be able to determine the sensor's resistance.  Somewhere in the sweep the amplitude will reach a peak.  This is the resonant frequency.  With the series resistor, sensor resistance and resonant frequency all being known at this point, you can determine the capacitance.  If I am not mistaken at resonance the series resistor will be equal to the parallel combination of the sensor resistance and capacitor reactance.  So use that as the basis to figure out the capacitance.

  • This is non-trivial, not sure how you expect to design and build this if you're completely new to circuit design!

    You could buy your way out of the problem, there are LCR modules that support the measurement range, but you're looking at a cost of approx $1k USD. For example, this module supports up to 1 Gohm apparently; but strongly suggest you discuss with the Analog Devices before buying it, to be 100% sure it will meet your needs: https://at.farnell.com/en-AT/analog-devices/admx2001b/messmodul-evaluationsboard/dp/4305964?ICID=I-RP-STM7REC-0

    Otherwise, you'd have to resort to looking at reference designs, those are frankly your best guides if you plan on studying and spending a significant amount of time (months) designing and building and coding and testing. Nearly all solutions will entail op-amp circuitry, high-res ADCs (20 bits or more) and signal processing software. Quite a lot to learn for a beginner.

    Example partial reference designs: TI https://www.ti.com/tool/TIDA-060029  (doesn't support 30 Mohm as-is, so you'd need to try to improve it (e.g. use lower-noise op-amps and lower-noise circuitry, which is likely part of the limitation).

    Also, it won't be fully automatic, you'll need to run calibration initially, and then from time to time (e.g. if the wiring changes).

    You should also realize it won't be really compact, if you're using prototyping boards like a DDS board, Arduino, etc. You may as well just buy an off-the-shelf LCR meter, for under $1000, and physically, it won't be an order of magnitude larger. And you'll save many months of work (easily 6 months if you're completely new to it).