How can I build an impedance measurement circuit?

Question

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. 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 direct digital synthesis (DDS) with an Arduino to generate a signal with a frequency of 1 kHz and an amplitude of 500mV, which meets my signal requirements. I am uncertain about how to proceed with the remaining circuit design.

Answer 1

The upper shown approach is good to with, you can design this simple circuit in terms to calculate the impedance parameters easily. Btw, Arduino has only 10 bit ADC keep that in mind, I will suggest to go with external ADS1115 16 bit.

Answer 2

Basically you have to supply a (sinusoidal) signal and measure the ratio of voltage applied to current flowing. This can be done with your Arduino, some opamps and some calculations.

For an example using an oscilloscope see https://github.com/jp3141/Vector-Network-Analyzer

Using this circuit, you need to measure V1 and V2. The current is (V1-V2)/R, and the voltage on the load is V2. These measurements and calculations are of complex quantities (i.e. real and imaginary parts). Basically, measure the values multiple times per cycle (say at > 10x the sinusoid frequency). Calculate the phase angle. Do the same for V2. You'll need to understand the concepts between real and imaginary signals to convert your ADC measurements to magnitude and phase and to complete these calculations.

With 30 MΩ, parasitic resistance and possibly capacitance will affect your accuracy. If you measure with NO load connected (i.e. just the parasitics), you can calculate that value, and then subtract it from your measurement with the load.