Impedance Spectroscopy Set-up Guide for DIY Instruments

[tommyers]

Hi,

I have designed and built my own impedance spectroscopy instrument, using a lock-in amplifier approach, with a reference signal. I know the basic theory behind impedance and impedance spectroscopy. However, I would like some thoughts on experimental set-up.

I have a reference source (generated by my instrument). To test I have a resistor (purely resistive element) placed between my reference to ground, with the input to instrument (vin) measuring the reference end of the resistor. I see a change in my magnitude AND in my phase, although this too can be seen on the scope between the ref signal and input signal - why for a resistor?

Also, supposing I want to measure a capacitor or inductor, then would I also place these between as I placed the resistor?

Does it depend on the values of the capacitance or inductance to how may the current and voltage lead or lag? or when any capacitance or inductance is in the path it immediately changes the phase by +90 or -90 deg?

As you can see, I need a good resource for impedance spectroscopy which describes experimental set-ups etc ... Pref. online!

Set-up diagram - correct - suggestions?

Ref > ------------- + ------------ > Vin
|
/
\ Resistor or element to be measured?
/
|
--- 0v

(please note if this diagram gets screwed when uploaded then the element should be connected to the part of the diagram marked with a '+')

Regards,
Tom

 

[berkeman]

What you are describing is more commonly called Impedance Analysis, using measurements made with an Impedance Analyzer:

http://cp.literature.agilent.com/litweb/pdf/5968-1947E.pdf

As the application note suggests, you will need to compensate out parasitic error factors in your fixture, in order to make accurate complex impedance measurements.

I'll move this thread to the EE forum for now. If you can post a more complete circuit diagram of your setup, we can offer some suggestions for how to compensate out some of the parasitic elements.

 

[oswaler]

Dear Tom,

It is great to hear that you have designed and built your own impedance spectroscopy instrument using a lock-in amplifier approach. I am always excited to see individuals taking an active role in creating their own scientific instruments.

To address your questions, it is important to understand that the impedance of a resistor is purely resistive, meaning there is no phase shift between the voltage and current signals. However, in your set-up, there may be some slight phase shift due to imperfections in the components or circuitry. This is likely what you are seeing on your scope.

When measuring a capacitor or inductor, it is important to take into account their reactive components, which can lead to a phase shift between the voltage and current signals. In this case, it is important to place the capacitor or inductor in series with the resistor, as you have correctly shown in your set-up diagram.

The amount of phase shift will depend on the values of capacitance and inductance, as well as the frequency of the signal being applied. At certain frequencies, the phase shift can be significant, while at others it may be minimal. It is important to choose a frequency range that is appropriate for the components you are measuring.

I would recommend consulting a reputable online resource or a textbook on impedance spectroscopy for more detailed information on experimental set-ups and techniques. It is also helpful to consult with experienced researchers in the field for guidance and advice.

Overall, it is important to carefully design and set up your experiments to ensure accurate and reliable results. I wish you all the best in your endeavors with impedance spectroscopy.

Best regards,