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Pierre

C-V measurements

Question

Hello,

I just discovered Analog Discovery 2...today and I'm very interested in impedance measurements capabilities.

I would like to perform capacitance measurements of semiconductors to characterize them.

Therefore I need to operate under AC small amplitude level (about 20mV) and under sweeping DC voltage (range of a few volts, say -4V to +1V).

So I have at least 3 questions :

- Is that kind of measurement record possible with this device ? Specific program needed ?

- Should I use the internal generator with some signal attenuation or use an external (fixed frequency) generator ?

- Other potential problems to take care of (concerning calibration for instance) ?

Many thanks.

Pierre.

P.S : I would like to perform I/V measurements as well but that should not be a problem I guess...

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Posted (edited)

Hi @Pierre

The last WF beta version adds option and example to perform Offset sweep:
https://forum.digilentinc.com/topic/8908-waveforms-beta-download/

Select Custom Mode, click 'Custom Amplitude', under Example select 'Offset sweep' and adjust the low/high offset values, amplitude.
Specify identical Start/Stop frequencies. The % X axis will represent low and high offset values.

Specially for low capacitance values (pF) the open compensation is recommended, click 'Compensation' under Options.
In case you are using external Wavegen signal amplification (#X or attenuation 0.#X) or Scope input attenuation (#X or amplification 0.#X) you can specify these under Options.

image.png.f222ff42a9294c489a438e49e1e996f2.png

 

Edited by attila

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That sounds very nice, thank you !

In fact, I just heard about Analog Discovery 2... two days ago and then did not buy anything yet.

But the (fast) increasing capabilities seem more and more interesting to me.

Questions still worry me :

- Is 20mVp signal amplitude still OK from point of view of noise ?

First, there could be generator noise for such a small amplitude -> that's why I asked fo possibility to use an external one to master this issue separately in case of integrated AWG performance limitations.

But more important may be voltage measurement noise issues because both impedance analyzer resistance and load mesured voltages will be lower than this value. So I wonder if op-amp instrumentation amplifiers would be needed or not ?

If yes, would it be possible to interface them between impedance analyzer and scope inputs ?

- I did not find anything precise concerning calibration processes in Analog Discovery 2 or WaveForms 2015 Reference Manual.

Did I miss something ?

Sorry for all those (tough) questions...

Pierre.

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Hi @Pierre

The Scope input resolution is ~300uV (at high gain 5V pk2pk) and the Wavegen output ~180uV (at low gain 2V pk2pk)

I don't think you will need, but theoretically you can use external amplifiers. 

Here you can see the measurement on a 39pF capacitor.
The signal is noisy but the IA uses FFT which highly reduces the effect of noise on measurement and you can also use averaging.

image.thumb.png.c1734a0aa83d4c943228ee89faa73d46.png

image.thumb.png.35fc684aa1ea05f628c33c5549413e21.png

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Not bad at all... thanks again for the demo.

It seems to me you used slightly DC shifted signal and 20mVpp (peak-to-peak) signal (amplitude setting is pp then).

I should use 20mVp (peak) signal, which means 40mVpeak-to peak signal -> I should get better signal from this point of view.

But the capacitances I intend to measure should be closer to 1-10nF (large diodes characterisation) with measurement frequencies ranging from 1kHz to 1MHz -> capacitance voltage will be weaker.

So if you have such a capacitance as a test in your lab, that's worth to try... By the way, if I had to use op-amp devices, how to use it and calibrate the analyser consequently ?

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Hi @Pierre

A 10nF in parallel with a diode looks like this with -4V to +1V offset sweep
C1 (yellow in time view) is on AWG 20mV 40mVpp 
C2 (blue in time view) is on reference resistor

image.thumb.png.ea0100476f7587e653cb6f3f42a14d69.png

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Thank you for your interesting measurement. It seems to be a beautiful tool...

There seems to be more signal noise : I suppose because there's less averaging than for first measurement (I see 100ms averaging vs. 500ms for 1st measurement) ?

But what I don't understand is the measured value in DC sweep around -4V :  why is it so disturbed ? There were not such a pb with 39 pF capacitance measurement...

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Hi @Pierre

There is a bit more noise probably because different frequency was used, 10k-100kHz. The reference resistor is also different.

The averaging is applied on the measurement, gain/phase. There is no averaging in time view.

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Hello @attila

OK, that's clear concerning the noise.

But I still don't understand well the capacitance evolution with voltage... maybe it is a +1V / - 4V sweep rather than -4 to +1V ?

High capacitance value at start may be explained by diode conduction (around 0.5-1V). This value dos not make sense but is due to low diode resistance -> high current.

Then capacitance value should not move in the 0 to 4V range -> voltage sweep seems to be reversed from -4 to +1V to +1V / - 4V.

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