Frequency Limitation on Impedance tool

[Cleber Borges]

In the AWG tool it is possible to set a wave with a minimum frequency of 100 uHz.
In the Impedance tool it is only possible to set a wave with a minimum frequency of 1 mHz.
What is the reason for this limitation for the generated wave in Impedance?
Thank you for your attention
Cleber Borges

[attila]

Hi @Cleber Borges

We could reduce the minimum frequency to 100uHz.
May I ask for what purpose do you need such low frequency? 
This is almost 3 hours of signal period, so a step in Impedance Analyzer would be done in ~45 hours, ~2 days...!

The Scope sample rate minimum is about 50mHz. The IA uses at least 16 periods for each step, this for FFT windowing and averaging/precision. The Scope with buffer of 8k needs about 500 samples per period, so the minimum IA signal frequency is about 100 uHz, 1000 seconds.

[Cleber Borges]

Hello @attila
Good Morning
Thanks for your response and patience :-)

I am a Professor of Chemistry  (University Researcher in Brazil) and I am interested in the area of:
EIS - Electrochemical Impedance Spectroscopy  ( below SITE reference in [1] ).

In an EIS study, the frequency region of 100kHz to 1uHz is very common.
It is the current practice of this area.

The amplitude of the signal is a few millivolts. Usually it's 10mV.
So my interest in low frequencies and low amplitudes

In WaveForms Help - AD2 (item 5: AWG) says:
The resolution is ~0.7 mV for amplitudes above 1 V, and ~0.18 mV for amplitudes of 1 V and lower.

Does the AD2 and sound card achieve a higher resolution than this?
Because I see that the low frequency is not limiting but the signal resolution at 10mV seems to me to be near the limit.

I am a layman in the subject and I am still starting in EIS
but I think that the hardware and the software could also be very useful in this field :-) !!!

Thanks again for your dedication!
Cleber Borges

[1] - https://en.wikipedia.org/wiki/Dielectric_spectroscopy

[Cleber Borges]

in addition: In the Biology and Food Sciences, the 1MHz to 20MHz region is widely used for viable cell counts - VCC [2]

[2] - Electrical impedance spectroscopy (EIS) for biological analysis and food characterization: a review

J. Sens. Sens. Syst., 6, 303-325, 2017
https://doi.org/10.5194/jsss-6-303-2017

[attila]

Hi @Cleber Borges

The AD2 AWG resolution is ~0.18mV and Scope ~0.3V.
To improve the AWG resolution a simple external voltage divider can be used, like 1k/10R which will reduce the 1V signal to 10mV and the resolution to 1.8uV
The Scope would require amplification which is a bit more complicated to do.
The sound card is an experimental feature and it depends on the audio AD/DAs of the computer. These inputs/outputs are AC coupled so it won't work for low frequencies.

Here is a comparison of 1V signal vs 10mV vs 10mV with averaging of 10, for an very small 39pF capacitor:

[Cleber Borges]

Ok, I got it. :-) @attila
Very cool. A voltage divider is a really a simple solution.
Some more questions:
1 - For amplification would a very well specified OpAmp be needed? Is transistor amplification n
2 - Some details of the sound card appear in the image: is 32bits or float more advantageous?
3 - In "WaveForms - Network" there is option of Nyquist Plot as visualization mode. Is it possible to access this view also in "Impedance"?
Many thanks
Cleber Borges

Edited March 12, 2018 by Cleber Borges

[attila]

Hi @Cleber Borges

1. I don't have expertise in electronics to recommend such.

2. The various options are added in case some of them are not working with a computer to have an alternative setting to try.
Theoretically 32 bit should be better, I think in float only 24bit are actually used.
It depends on the physical audio ADc/DAc of the computer. I don't see any different between 16b, 32b and Float, so my PC has 9-16 bits mic input.
The microphone input is usually mono or eventually stereo, 1 or 2 channels.

3. There is no Nyquist plot in Impedance analyzer but you can create such with the Script tool.

plot1.X.data = Impedance1.Traces.Trace.getData("Rs")
plot1.Y1.data = Impedance1.Traces.Trace.getData("Xs")

[attila]

Hi @Cleber Borges

You can apply lower frequencies with WF v3.7.15:
https://forum.digilentinc.com/topic/8908-waveforms-beta-download/

 

[Cleber Borges]

Hello @attila,

I'm seeing the updates now ...
Thank you very much for the new limits and Nyquist Plot.  :-)

the item:  1) - Wavegen, Network/Impedance Analyzer: external Amplification option

Will it be possible to have tips in the future?

the item: 2) Sound Card device CPU usage:  I'm still looking for where to see this feature! ... :-)

The AWG for my sound card worked fine in the line-in (Scope)

thank you so much  

Cleber Borges

 

[attila]

Hi @Cleber Borges

1. With the external Amplification option you can specify the externally applied gain (opamp, voltage divider...). Like if you amplify the AWG signal 10 times (10x) the application will take this in consideration, so setting 10V it will output 1V which amplified will result 10V, or for voltage division of 1/100 (0.01x) setting 1mV the AWG will output 100mV, which attenuated will give 1mV signal...

2. This is just a fix. In the previous versions the CPU usage was high when using sound card.

I had a hard time to setup the drivers an old laptop which has real line in and out to test the Impedance Analyzer with sound card.
The result in ideal cases look good, but this does not replace the Analog Discovery.
With sound card the frequency range is limited, the channels are AC coupled, so you have no DC information in scope, you can't set output offset...

The reference traces named SC were captured with sound card and the other ones with AD:

 

 

Edited March 15, 2018 by attila

[mmachtey]

Hi,

First, thank you for all the shared information.

As Cleber said, Also I need to plot a Nyquist as traditional form "-ImaginaryZ" vs "RealZ". For example: the next figure, is a simulated nyquist for a R= 2,7 kOhm  and C= 1 uF (in parallel). As you can see, the graph have positive value for both, "Y" and "X", axes.

 

I'm using the "waveform-Impedance Analyzer" with the adapter (AD2 + IA). 

I try using the recommended script, but didn't work. Please, Can you explain me how to do it? 

plot1.X.data = Impedance1.Traces.Trace.getData("Rs")
plot1.Y1.data = Impedance1.Traces.Trace.getData("Xs")

Thanks

[attila]

Hi @mmachtey

In newer WaveForms version you can find in Impedance Analyzer Nyquist as well Custom plots.

https://reference.digilentinc.com/reference/software/waveforms/start

https://forum.digilentinc.com/topic/8908-waveforms-beta-download/

 

Edited December 15, 2020 by attila

[mmachtey]

Hi Attila, 

I know about the custom plot option. But neither the nysquit or custom plot are reals nyquist plot. Please see that the imaginary "y" axe have negative values (thats incorrect) and in the custom plot you share have frequency units in the "x" axe. It should be the real Z (in Ohm units).

thanks you again

Edited December 16, 2020 by mmachtey

[mmachtey]

Please, Can you help me to do the -Zimg vs ZReal nysquit plot as we need. 

Happy new year!

Matías

[JColvin]

Hi @mmachtey,

Much of the Digilent staff is out of office these last couple of weeks of December, so it may be some time before we get a response and update for you.

Thanks,
JColvin

[attila]

Hi @mmachtey

See the following: default.dwf3work

plot1.X.data = Impedance.Traces.Trace.getData("Rs")

var rs = []
Impedance.Traces.Trace.getData("Xs").forEach(
    function(x){rs.push(-x)}
)
plot1.Y1.data = rs

[3D_loves_eng]

On 1/4/2021 at 1:49 PM, attila said:

Hi @mmachtey

See the following: default.dwf3work

plot1.X.data = Impedance.Traces.Trace.getData("Rs")

var rs = []
Impedance.Traces.Trace.getData("Xs").forEach(
    function(x){rs.push(-x)}
)
plot1.Y1.data = rs

Hi @attila

Is it possible export the plot window to a .PNG or .BMP files by script?

Kind regards,

 

[attila]

Hi @3D_loves_eng

Yes, like: Impedance.Export("~/Desktop/imp.png", "Impedance")