Author Topic: Impedance Analyzer Build and Experiments  (Read 109007 times)

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #75 on: January 01, 2015, 01:37:57 am »
Been a while, but I think I got as much as possible out of the AD5933, nice chip for impedance analysis up to 100kHz+

New toy should allow me to expand on that range. Using the digilent Analog func gen and scope shoudl allow up to near 10MHz, however not quite what I need just out of the box.

Here a nice capture of a 220nF capacitor using nothing but a 50R resistor and various cables:

 
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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #76 on: January 01, 2015, 01:42:29 am »
So while pretty nice, it does show a number of issues, like in dB, not Ohm, and very small area being actually reliable (400k to ~ 1MHz) and of course missing many of the interesting traces like ESR etc.

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #77 on: January 01, 2015, 02:11:47 am »
Picture of 220nF film capacitor for reference:


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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #78 on: January 01, 2015, 02:15:58 am »
And using a very low impedance 25uF capacitor, things get a bit noisy:

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #79 on: January 01, 2015, 02:20:13 am »
Picture of large 25uF film capacitor for reference:



For comparison, you can still see the "smallish" 220nF film at the edge. This is a very low impedance capacitor, never mind the "audio" label and 1% precision.

But this is a capacitor designed for precision speaker crossover design. While I'm aware of the general ignorance here on EEVBlog forum of the importance of using quality components for audio. I have to point out that passive crossovers is one of the areas where you cannot get away with shitty +/-20% electrolytic capacitors as you need to precisely align the crossover frequency with the mechanical phase alignment and phase errors of drivers to achieve a repeatably successful design, and we are working in the preferably <1% area for good results here.


Edit: Rant over, Triggered by a capacitor, sorry, I just read too much "shit" from "engineers" believing they always know what they are talking about.
« Last Edit: January 01, 2015, 06:25:08 am by jaxbird »
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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #80 on: January 01, 2015, 02:53:03 am »
Anyway, the question got to be, should I add a pre-amp or an amp to increase the sensitivity of the scope.

Hard choice, I have experience of building a high frequency constant current Howland based drive for the driver, but my experience with the Howland is that you cannot control the offset and it's very difficult to balance, as in you need some serious precision resistors for reliable results.

I think I could amplify the differential output using some e.g. AD8051 in stages of x2, maybe 3 stages in total for optimal results, then using some 74hc405x to switch various loads and gains for extended ranges, all controllable by the device using the available digital IO.


 
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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #81 on: January 01, 2015, 03:46:36 am »
Interesting old school 100nF versus multilayer 100nF:





Edit: 100nF not uF ( :palm:)
« Last Edit: January 01, 2015, 03:56:19 am by jaxbird »
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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #82 on: January 01, 2015, 03:49:41 am »
Pictures of above measurements:

Oldschool:



Multilayer:

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #83 on: January 01, 2015, 04:29:22 am »
Next target, not sure if I can capture it, don't have kelvin clips that small, but it's 0805 100nF for comparison to the previous measurements:



edit: not uF, but nF, sorry
« Last Edit: January 01, 2015, 06:53:40 am by jaxbird »
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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #84 on: January 01, 2015, 05:06:23 am »
Not easy, but managed to capture the tiny 0805 100nF cap:



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Offline miguelvp

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Re: Impedance Analyzer Build and Experiments
« Reply #85 on: January 01, 2015, 05:14:38 am »
Friend of mine in Spain that is a ham as I was calling him to wish him a happy new year pointed me to this:

http://miniradiosolutions.com/

And this:

https://greatscottgadgets.com/hackrf/

last not related but awesome too :)
 

Offline c4757p

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Re: Impedance Analyzer Build and Experiments
« Reply #86 on: January 01, 2015, 05:18:39 am »
Ooh, I hadn't seen this before, this project looks nice. I'll have to bookmark it. I'm working on something quite similar but over a 1 kHz to 150 MHz range as my BSEE senior project - perhaps I'll make a post about that too.
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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #87 on: January 01, 2015, 05:40:59 am »
Friend of mine in Spain that is a ham as I was calling him to wish him a happy new year pointed me to this:

http://miniradiosolutions.com/

And this:

https://greatscottgadgets.com/hackrf/

last not related but awesome too :)


Thanks, but I think those are slightly higher in frequency and lower in power than what I am trying to achieve.  My goal is more in the 10Hz to 10MHz range, but being able to provide plenty of current to measure low impedance at these frequencies.

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #88 on: January 01, 2015, 05:50:51 am »
Ooh, I hadn't seen this before, this project looks nice. I'll have to bookmark it. I'm working on something quite similar but over a 1 kHz to 150 MHz range as my BSEE senior project - perhaps I'll make a post about that too.

Thanks, but keep in mind this project was originally intended to push the AD5933 to it's limits, which I pretty much did earlier with reasonable results, but now I have switched to using a pre made Analog Discovery Unit from Digilent to aim even higher, as in beyond the 100-500kHz limit of the AD5933. Hoping to achieve close to 1Hz - 10MHz instead. And hopefully being able to probe down to near 10mR.

My goal now is primarily to write some software and create a module for the digilent unit for precision impedance analysis across a wide range, including plotting charts for impedance, capacitance, inductance, ESR, etc.



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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #89 on: January 01, 2015, 06:16:34 am »
Interesting old school 100nF versus multilayer 100nF and 0805 100nF x7r (50V of questionable origin, but claimed Samsung)

Clear winner is the smd 0805 x7 50V, but closely followed by the multilayer 100uF through hole and as a sloppy last is the old school single layer 100uF ceramic.

« Last Edit: January 01, 2015, 06:18:54 am by jaxbird »
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Offline dannyf

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Re: Impedance Analyzer Build and Experiments
« Reply #90 on: January 01, 2015, 12:03:09 pm »
Quote
Pictures of above measurements:

Those kelvin clips of yours are just nice, :)
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Offline dannyf

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Re: Impedance Analyzer Build and Experiments
« Reply #91 on: January 01, 2015, 05:29:29 pm »
Those chips have a big limitation: they assumed that the signal applied to the dut is the same as from the chips. AD could have implemented another ADC channel and to get a separate voltage measurement and do FFT from there. That would have made those chips infinitely more versatile.

On that front, have you experimented with a discrete buffer on the excitation pin? Essentially, run a pair of current amps (pnp+npn, with a resistor in between the bases and emitters).

The gain loss should be small. and can be corrected in software if you allow calibration.
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Offline robrenz

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Re: Impedance Analyzer Build and Experiments
« Reply #92 on: January 01, 2015, 05:35:06 pm »
Quote
Pictures of above measurements:

Those kelvin clips of yours are just nice, :)

Yes they are awesome! Anyone have a link where I could get a set?  :-DD

Offline dannyf

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Re: Impedance Analyzer Build and Experiments
« Reply #93 on: January 02, 2015, 12:55:30 pm »
My kelvin clips - practically free.

I could have squeezed in a 1/8w 100ohm resistor there.
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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #94 on: January 07, 2015, 02:23:59 am »
Those chips have a big limitation: they assumed that the signal applied to the dut is the same as from the chips. AD could have implemented another ADC channel and to get a separate voltage measurement and do FFT from there. That would have made those chips infinitely more versatile.

On that front, have you experimented with a discrete buffer on the excitation pin? Essentially, run a pair of current amps (pnp+npn, with a resistor in between the bases and emitters).

The gain loss should be small. and can be corrected in software if you allow calibration.

The AD5933 is somewhat limited, but works well within 1kHz to 200kHz. I used a Howland current source on the output with reasonably good results. I haven't attempted a discrete buffer.

I have now switched to experiment with an Analog Discovery unit from Digilent, basically 2 x 100Msps 14 bit ADC, plus 2x100Msps 14 bit DAC

(http://www.digilentinc.com/Data/Products/ANALOG-DISCOVERY/Discovery_TRM_RevB_1.pdf)

It is of course 10x the cost of AD5933 (http://www.microchipdirect.com/ProductSearch.aspx?Keywords=TDGL023), but much better suited and useful for many other purposes. With the dual ADC I can do a DFT and get absolute values.

But I probably cannot get away with a purely passive solution (simple shunt resistor) as it isn't happy driving more than 50R load at 2Vpp amplitude (already rolling off a bit early at that load) and the ADCs LSB is something like ~300uV at highest sensitivity.

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #95 on: January 07, 2015, 02:27:52 am »
Quote
Pictures of above measurements:

Those kelvin clips of yours are just nice, :)

Yes they are awesome! Anyone have a link where I could get a set?  :-DD

Hehe, yeah well, it works ;D

I have added some actual kelvin clips now, the other solution was a bit fiddly.

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #96 on: January 07, 2015, 02:41:21 am »
So I've been playing around with the software, rewriting it a few times before getting to good results. My first attempt was to simply use 0 crossing and RMS on voltage on the sampled data to calculate phase and amplitude, but, while it works, it isn't easy to get accurate phase at higher frequencies even with some interpolation to improve the accuracy.

The solution where I get best results is using DFT. And a few tricks of averaging multiple sample buffers depending on amplitude. That does give reliable results even when the signal is down to 1mVpp. But still it does get too noisy when attempting to measure down near 10mR.

Here a capture of the new software with a film capacitor:



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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #97 on: January 07, 2015, 02:48:55 am »
And here an electrolytic capacitor:



Still need to add ESR and other interesting values to make it more useful. Great thing is that this approach does not need any calibration as the Digilent/Analog unit got good accuracy.

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #98 on: January 07, 2015, 03:57:53 am »
With the Kelvin clips shorted and 50R shunt it measures ~13mR, so that is the lower limit in this configuration, the 13mR probably come from leakage between the channels.




So to get below 10mR I'll need either more current or higher sensitivity or maybe both.

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Offline jaxbirdTopic starter

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Re: Impedance Analyzer Build and Experiments
« Reply #99 on: January 08, 2015, 11:13:47 pm »
Did some work on cleaning up the software while waiting for parts:


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