Impedance Analyzer Build and Experiments

[jaxbird]

I have very recently received a nice bag of goodies, it was generously sent by The Electrician:



5 capacitors and 2 inductors, including captures of measurements on his high end Hioki analyzer.

Very nice references for this project, huge thanks to The Electrician.

[jaxbird]

This is great because it means no more fumbling around in the dark on whether this device will perform usable measurements or not.

Here first using the small low impedance electrolytic Sanyo 22uF/16V OS CON.

Reference measurements:





And my measurements:

[jaxbird]

The measurements are much more noisy at low impedances, but accuracy is reasonably good.

Comparison based on the cursors at 120Hz:

Unit	Reference	Measured
|Z|	57.589R	57.347R
Rs	923.101mR	887.992mR
D	0.014433	0.015486

[jaxbird]

and at ~100kHz:

Unit	Reference	Measured
|Z|	79.990mR	73.992mR
Rs	21.821mR	19.488mR
D	0.282733	0.284133

[The Electrician]

Hi, Jaxbird!

I'm glad they came through OK.  Did they make you pay any customs duty? 

First thing I notice is the SRF of your measurement appears to be at about 400 kHz, whereas the sweeps I included indicate about 750 kHz.  I made sure the parts were inserted into the fixture with the absolute shortest leads possible, which typically is right up against the body of the part in the case of the capacitors.  I suspect you have just a little more lead length involved in your measurement, or perhaps your compensation for the parasitics of your clips is a little off.

But, besides that, this is a really great project!  Wouldn't it be nice to be able to get reasonably accurate impedance sweeps for only a few hundred dollars instead of 10's of thousands?   

[jaxbird]

Hi, Jaxbird!

I'm glad they came through OK.  Did they make you pay any customs duty? 

First thing I notice is the SRF of your measurement appears to be at about 400 kHz, whereas the sweeps I included indicate about 750 kHz.  I made sure the parts were inserted into the fixture with the absolute shortest leads possible, which typically is right up against the body of the part in the case of the capacitors.  I suspect you have just a little more lead length involved in your measurement, or perhaps your compensation for the parasitics of your clips is a little off.

But, besides that, this is a really great project!  Wouldn't it be nice to be able to get reasonably accurate impedance sweeps for only a few hundred dollars instead of 10's of thousands?   

Thanks for sending the references, I very much appreciate it  No duty luckily I hardly every have to pay extra duty (only when I receive items via FedEx or DHL)

Yeah, I have tried to study the comparison of the measurements, I believe I got some residual inductance I need to deal with (probably around 10-20nH or there about), here an overlay of the 2  measurements on same scale:



I hope to be able to construct and share such a device, sure, it will not be a reference device suitable for publishing datasheets, but something generally useful to get a good idea of behavior/performance.

[The Electrician]

I hope to be able to construct and share such a device, sure, it will not be a reference device suitable for publishing datasheets, but something generally useful to get a good idea of behavior/performance.

I think it's doing quite fantastic!  Don't forget, by the way, especially for electrolytics, the ambient temperature effects.  It's instructive to run the analyzer in continuous sweep mode and heat up a component with a "warm" air gun, or even just grasp it, and watch the ESR of an electrolytic change; of course, you can do that without needing a sweep.

People buy a DE-5000 for $100.  I'd easily pay a few hundred for sweep capability.  A person learns after playing with an impedance analyzer that you can tell the health of an electrolytic in an instant with a single sweep.

[AlfBaz]

I hope to be able to construct and share such a device, sure, it will not be a reference device suitable for publishing datasheets, but something generally useful to get a good idea of behavior/performance.

I think it's doing quite fantastic!  Don't forget, by the way, especially for electrolytics, the ambient temperature effects.  It's instructive to run the analyzer in continuous sweep mode and heat up a component with a "warm" air gun, or even just grasp it, and watch the ESR of an electrolytic change; of course, you can do that without needing a sweep.

People buy a DE-5000 for $100.  I'd easily pay a few hundred for sweep capability.  A person learns after playing with an impedance analyzer that you can tell the health of an electrolytic in an instant with a single sweep.

+1
I've been messing around trying to use FG and scope under automation to do bode plots and impedance sweeps.
Some success with bode plots on two port systems. Nothing noteworthy doing impedance sweeps on single port DUT's. Many, many sources of errors/limitations and assumptions in my methods

[jaxbird]

I hope to be able to construct and share such a device, sure, it will not be a reference device suitable for publishing datasheets, but something generally useful to get a good idea of behavior/performance.

I think it's doing quite fantastic!  Don't forget, by the way, especially for electrolytics, the ambient temperature effects.  It's instructive to run the analyzer in continuous sweep mode and heat up a component with a "warm" air gun, or even just grasp it, and watch the ESR of an electrolytic change; of course, you can do that without needing a sweep.

People buy a DE-5000 for $100.  I'd easily pay a few hundred for sweep capability.  A person learns after playing with an impedance analyzer that you can tell the health of an electrolytic in an instant with a single sweep.

Thanks, this is good advice  Also as you mentioned earlier, not being sloppy with the calibration makes a huge difference .. I've made a few changes, better Kelvin Clips, shielded sense wires and a few more details. All contributing to better results.

Current setup looks like this: (still need to add the low level amplification opamps etc)

[jaxbird]

Here a measurement of the 6.8uF film capacitor:



Reference measurements:





My measurement:



Overlaying the measurements:




Edit: adding images

[The Electrician]

This is most excellent!

See that bump in the ESR and DF at about 3 MHz?  The first time I saw that on a large film cap, I thought something was wrong with the analyzer or the fixture.  But it's real; it looks just like that on an Agilent 4294 and on the Wayne-Kerr.  I think it's an internal resonance due to the nature of the metallization sprayed on the ends of the extended foil.

Your analyzer shows it perfectly!

[jaxbird]

This is most excellent!

See that bump in the ESR and DF at about 3 MHz?  The first time I saw that on a large film cap, I thought something was wrong with the analyzer or the fixture.  But it's real; it looks just like that on an Agilent 4294 and on the Wayne-Kerr.  I think it's an internal resonance due to the nature of the metallization sprayed on the ends of the extended foil.

Your analyzer shows it perfectly!

Thanks, I'm pretty happy with this performance, but it's still got some areas where the device does not perform this well. I'll try post a few examples. E.g. when doing inductor measurements the Rs does not quite follow. I'm experimenting with some dynamic test current adjustments (voltage/shunt resistance) based on impedance. That should also extend the range up to ~500k Ohm.

I was wondering about that bump, seems like nearly all film capacitors 1uF+ got this behavior. I was thinking they had maybe a smaller 2nd capacitor included either intentionally or due to mechanical construction. Thanks for providing the actual explanation 

 

[jaxbird]

Got the amplifier part of the board populated, it's not going to win any "pretty soldering award", but it works:



It should allow measurements down below 1mR, here with a large film capacitor:



And zoomed in on the resonance frequency:

[jaxbird]

Still needs some work on the calibration to get the amplification completely working, I get some phase offset although the amp circuit should be very close to 0 degrees from 1 to 20+MHz.

Anyway, here with a piece of ~1mR wire:



And a 100kR 1% resistor:

[jaxbird]

Spent a few evenings getting the amplification to behave, according to simulation it should be flat to 20MHz with almost no phase shift. But after measuring it, in reality there is an ~8 degree phase shift already at 10MHz, probably should have used more isolation between traces on the PCB.

Anyway, I can compensate for the phase shift during calibration, as the instrument is capable of measuring itself.

The great thing is that with 16x amplification it is now capable of measuring very low impedance, down to below 1 milliohm.

Here a test using the large 20uF reference capacitor from The Electrician:

[jaxbird]

Reference measurements for the 20uF:

[jaxbird]

And my measurement:




I don't think it will get much better using the current hardware, but there are still a few tweaks I want to play with.

[The Electrician]

That is astounding! 

It matches the reference curves so well it's hard to believe you can get such good performance with so little hardware.

This shows that a low priced USB based box could be sold for a few hundred dollars--certainly well under $1000!

This has got to be one of the neatest projects on the forum.

[kripton2035]

and the $250 of the dev board he uses could easyly be replaced with a small mcu + good adc board self made for some $50 IMHO.
the dev board helped to develop the thing faster, but is overpriced for how he uses it ?

[Kevin.D]

Been following this interesting thread/project. Jaxbird .
What's your plans when completed project. ? 

[kripton2035]

and the $250 of the dev board he uses could easyly be replaced with a small mcu + good adc board self made for some $50 IMHO.
the dev board helped to develop the thing faster, but is overpriced for how he uses it ?

He already has it, so its adding addtional functionality to the tool. The discovery can be had for 99 bucks, that all I paid for mine. I would be interested to see your 50 buck implementation though.

the $99 is for an US student only ... otherwise it's $279

To qualify for student pricing, the customer must be a student at a US academic institution and be required to purchase the board for a class. The quantity is limited to one board per student.

[jaxbird]

That is astounding! 

It matches the reference curves so well it's hard to believe you can get such good performance with so little hardware.

This shows that a low priced USB based box could be sold for a few hundred dollars--certainly well under $1000!

This has got to be one of the neatest projects on the forum.

Thanks, I'm happy the amplification was quite successful. It's been a long, interesting, journey (I actually find it relaxing to work on ), while the instrument is quite usable, there are a few things I want to change, so I think it might be time for a new prototype soon.

The credit for the good performance has to go to the Analog Discovery unit, it is a very nice little device that packs a lot of power in a very small footprint. The dual 14 bit 125Msps ADC (AD9648) is pretty amazing.

But I might have painted a little too rosy picture of the performance in my eager to show what it can do. Under certain circumstances the results look significantly worse. e.g. measuring Rs of a small capacitor at low frequency. And Rs on inductor measurements is lower that what it should be.

Also you can see that the Dissipation factor is slightly below what it should be, but as long as it tracks nicely, I can easily live with that.

It is a bit time consuming to get a measurement like the one I posted earlier: I need to first let everything warm up, then do an open/short calibration, and then I can do the measurement.

And as you advised, being careful with calibration does indeed pay of, I have found that even the few nH in the tips of the Kelvin clamps can make a difference, so to make sure I get open/short at the same point I do it like this:

Open:


Short:


Do you only use open/short calibration, or do you use a reference as well?

Another issue I would like to have a look at is compensating for longer cables. Currently I have to use very short cables to get good results.

[jaxbird]

and the $250 of the dev board he uses could easyly be replaced with a small mcu + good adc board self made for some $50 IMHO.
the dev board helped to develop the thing faster, but is overpriced for how he uses it ?

You would not be able to replicate the performance of the discovery module for $50, the ADC alone is something like $80. However it could be possible to do something clever and use the same technique as sampling scopes, as what we want to sample is a repeating wave (using scan and not sweep) and we already know the frequency, so maybe a dual DAC, one channel to generate the output and one channel combined with a fast comparator to sample the signal. Should be possible and a lot cheaper than high speed ADC and FPGA, mem etc.

Might also be possible to use a completely different approach with MLS sequence to sample everything in one go.

While fun and interesting, I don't think I'm prepared to go that far

[jaxbird]

Designed a new version, slightly simplified but should have equally good performance.

Here ready for engraving on some double sided FR4:

[jaxbird]

Been following this interesting thread/project. Jaxbird .
What's your plans when completed project. ?

Thanks, well, I don't really have any plans, it's just something I found interesting to pursue. A very useful instrument to have in the collection was my motivation . Surely not something anyone would use on a daily basis, but occasionally you just want to know the characteristics of e.g. a capacitor or inductor, plus all the other areas where impedance analyzers are useful. I'd like to make a probe I can stick in a PCB, not so much for fault finding, more to check designs and get better insight into various behaviors.