Author Topic: EEVblog #626 - Ceramic Capacitor Voltage Dependency  (Read 37196 times)

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Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #50 on: June 08, 2014, 03:37:37 am »
One more thing...

MLCC value drifts over time. As the crsytal structure settles under dc bias the cap will exhibit drift. It takes about 5 years at dc level for the cap to become stable.
It is an exponential decay with the largest change happening the first few months in operation.

Here is the trap: heating the dielectric above its curie point resets this decay!
And the curie point is 148 degreee c for BaTi based caps.
This means : solder it and you reset the decay!
So you can forget using these as precision timing!
Worse, you got a system returning from the field after 6 months with drift. Touch it with a soldering iron and you are screwed... You reset the decay.
The one year decay is about 8%.

By the way: daves blue capacitor in the video is simply a surface mounted cap with legs spotwelded on and dipped in epoxy.
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Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #51 on: June 08, 2014, 03:42:03 am »
Last thing: if you thought you heard it all : dave says in the video 'its only a resistor, the most basic of parts' ...
EEuuuuuuuuurt (big fail buzzer going off)

If you think this dc bias stuff of caps is bad, wait till you discover all the crap you can run into due to fringe effects in resistors... Noise due to sintering, iregularities in deposit, working voltage, hygroscopy, inductance in spiral cut parts, drift due to dendrite formation in voltage overstressed laser trimmed parts...  Don't get me started on that stuff...
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Offline WarSim

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #52 on: June 08, 2014, 04:18:01 am »

These fundamental Friday videos are beginning to worry me. 
There has not been a single one that taught me something new.

Well, the title says it all, fundamental. If you know your fundamentals then there is nothing to learn. That's just the way it is. Sometimes I do lesser known ones like this, but if you know those too oh well  :-//

Please don't get me wrong. 
My statements are against the many egomaniac engineers I have had to work with that seeming haven't grasped the fundamentals not this video. 
I have to remind myself of the several excellent engineers that I had the pleasure of working with.  Sometimes I forget that those many are not all of them.
 
Any good engineer will learn from any source no matter what it is.  That is why they learn the fundamentals first.  So even if a lowly Technologist explains an issue with physics, an engineer should be able to verify or dispute the issue.  Not attack the source with argument based on their status. 

Oops another rant starting, I better stop now :).

Thanks for all the videos.  I will never avoid an information source like the bad examples mentioned above, just surprise me someday with something new, please. 



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Offline jesuscf

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #53 on: June 08, 2014, 04:38:25 am »
I wonder how practical would it be to make a VCO using the voltage dependency of ceramic capacitors.   Something along the lines of a VCO with a varicap, but at a much lower frequency.
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Offline WarSim

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #54 on: June 08, 2014, 04:47:58 am »
Yes everyone listen to free electron.  These issues are present in all capacitors that are created with any type of particle suspension.  The only thing in question is the manufacturing steps taken to minimize the issues.  In general this issue is lowest in class 1 of the ceramic capacitors, not nonexistent.  So in extension, a manufacturing variance could inflate these issues even if the general rule says it's best.  Also why the capacitor manufactures with reliable and repeatable manufacturing processes are trusted the most. 
There is no such thing as a perfect capacitor, just the right one for each purpose.  Using only molecularly controlled grown crystal capacitors everywhere at $100-10,000 each would make any project cost prohibitive.  I have never seen one myself only helped make a device for the research department to develop them about a decade ago.


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Offline 13hm13

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #55 on: June 08, 2014, 06:03:14 am »
Anyone know if any of the circuit-simulation software suites (SPICE, etc.) have either native or avail plug-ins for simulating these type of "ceramic distortions"?
 

Offline 13hm13

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #56 on: June 08, 2014, 06:36:04 am »
So as a general rule of thumb, get the physically largest capacitor you can that is practicable for your electrical specification needs.
This is part of it ...
On audio DIY forums/blogs, a somewhat popular project is to mod low-cost (but otherwise high-performance Asian DAPs [aka digital audio players, like iPods] **)
Below is a stock (unmodded) PCB from a decent-sounding DAP by Chinese manuf Teclast:


And after a bit of experimenting -- via o'scoping, listening tests and other metric--some modders on a Russian electronics DIY audio forum made some changes that apparently improve both objective (measurable) and subjective (listening test) performance (of interest to this EEVblog topic are the cap mods. SORRY: the image below is 180-degrees opposite from above):


** Not all Chinese products are "Wung Hun Lo". There are quite a few high-performance brands that do not make it to USA/Europe/Australia mass-market stores, but that one can on eBay or even Amazon.

====

FYI, Russian modders' total list of mods done are below:
Replacement / differences from stock:
 
replacement negative power inverters in headphones AMP and LPF
    voltage regulators headphonesAMP and LPF with replacement filter capacitor Cbypass (is good for filtering high frequency interference)
    an additional negative regulator shoulder headphonesAMP, along with the replacement of the capacitor in the inverter on Cfly X5R 4.7uF
    replacement signal resistors located between DAC outputs and LPF
    capacitors in the LPF replacement for ceramic NP0
    replacement 10uF 1uF ceramic filters in power modulators DACs to 0,1 uF NP0
    ceramic replacement shunts opamp LPF and headphonesAMP at 0,1 uF NP0
    the signal from the low pass filter to the ear specialist now passes through the wire MC16-13(silver-plated copper) 0.05, cut lead-in track, contacts potentiometer raised and do not touch the board. Still have access to the line-out, installed protective resistors 50 Om
    removed the low quality resistors in the signal path to the headphonesAMP and line-out
    increased power amplifier to your ear +-4V
    filtering power before and after the dc-dc TD1410 (power supply for headphonesAMP)
    opamp LPF LM4562
   headphonesAMP made of LM4562+THS4222 - output up to 100mA per channel, 14mA quiescent current per channel(most portable headphones will work in Class A)
    power cap for headphonesAMP is 2x220uF Panasonic FK, LPF 2x47uF Panasonic SU(bipolar capacitors with very low distortion)
    measuring resonator 24MHz, the structure in tact CPU PLL, a generator with low jitter
    stock replacement batteries for the assembly of two LP473850 1090mAh, setting equalizer to equalize pressures between the banks
    sealed cables and sensor module aluminum tape to avoid interference on the part of the signal
« Last Edit: June 08, 2014, 06:48:34 am by 13hm13 »
 

Offline The Electrician

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #57 on: June 08, 2014, 07:08:57 am »
Have a look at this:

 

Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #58 on: June 08, 2014, 07:36:37 am »
Anyone know if any of the circuit-simulation software suites (SPICE, etc.) have either native or avail plug-ins for simulating these type of "ceramic distortions"?

The software made available by the cap makers can spit out a spice model of the cap you pick.
Every cap maker has his own secret sauce dielectrics. This isnt calculated. The cap goes through a waterfall analysis and a model is constructed.

Just like there are s parameters and ibis models for connectors, there is a model for caps. I have ran sims with these models and they are pretty damn close to measure. Og course the simulator needs accurate models for everything. As silicon maker we have the real circuit to run it on. I doubt you can do it at home...
« Last Edit: June 08, 2014, 07:59:21 am by free_electron »
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Offline 13hm13

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #59 on: June 08, 2014, 08:03:40 am »
Have a look at this:


Testing JUST the cap, out of ckt, may not be realistic no matter how accurate the testing routine (e.g., using correct meter(s) correctly). Of course, the cap in its permanent topology (the ckt) is not going to be as easy to model.
What TDK tests is a start but still low-hanging fruit.
In situ testing is most important ... NASA or USA military will have some answers?
« Last Edit: June 08, 2014, 08:05:35 am by 13hm13 »
 

Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #60 on: June 08, 2014, 09:07:22 am »
Any decent engineering kit does that. Either during design or during emc test. For mass production the circuit is checked to see there are no out of band points that could be problematic during emc test. Then circuit is optimised for bom cost by stripping unneccesary caps. If a regulator calls for 10uf then this means it needs to see 10 effective uf. Take the caps directly around it plus all other decoupling in the board and you may actually have waaaay too much. So bom cost can be tweaked by removing extraeneous parts. Deciding which ones and where takes experience and skill. Sweep the board at proper dc bias and measure both bulk capacitance and impedance. There is a reason proper lcr merers can either supply dc bias or reject it during measurement. A 4367 lcr bridge and a network/impedance analyser are required for such work.
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Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #61 on: June 08, 2014, 09:26:54 am »
Doug Ford reminded me of an old article of his that shows the effect of XR7 capacitor non linearity on audio distortion.
http://www.dfad.com.au/links/DFAD_PASSIVE_SURROUND.pdf
 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #62 on: June 08, 2014, 09:34:03 am »
One very important thing is regulators. To meet performance a regulator requires a certain output and input capacitance. This is given in the spec. You NEED to compensate the caps for dc ! The numbers in the spec are ideal values. It is up to the designer to derate the caps.

Not always the case.
I've seen regulator datasheets give a minimum capacitance and then proceed to specify and recommending that same value cap (including part number) in the app circuit that has no chance of meeting that minimum capacitance.
So you just don't really know whether their actual minimum value is theoretical, or practical derived using typical parts.
To be on the safe side of course you assume it's theoretical and derate the part suitably. In which case the application circuits could be in theory, wrong.
« Last Edit: June 08, 2014, 09:53:20 am by EEVblog »
 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #63 on: June 08, 2014, 09:42:20 am »
Thanks for all the videos.  I will never avoid an information source like the bad examples mentioned above, just surprise me someday with something new, please. 

Geeze, well just how a I supposed to do that?
This is probably one of the more obscure "fundamental" issues out there, and you want me to surprise you with something fundamental you don't know?  :-//
 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #64 on: June 08, 2014, 09:45:17 am »
Last thing: if you thought you heard it all : dave says in the video 'its only a resistor, the most basic of parts' ...
EEuuuuuuuuurt (big fail buzzer going off)

I was of course referring to the basic applications we were looking at here.
It's part of the story telling  ;)
 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #65 on: June 08, 2014, 09:49:48 am »
So all of this does *NOT* apply to Class I capacitors?

In theory, yes it can (to a small degree). In practice, no, it's not something you generally have to worry about with a class 1 cap. Even the worst class 1 caps are an order or two better than class 2 in this area.
 

Offline 13hm13

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #66 on: June 08, 2014, 10:29:54 am »
Another approach to addressing this issue is to base one's choice of cap type based on application, using a succinct "laundry-list" format . So ...

The voltage-dependency issue may be mostly a NON-ISSUE in the following applications:
(1)
(2)
(3)

The voltage-dependency issue is IMPORTANT in the following applications:
(1)
(2)
(3)

===

And the results of the lists above can be later turned into charts, tables, info-graphics, etc.

Yes, some topical charts and datasheet info have already been pointed out by Dave/others . But info "hidden" in the obscure sections of datasheets, and web sites are not ideal for designers (I'm sure the obscurity is partly driven by marketing and economics!)
« Last Edit: June 08, 2014, 12:50:22 pm by 13hm13 »
 

Offline David_AVD

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #67 on: June 08, 2014, 11:26:09 am »
Doug Ford reminded me of an old article of his that shows the effect of XR7 capacitor non linearity on audio distortion.
http://www.dfad.com.au/links/DFAD_PASSIVE_SURROUND.pdf
That's a great reference and is the type of thing I was thinking about when I questioned the cap's effect in coupling and filter application.

If the capacitance changes with AC level, it's not surprising that it would result in distortion of some sort.

The filter corner frequencies could also shift depending on the overall signal level I assume.
 

Offline JackOfVA

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #68 on: June 08, 2014, 11:45:50 am »
It's actually worse, in some respects, than Dave suggests, as the dielectric has 'memory' - probably what Robert Pease discussed in his 'dielectric soakage' notes.

A few images below from my capacitance versus voltage work.

Cap 25 & 50V Plot - Capacitance versus voltage for 25 and 50V rated X7R 1u0 1206 surface mount parts. Note the hysteresis seen in the plot - the actual capacitance value at a particular bias voltage depends on the history of the applied voltage.  This data was taken with relatively fast voltage steps, using an HP4192A with built-in bias generator.

Z5U Three Time Steps - To see if dielectric soakage explained the hysteresis, I ran a Z5u 0u1, 50V 1206 surface mount part over +/- 35V bias range, but with varying time interval between successive voltage steps. Regardless of the time between voltage steps, the same 'butterfly' pattern is apparent. There is a change in absolute capacitance but in general the same hysteresis pattern is repeated for step times of 1, 10 and 100 seconds.

Full Size - The Z5U 0u1 1206 surface mount part was exposed to alternating bias voltage of +35V and -35V for an extended time period and relatively slow intervals between bias voltage reversals and the capacitance observed.

Expanded - An expanded view of one of the cycles in the above plot. This plot is the best illustration of dielectric soakage.

CapvsV - This is an older data analysis (2001) made with a manual General Radio 1650A RLC bridge and an external bias supply, with a random selection of capacitors from my parts bin.  As might be expected, the C0G and film parts are unchanged with bias voltage, within the limits of the 1650A bridge. This data does not show hysteresis because it was taken with one continuous voltage run, slowly as the process was to adjust the bias supply, null the bridge and record the voltage, capacitance and dissipation factor. Repeat until the desired end voltage point is reached.

D versus V - Same parts and test setup as above, but this time looking at the dissipation factor. It also changes with applied voltage, and in a non-obvious fashion.

ExpFit - lastly, I tried an exponential curve fit to the 2001 data, with decent results for C versus V.


« Last Edit: June 08, 2014, 11:50:54 am by JackOfVA »
 

Online Bud

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #69 on: June 08, 2014, 03:09:24 pm »

Serious design work requires characterising the power decoupling . I use a agilent 4395a network/impedance analyser. This allows me to plot the impedance over frequency with dc bias applied. Cap  values are then inserted depending on where the impedance is out of whack.

Is this done on the fully populated board or just one with only decoupling caps on the power rails in place?

Also, where is the analyzer is connected to, just to the power supply side or to each critical area of board requiring good decoupling?
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Offline 13hm13

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #70 on: June 08, 2014, 03:45:13 pm »
I like the earlier suggestion that an important reason for this effect may be SIZE related. So a followup by Dave/anyone, in which same value/diff.-size ceramics are compared, would be useful.

In any case, WRT the overall importance of this phenomenon ... I guess I'm still not convinced it is anything more than a marginal issue (more of a conversation piece and a fascinating scientific phenomenon).
I am involved in the wanky world of audiophile design ... and of any consumer-electronic fields, audiophilia would be the first to know about such issues (we've certainly got our share of ridiculous pre-amps and speakers with boutique, super $$ caps by Mundorf and BlackGate).
Maybe the obscurity of the "ceramic voltage dependency" issue (such as those obscure/hard-to-find datasheet graphs) is DESERVED and REALISTIC.
Convince me to the contrary, folks ;)
 

Offline JackOfVA

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #71 on: June 08, 2014, 04:27:56 pm »
I like the earlier suggestion that an important reason for this effect may be SIZE related. So a followup by Dave/anyone, in which same value/diff.-size ceramics are compared, would be useful.

In any case, WRT the overall importance of this phenomenon ... I guess I'm still not convinced it is anything more than a marginal issue (more of a conversation piece and a fascinating scientific phenomenon).
I am involved in the wanky world of audiophile design ... and of any consumer-electronic fields, audiophilia would be the first to know about such issues (we've certainly got our share of ridiculous pre-amps and speakers with boutique, super $$ caps by Mundorf and BlackGate).
Maybe the obscurity of the "ceramic voltage dependency" issue (such as those obscure/hard-to-find datasheet graphs) is DESERVED and REALISTIC.
Convince me to the contrary, folks ;)

I've looked at intermodulation distortion at radio frequencies when signals are passed though X7R coupling capacitors and compared the 2nd and 3rd order intercepts with the same circuit but (a) with C0G parts and (b) with film capacitors.

Typically one side of the coupling capacitor is at +15V and the other side at 0 or in some circuits +20V. RF levels are around 0 dBm and the RF load impedance is 50 ohms. 

At 20 KHz, Xc (assuming for a moment the X7R is still 1uF with this DC bias on it) = about 2 ohms. Current through the blocking capacitor is on the order of 4.5 mA and the corresponding AC voltage drop across the 2 ohm blocking capacitor is 9 mV. (All RMS).

At 200 KHz, Xc is 0.2 ohms and the AC voltage drop is 0.9mV.

Analysis of this says you neglect the DC bias except in so far as it alters the capacitance value, and consider the non-linearity caused by an RF 'wiggle' of 9mV at 20 KHz or less than 1mV at 200 KHz and above.   My measurement floor is a 2nd order output intercept of +120 dBm and 3rd order output intercept around +52 dBm, limiting value is intermodulation caused in band reject filters between the device under test and the spectrum analyzer.

To date, I've not seen any significant difference between X7R 1uF 1206 SM parts and film or a bunch of parallel 0u1 C0G 1206 SM parts. Hence I continue to use X7R parts in the RF amplifiers I build and sell.  The best HF preamplifiers I build show OIP2 >= +120 dBM and OIP3 ~+50 dBm with test tones of 3 & 4 MHz, measured at 0 dBm output.

At least in this particular application, I've not found an issue with X7R coupling capacitors. I've taken a look at test tones in the medium wave band (500 & 700 KHz) and see the same thing - no change with capacitor type. It may be that lower frequencies will show degraded OIP2/3, but at the frequencies where these amplifiers are used, no degradation.

 

Offline w2aew

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #72 on: June 08, 2014, 06:26:13 pm »
A nifty measurement shortcut you can use when measuring the RC time constant, particularly on an old analog scope without cursors, is to adjust the vertical scale vernier so the the waveform occupies 8 vertical divisions. Then, 1 RC time constant is when the waveform crosses the 5th division vertically.
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Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #73 on: June 08, 2014, 09:33:11 pm »
One very important thing is regulators. To meet performance a regulator requires a certain output and input capacitance. This is given in the spec. You NEED to compensate the caps for dc ! The numbers in the spec are ideal values. It is up to the designer to derate the caps.

Not always the case.
I've seen regulator datasheets give a minimum capacitance and then proceed to specify and recommending that same value cap (including part number) in the app circuit that has no chance of meeting that minimum capacitance.
So you just don't really know whether their actual minimum value is theoretical, or practical derived using typical parts.
To be on the safe side of course you assume it's theoretical and derate the part suitably. In which case the application circuits could be in theory, wrong.
correct, not always the case. that is why i explicitly said 'apps engineer worth his salt' and 'some manufacturers' and 'sometimes dpeending on department' ...

it is a crapshoot.
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Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #74 on: June 08, 2014, 09:41:35 pm »

Serious design work requires characterising the power decoupling . I use a agilent 4395a network/impedance analyser. This allows me to plot the impedance over frequency with dc bias applied. Cap  values are then inserted depending on where the impedance is out of whack.

Is this done on the fully populated board or just one with only decoupling caps on the power rails in place?

Also, where is the analyzer is connected to, just to the power supply side or to each critical area of board requiring good decoupling?

Fully populated board , powered up and running ( devices consuming current when in operation , not when idle or twiddling their thumbs )
impedance plot is taking at every power entry of the system or system section:

power enters somewhaere at the board edge. that is to be measured.
any local regulator on the system feeds a power section. so that is where we measure again.
in essence all power rails need to be tested and there should be no 'invisible capacitors' shielded by an intermediate regulator ( or a ferrite bead ! )

you need the impedance option for the machine (option 10) and the proper testjig that lets you connect to a dc biased system i use the component testjig that allows dc biasing.

you can make this jig yourself. the schematics are out there. simply calibrate it by taking an unconnected sweep and storing that in cal memory. the 4395 then automatically compensates for all the interconnect crap so your referenc eplane is at the jig terminal.
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