New MicSig/EEVblog DP10007 HV Differential Probe

[Martin72]

Found one.

https://blog.teledynelecroy.com/2021/04/how-to-test-cmrr-of-differential-probes.html

[EEVblog]

Now I got one, how can I test it to see if it´s still a problem ?

Function gen into a 50 ohm load into a scope, largest voltage you can generate.
Connect both diff probes to the positive side of the 50ohm load, make sure they are twisted.
Measure the diff probe output amplitude in ratio to the input with 20*log(Vout/Vin)
10MHz spec is -40dB IIRC.

[PartialDischarge]

Measure the diff probe output amplitude in ratio to the input with 20*log(Vout/Vin)

One trick here is that the attenuation ratio of the diff probe has to be taken into account, ie if the input is 1Vrms and the output at 1:1 scope setting is 1mV, then the CMRR is not 60dB but 60dB-probe att.

I never know if manufacturers are playing with this, and for example in the Lecroy image the CMRR for the x50 and x500 look suspiciously similar, for x500 the CMRR should be at least 20 dB worst, although this can depend on the actual behaviour of the probe... best to test

[EEVblog]

Measure the diff probe output amplitude in ratio to the input with 20*log(Vout/Vin)

One trick here is that the attenuation ratio of the diff probe has to be taken into account, ie if the input is 1Vrms and the output at 1:1 scope setting is 1mV, then the CMRR is not 60dB but 60dB-probe att.

I never know if manufacturers are playing with this, and for example in the Lecroy image the CMRR for the x50 and x500 look suspiciously similar, for x500 the CMRR should be at least 20 dB worst, although this can depend on the actual behaviour of the probe... best to test

Ah, yes, of course. You set that as the probe ratio on the scope.
The manufacturer could very well diddle this. It should be specifically mentioned in the datasheet if it's output referred, but it never is.

[Martin72]

@Dave:

 

[EEVblog]

Response for the DP10007 vs the HVP70
Just like I told Micsig right back at the start, it didn't meet the -40dB spec at 10MHz, and they said they'd look into it and they never got back to me.
Phase and gain response is not smooth like the HVP70. But as expected, bets are are off over 10MHz with such probes which is why the spec stops there.

[ojete]

Thanks Dave, I was interested in the DP10007 but this measurements don´t look very good.
This problem was "supposedly" fixed in a later revision of the probe but I don't know if this is true, maybe if someone has a recent unit can´t test it and tell us if something has changed.
This cmrr loss and abrupt phase shift at 6 khz, maybe can be because of a resonance? and maybe because of the ceramic capacitors and the piezo effect?

[Martin72]

Hi,

Got one bought a few months before, will test it in the next days.

[EEVblog]

Thanks Dave, I was interested in the DP10007 but this measurements don´t look very good.
This problem was "supposedly" fixed in a later revision of the probe but I don't know if this is true, maybe if someone has a recent unit can´t test it and tell us if something has changed.
This cmrr loss and abrupt phase shift at 6 khz, maybe can be because of a resonance? and maybe because of the ceramic capacitors and the piezo effect?

Interesting, they didn't convey that to me, and they knew I waiting for their response before I ordered them.
Mine is certainly an original unit.

Video of the testing:

[dreamcat4]

ok so if this transpires that there was a product revision that improves the cmrr. then it would be great to see a teardown comparison. to know if there were any internal changes if its possible, maybe to figure out how to mod older units. or if not possible for example without breaking the calibration etc?


 

[EEVblog]

ok so if this transpires that there was a product revision that improves the cmrr. then it would be great to see a teardown comparison. to know if there were any internal changes if its possible, maybe to figure out how to mod older units. or if not possible for example without breaking the calibration etc?

My teardown photos are here:
https://flickr.com/photos/eevblog/albums/72157719721698708

[trp806mo]

My DP1007 from 2 years is in spec : (raw file, 10/20dB must be added) <-40dB@10MHz

[ojete]

Dave, you could ask the guys of MicSig to send you a new unit of the DP10007. If they have fixed the resonance problem I'm sure they would do it so you can test it and sell it in your store if you want.

[Martin72]

Preparing...

[ojete]

Thank you so much. I'm really interested in this probe and I hope they have fixed the resonance problem.

The Sapphire probe Dave sells it's very tempting but with shipping and the customs here in Europe the price goes up very quickly.

[Martin72]

Hi,

Quickshots of today, tomorrow afternoon carrying on for further playings.
Raw values without adding something.
Probe is setting to 10x.
Setup:
-Siglent SDS2504X HD
-Siglent SDG2122X
-Huber&Suhner 50 ohm termination.
Bode plot amplitude setting to max.

CMRR Plots 20Mhz and 120Mhz (thanks to the SDG2122X)
CMRR Value rms Ch1 and Ch2 at 10Mhz
Remarkable:
Calculated value meets the value in the bode plot. ( 20*log10(Ch2/Ch1) ) edit : phase also !
As mentioned before, further play arounds tomorrow - If you want to see something else with this setup, any comments on what I did today,  feel free...

Martin

[Martin72]

Another play before visiting relatives...
Using the formular editor of the scope, F is displaying 20*log(C2/C1).
In retrospect, I don't even know if I need to declare log10 additionally in the editor than just log.
I'll test it later.

[PartialDischarge]

The phase of the CMRR is meaningless, what matters is the amplitude. Also the phase response in differential mode is also not very interesting, the pulse response in time axis is however more indicative

"Using the formular editor of the scope, F is displaying 20*log(C2/C1)"
It has to be the ratio of the rms value of the signals, not the ratio of instantaneous voltages.

[Martin72]

Nevertheless, I like the way it fits together, both in Bode and measured in this way.

It has to be the ratio of the rms value of the signals

Yes, that also occurred to me when I was on the road.
But at the moment I don't know how to include this in the formula editor.

[Martin72]

I got a second diff.-probe, a testec si-9002, 25Mhz.
Pics:
CMRR plot and "normal" bode plot up to 30Mhz.
Looks good and was expected.
To avoid "picture shuffling" I´ll post the bandwith results of the Micsig in a second post.

[Martin72]

Here the bandwith of the micsig, first up to 30Mhz, then 120Mhz.

[Martin72]

Again the bandwith of the micsig probe, but in "higher resolution"...
The linearity between 20Mhz and 100Mhz is...not given.
At work we got some probes from testec, also with 100Mhz, will take one of them at home in the next days.

[ExaLab]

Relatively recently Micsig changed its specification regarding the CMRR of the DP10007 probe from the old 50db@1MHz to 45dB. Furthermore, the CMRR @10MHz that in the past was 40dB is no longer specified!!
It can be deduced that although they are aware of the problem, they have no intention of solving it...

I therefore decided to proceed on my own by making slight modifications to the probe which gave me rather good results (see the below graph) both on the x10 range, where at 10MHz I brought the CMMR from 39dB to 51dB (+12dB) and on the x100 range where the CMRR went from 22dB to 50dB (+28dB).
For your information, my probe has the same PCB version as the one presented in detail by Dave in his various posts (i.e. DP700V_V1X 2020.09.22).

Therefore, if your probe has this PCB version (or a similar version that includes the same schematic...) and you are interested in converting your poor CMMR DP10007 into a "Super Probe", I will provide with all the information necessary to do so (naturally not taking any responsibility and not being able to give you any guarantee regarding the results...).

Stay tuned!

[EEVblog]

Relatively recently Micsig changed its specification regarding the CMRR of the DP10007 probe from the old 50db@1MHz to 45dB. Furthermore, the CMRR @10MHz that in the past was 40dB is no longer specified!!
It can be deduced that although they are aware of the problem, they have no intention of solving it...

That's why I never stocked it and went back to the more expensive Saphire probe.

[ExaLab]

Before starting with the discussion, I would like to make some considerations about the probe bandwidth.

As noted by many users of this thread, the probe bandwidth is far from the declared 100MHz!
To verify this avoiding the artifacts of the input resonances it is sufficient to remove the input leads and test the transfer function (TF) of the device in this status.
The bandwidth of the "pure" probe more or less will be near 40-50Mhz.
As shown in the below graph, the TF of the full device (i.e. provided of the input leads) is due to the sum of the "pure" probe TF with the input LC TF.
The resonance frequency of this LC network is about 110MHz but, twisting the leads, other "modes" will overlap to the main resonance complicating the situation...
My feeling is that this resonance was intentionally used to attribute the nominal 100MHZ bandwidth to the probe although, in practice, the behavior above 40MHz is highly variable and conditioned by the geometry and mutual position of the input leads.

A further fundamental aspect that few people know is that the resonance associated with the capacitance of the input stage can also substantially influence the CMRR measurement at high frequencies.
Therefore, always give due weight to the resonant frequency originating from the common mode capacitance of the two inputs (in this device is approximately 5pF) and the cumulative series inductance of the reference path (that usually coincides with the ground path of the measuring bench).
If this resonance frequency is close to the CMRR frequency you are investigating, the effect is that you'll measure a CMRR worse than the actual value.
As example, if the measurement bench is organized as in the below drawing, the resonance frequency can be estimated at around 25MHz and can strongly affect the 10MHz CMRR measurement.
To limit this error it is usually sufficient to reduce the inductance of the reference loop by making a direct ground connection between the oscilloscope and the signal generator lead and making this auxiliary connection close to the input and output leads of the probe.
This will move the resonant frequency further to the right, away from the frequency under investigation.
In practice, this operation can be easily accomplished by simply connecting a probe between the oscilloscope and the output lead of the signal generator (generally this connection is unconsciously present since the amplitude of the input signal is measured...)

Remark / Important Note: in case of significant high frequency common mode components, the above precautions must also be adopted during the usual differential measurements (this to avoid to degrade the expected high frequency CMRR of the probe).

OK, enough with the premises!
Next time I will explain how to improve the CMRR above 100KHz.
Do not miss!!