EEVblog 1744 - NEW Micsig DP700 High Voltage Differential Probe

[EEVblog]

The DP700 is Micsig's third suck of the sav on a high voltage differential probe.

https://www.micsig.com/DPA/

00:00 - Comparison to the DP10007 and MDP700
03:58 - Unboxing of the DP700
05:52 - CMRR measurement hardware setup
09:52 - Sanity check gain and bandwidth check.
11:29 - Zero function
12:25 - Measuring the CMRR
16:30 - Trap for young players. RMS vs AC RMS, it's all in the sigma
17:44 - Another trap for young players, double sigma
19:07 - CMRR at 100MHz
22:41 - Real world test on a 100V GaN MOSFET
25:28 - Teardown
29:44 - PCB inspection

[Kleinstein]

If the measured CMR is so sensitive to touching one cable or the other, is there a need for better CMRR specs ?

For the CMRR measurement one would ne actually need the 50 ohms termination. Without the genrator would give more amplitude out.

A point that could help for the higher frequency CMRR could be a clip on ferrite on the cables (coax and power supply).

The nearly air tight case around the input resistors is only helping for a short time scale. Over time humidity would still find a way in.
It is still nice to avoid condensation when comming from cold to warm (and humid). It may be more a safety thing than actual precission.

[LooseJunkHater]

Was quite surprised that none of the part numbers were scratched off, as well as that the input op-amps have such a high input bias current of about 1uA. I was under the impression that most high bandwidth high voltage differential probes required significantly lower input bias currents (in the pA or fA range).

Hopefully someone reverse-engineers this probe in the near future and makes a schematic of it! Seems like it would make for a great DIY low-cost probe.

[ballsystemlord]

Hello,
I have a few questions.

I'm still confused, why are there two op-amps on the board?

Thanks!

[rob77]

Hello,
I have a few questions.

I'm still confused, why are there two op-amps on the board?

Thanks!

both have the same input and one is doing 10x gain compared to the other. then you switch between the outputs of the 2 when selecting gain on the probe.

[MTronic]

Seems like it would make for a great DIY low-cost probe.

The AD8130 seems to be quite popular for DIY probes. On this forum, as early as in 2015!, the AD8130 was used for a "lean" low cost DIY Diff probe here: https://www.eevblog.com/forum/projects/cheap-differential-probe-ad8130/.

@Dave Would be very interesting to many of us ... ... if you analyze the circuit for a YT vid, esp. in the context of op amp switch for the different attn/gain settings

[Kleinstein]

The AD8130 is still a rather odd choice for the probe.  It has quite high current noise and thus not well suited for a high impedance signal. It may require relatively large capacitors at the input divider.

The AD8129 as a 2nd stage amplifier for a gain of 10 could make some sense - both amplifers in parallel at the input side not so much.

[scopeman]

On the airtight case perhaps one should coat the gasket with silicone vacuum grease and also coat the resistors with an acryllic based conformal coating. That should fix that problem. Humiseal 1B73 comes to mind as it has a UV indicator in it.

I have used 1B73 in charge amplifier circuits with no issue as long as the board is squeeky clean. I only use it on boards with aqueous cleanable soldering flux and never use it
with a no-clean flux.

One of my PCB assemblers found out the no-clean means can't clean for circuitry that demands low leakage on the PCB. They had to send a few hundred PCB's out to a special
board cleaning service when they used the wrong solder paste.

Sam
W3OHM

[David Hess]

The nearly air tight case around the input resistors is only helping for a short time scale. Over time humidity would still find a way in.
It is still nice to avoid condensation when comming from cold to warm (and humid). It may be more a safety thing than actual precission.

I agree.  I know exactly what they are trying to solve, and it is not going to work; the o-rings are not going to provide hermetic sealing on an FR4 like substrate against atmospheric pumping so humidity is still going to affect the printed circuit board material.  The solution is to use either a better FR4 type of material, or a different substrate material entirely.

I doubt they needed to go to that much effort to preserve safety when nobody else has had to do that.  On the other hand, practically all of these modern inexpensive differential probes suffer from degraded common mode rejection as they age because the printed circuit board substrate is susceptible to humidity and worse.

On the airtight case perhaps one should coat the gasket with silicone vacuum grease and also coat the resistors with an acryllic based conformal coating. That should fix that problem. Humiseal 1B73 comes to mind as it has a UV indicator in it.

I have used 1B73 in charge amplifier circuits with no issue as long as the board is squeeky clean. I only use it on boards with aqueous cleanable soldering flux and never use it
with a no-clean flux.

One of my PCB assemblers found out the no-clean means can't clean for circuitry that demands low leakage on the PCB. They had to send a few hundred PCB's out to a special
board cleaning service when they used the wrong solder paste.

The problem is not leakage so much as changing dielectric properties of FR4 like printed circuit board materials, including printed circuit board hook.

[EEVblog]

The nearly air tight case around the input resistors is only helping for a short time scale. Over time humidity would still find a way in.
It is still nice to avoid condensation when comming from cold to warm (and humid). It may be more a safety thing than actual precission.

I agree.  I know exactly what they are trying to solve, and it is not going to work; the o-rings are not going to provide hermetic sealing on an FR4 like substrate against atmospheric pumping so humidity is still going to affect the printed circuit board material.  The solution is to use either a better FR4 type of material, or a different substrate material entirely.

Or just conformal coat it.

[David Hess]

The nearly air tight case around the input resistors is only helping for a short time scale. Over time humidity would still find a way in.
It is still nice to avoid condensation when comming from cold to warm (and humid). It may be more a safety thing than actual precission.

I agree.  I know exactly what they are trying to solve, and it is not going to work; the o-rings are not going to provide hermetic sealing on an FR4 like substrate against atmospheric pumping so humidity is still going to affect the printed circuit board material.  The solution is to use either a better FR4 type of material, or a different substrate material entirely.

Or just conformal coat it.

Conformal coating works well for surface leakage, but would have too much effect on the dielectric constant and would not stop atmospheric pumping into the bulk of FR4 types of fiberglass.  If it was a solution, then someone would have successfully used it to solve this problem long ago.

The easiest solution is to just choose the correct printed circuit board material.  I have often wondered if the less expensive materials made by Rogers would work, but never found anything online about it.  It would be interesting to make some layouts to test them for high impedance attenuator applications.

[EEVblog]

Conformal coating works well for surface leakage, but would have too much effect on the dielectric constant and would not stop atmospheric pumping into the bulk of FR4 types of fiberglass.  If it was a solution, then someone would have successfully used it to solve this problem long ago.

Can be gotten around by routing slots like they have already done. Then have very thin slivers to hold the series array together and then gunk it.
PCB material isn't a problem if there is no PCB material under the high impedance resistor to cause a problem. And the outside paths around them are so far out and thin.

[David Hess]

Conformal coating works well for surface leakage, but would have too much effect on the dielectric constant and would not stop atmospheric pumping into the bulk of FR4 types of fiberglass.  If it was a solution, then someone would have successfully used it to solve this problem long ago.

Can be gotten around by routing slots like they have already done. Then have very thin slivers to hold the series array together and then gunk it.
PCB material isn't a problem if there is no PCB material under the high impedance resistor to cause a problem. And the outside paths around them are so far out and thin.

I have never seen any examples where they did that to solve this problem.  I suspect it is just easier to use a better printed circuit board material, which is how they solved it decades ago.