New low-cost ($170) 100MHz Differential scope probe from Micsig

[nctnico]

DC-DC part number is AH4V55V5S-1W, made by "SUCCEED"

Google can't find it.
Wonder how much isolation it provides ? Could it be the limiting component for this units voltage ratings ? 

Perhaps someone can post a picture to see if it has the right UL markings.

[wraper]

DC-DC part number is AH4V55V5S-1W, made by "SUCCEED"

Google can't find it.
Wonder how much isolation it provides ? Could it be the limiting component for this units voltage ratings ? 

Why would it matter a tiny bit. The only thing it isolates is 5V power from USB. Output to the scope is directly connected to the rest of the circuit.

[nctnico]

DC-DC part number is AH4V55V5S-1W, made by "SUCCEED"

Google can't find it.
Wonder how much isolation it provides ? Could it be the limiting component for this units voltage ratings ? 

Why would it matter a tiny bit. The only thing it isolates is 5V power from USB. Output to the scope is directly connected to the rest of the circuit.

After some thought I have to agree. IMHO the DC-DC converter is there to prevent ground loops and not to provide a safety isolation barrier.

[exe]

Why would it matter a tiny bit. The only thing it isolates is 5V power from USB. Output to the scope is directly connected to the rest of the circuit.

Wait-wait-wait, so this thing is not safe? Like, if I touch touch-screen on my scope I can get electrocuted? Or if I touch ground lead of another probe?

Also, the scope itself is only cat 1...

[alm]

The input impedance of the probes should be sufficiently high (>= 4 MOhm to common?) to limit the current through them to a safe level. But it is often recommended to make sure the scope is grounded when using a differential probe. Since the differential probe is not isolated, hooking both probes to a high potential could still pull the scope to the same high potential. Depending on the capacitance, that could be painful.

The opposite problem is if the scope floats too far from the device under test. Then you may exceed the maximum common mode voltage of the probes, distorting the signal. By tying the scope to ground, you make it more likely that it will be close(ish) to the device under test (if that is also ground referenced somehow).

I do not know what the Micsig manual says about ground connections. If you make sure the scope is grounded, then you should be able to safely touch the scope without fear of shocks.

[nidlaX]

The input impedance of the probes should be sufficiently high (>= 4 MOhm to common?) to limit the current through them to a safe level. But it is often recommended to make sure the scope is grounded when using a differential probe. Since the differential probe is not isolated, hooking both probes to a high potential could still pull the scope to the same high potential. Depending on the capacitance, that could be painful.

The opposite problem is if the scope floats too far from the device under test. Then you may exceed the maximum common mode voltage of the probes, distorting the signal. By tying the scope to ground, you make it more likely that it will be close(ish) to the device under test (if that is also ground referenced somehow).

I do not know what the Micsig manual says about ground connections. If you make sure the scope is grounded, then you should be able to safely touch the scope without fear of shocks.

Last I recall, the manual does ask the user to ensure the scope is properly grounded. Then again, all of their scopes can be battery powered, so I'm not sure how many people will be following those guidelines.

[exe]

Last I recall, the manual does ask the user to ensure the scope is properly grounded.

My scope has 1k resistance to ground when powered from mains. Is this a proper ground?

[nidlaX]

Last I recall, the manual does ask the user to ensure the scope is properly grounded.

My scope has 1k resistance to ground when powered from mains. Is this a proper ground?

Well, the Micsig scopes use a DC input, no direct chassis ground. Someone else can comment on safety in this situation. If you were using a traditional scope, it should have a direct connection between the chassis shell to earth.

[alm]

I think it should be sufficient in combination with the differential probe, since 1 kOhm is still low compared to the MOhm impedances from the probe.

I am not so convinced it is also safe with passive single-ended probes. If you clip the ground lead to say 230 V, it may trip the GFCI, but not every circuit is protected by a GFCI. The current of 230 mA will not be enough to blow a fuse. So the ground connection is not ensuring the scope is at a safe potential. This assumes that the connection to ground is purely resistive, Things might be better if there is an element that decreases in resistance at higher voltages (e.g. TVS, NTC). Maybe someone more familiar with these scopes can comment.

[David Hess]

If a differential probe is used with an isolated input or an oscilloscope with a missing ground connection, then the input or the oscilloscope if its ground connection is missing will charge to the common mode input voltage which can be considerable resulting in potential damage to the oscilloscope.  Tektronix includes a specific warning with their high voltage differential probe not to use it with an isolated oscilloscope input like on their TPS series.  Note that the attenuation of the probe make no difference since the attenuators go to the now isolated ground.

I suspect the isolated DC to DC converter is not there for safety but to prevent a ground loop between the oscilloscope's own USB connection and its vertical input and between any other USB power supply if used and the oscilloscope since they will likely share ground through the AC power connection.

[2N3055]

Hi, Just spoke with Telonic in the UK after purchasing a TO1104Z they are still waiting for there first delivery of the probes, the impression i got is that it is a Micsig probe.

https://www.telonic.co.uk/Micsig-DP10013-High-Voltage-Differential-Probe-p/dp10013.htm

At our Italian friends you have a special... You will get 2 for cca. 165 GBP ... Just saying... And they had it on stock, shipped same day.. 
Price for one is pretty much the same..
Just saying..

[boborich]

Anyone tested the bandwidth of the probe? I brought one and tested the bandwidth and I had a difficult time to get 100Mhz out it. My test rig is siglent SDG2122X(hacked from 2042) with sine wave set to 5V P-P without any DC offset, A MDO 3054 500Mhz scope(optioned from 3024). Hock the probe to the scope and set the scope input impedence to 1M without any bandwidth limit. Here is what I got:

At 50Mhz, I got 5.2V P-P.
At 70Mhz, I got 6.8V P-P.
At 80Mhz, I got 7.6V P-P.
At 90Mhz, I got 4.7V P-P.
At 100Mhz, I got 2.7V P-P.

There is a big peaking at 80Mhz starting from 60Mhz, and fall off at 9xMhz at -3db. I don't think this qualifies for a 100MHz probe.

Any thoughts? Am I test it the wrong way?

[TheSteve]

What does it look like if you probe a 50 ohm load on the output of the AWG?

[capt bullshot]

Anyone tested the bandwidth of the probe?
There is a big peaking at 80Mhz starting from 60Mhz, and fall off at 9xMhz at -3db. I don't think this qualifies for a 100MHz probe.

I've seen the same effect with some other 100MHz differential probe.
The root cause for this behaviour are the probes test leads, they are simply too long for a 100MHz bandwith. Try moving them around or twisting them, this heavily influences the frequency response above 60MHz.
IMO a diff probe claiming 100MHz BW but using such long test leads is just nonsense. The probe circuit itself would do a nice 100MHz -3dB rolloff, but using the non-removable leads spoils it all. Same thing as with the ground clip of an ordinary x10 probe, they tend to resonate around 100MHz.

[capt bullshot]

Well, the Micsig scopes use a DC input, no direct chassis ground. Someone else can comment on safety in this situation. If you were using a traditional scope, it should have a direct connection between the chassis shell to earth.

This is not a big deal, a "safety impedance" from the high voltage potential to touchable circuit ground is sufficient. Along with all the minimum distance and creepage requirements. The "safety impedance" is allowed to cross the isolation barrier and is part of it. AFAIR you can find this somewhere in IEC 61010 standards.

[wraper]

Anyone tested the bandwidth of the probe? I brought one and tested the bandwidth and I had a difficult time to get 100Mhz out it. My test rig is siglent SDG2122X(hacked from 2042) with sine wave set to 5V P-P without any DC offset, A MDO 3054 500Mhz scope(optioned from 3024). Hock the probe to the scope and set the scope input impedence to 1M without any bandwidth limit. Here is what I got:

At 50Mhz, I got 5.2V P-P.
At 70Mhz, I got 6.8V P-P.
At 80Mhz, I got 7.6V P-P.
At 90Mhz, I got 4.7V P-P.
At 100Mhz, I got 2.7V P-P.

There is a big peaking at 80Mhz starting from 60Mhz, and fall off at 9xMhz at -3db. I don't think this qualifies for a 100MHz probe.

Any thoughts? Am I test it the wrong way?

Did you twist the input leads as recommended in manual?

[nctnico]

Perhaps the approx 15pf input capacitance of the scope is also an issue.

[boborich]

I did more testing and here are the findings:
1. Twisted the lead make things worse at 100MHz, now the P-P value drops more to 1.6V when twisted.
2. Tried to increase the signal output from 5V P-P to 10V P-P, same issue, no improvement.
3. Load the arb gen with 50ohm, the peaking seems better but still the same trend at 100MHz, drop to the toilit.
4. Try using seprate USB power to the probe, no change.
5. Tried on a different scope (DZ1054 hacked to 100MHz), same issue.

My conclusion is this probe is really usable at 50MHz with the standard leads....

BR,

Richard

[IAmBack]

It would be interesting to see this kind of test with probes from different manufacturer, like Tek.

[tautech]

I did more testing and here are the findings:
1. Twisted the lead make things worse at 100MHz, now the P-P value drops more to 1.6V when twisted.
2. Tried to increase the signal output from 5V P-P to 10V P-P, same issue, no improvement.
3. Load the arb gen with 50ohm, the peaking seems better but still the same trend at 100MHz, drop to the toilit.
4. Try using seprate USB power to the probe, no change.
5. Tried on a different scope (DZ1054 hacked to 100MHz), same issue.

My conclusion is this probe is really usable at 50MHz with the standard leads....

BR,

Richard

We don't know if this differential probe is affecting the integrity of the AWG sine waveform as we have no parallel reference for your measurements.

What I suggest is the SDG be set to HiZ output so that the signal amplitude matches that on its display (still 50 source) and a passive probe be also used as reference on another channel.

As the SDG BNC shells are mains ground referenced and so are the scope inputs this setup will compromise a truly differential measurement but still may offer some insights to accuracy...........or not.

[nctnico]

I did more testing and here are the findings:
1. Twisted the lead make things worse at 100MHz, now the P-P value drops more to 1.6V when twisted.
My conclusion is this probe is really usable at 50MHz with the standard leads....

Retest with a 50 Ohm termination (preferably on a scope with real 50 Ohm inputs).

[boborich]

Tried on the scope end with 50, 75, or 1M termination, doesn't affect the result, only the amplitude changes.

MDO3000 has internal termination selections. Definately neither the cause nor the solution to the bandwidth.

[capt bullshot]

It would be interesting to see this kind of test with probes from different manufacturer, like Tek.

Not exactly the same kind of test. Look at the attached waveforms:
Both were simultaneously acquired at the very same test point, a real world signal (gate drive to a power MOSFET)

Red trace is one of these: http://www.tek.com/isolated-measurement-systems (the 500MHz BW version), connected by the provided probe accessory to the DUT (coaxial cable, shield to MOSFET source, signal was run through a SMB connector soldered directly to PCB)

Blue trace is a Yokogawa branded 100MHz diff probe, with the two attached test leads clipped to the source and gate pin of the MOSFET

The ringing visible at the lead-out of the pulse is at 39MHz, well within the specified BW of the diff probe. BW limiting the tek probe to 250MHz didn't change a thing, next available BW limit was 20MHz, the ringing was gone (as expected).

As some of use were puzzled by this view (a reprensentative of Tek did a in-house demo of the Isovue probe), I verified the result by measuring the pulse using one of our old TPS2024 scopes (these have fully isolated inputs) - the same 39MHz ringng is visible with the same amplitude as the Isovue probe. Next I checked the BW of the diff probe using a signal generator. The result was quite the same as seen here by boborich, clearly related to the long input leads. Together with the input capacitance of the diff probe they form all kinds of antennas, resonant circuits and whatever, grossly varying the system (sig gen output impedance, leads, diff probe input impedance) frequency response above 50MHz.

So your usable BW of such kind of a diff probe heavily depends on the sources impedance and the test leads. For a defined frequency response above some 10s of MHz you'll have to use something like the Isovue probes.

[IAmBack]

I did more testing and here are the findings:
1. Twisted the lead make things worse at 100MHz, now the P-P value drops more to 1.6V when twisted.
My conclusion is this probe is really usable at 50MHz with the standard leads....

Retest with a 50 Ohm termination (preferably on a scope with real 50 Ohm inputs).

I think this probe is not intended to work with 50 Ohm input.

[IAmBack]

It would be interesting to see this kind of test with probes from different manufacturer, like Tek.

Not exactly the same kind of test. Look at the attached waveforms:
Both were simultaneously acquired at the very same test point, a real world signal (gate drive to a power MOSFET)

Red trace is one of these: http://www.tek.com/isolated-measurement-systems (the 500MHz BW version), connected by the provided probe accessory to the DUT (coaxial cable, shield to MOSFET source, signal was run through a SMB connector soldered directly to PCB)

Blue trace is a Yokogawa branded 100MHz diff probe, with the two attached test leads clipped to the source and gate pin of the MOSFET

The ringing visible at the lead-out of the pulse is at 39MHz, well within the specified BW of the diff probe. BW limiting the tek probe to 250MHz didn't change a thing, next available BW limit was 20MHz, the ringing was gone (as expected).

As some of use were puzzled by this view (a reprensentative of Tek did a in-house demo of the Isovue probe), I verified the result by measuring the pulse using one of our old TPS2024 scopes (these have fully isolated inputs) - the same 39MHz ringng is visible with the same amplitude as the Isovue probe. Next I checked the BW of the diff probe using a signal generator. The result was quite the same as seen here by boborich, clearly related to the long input leads. Together with the input capacitance of the diff probe they form all kinds of antennas, resonant circuits and whatever, grossly varying the system (sig gen output impedance, leads, diff probe input impedance) frequency response above 50MHz.

So your usable BW of such kind of a diff probe heavily depends on the sources impedance and the test leads. For a defined frequency response above some 10s of MHz you'll have to use something like the Isovue probes.

Thank You. This kind of behavior is expected with this kind of leads.