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

[Hydron]

Yep, had the same issue with one of mine, just started doing it when I got it out of the drawer one day. Annoyingly it's the one that I did some hacking on (shortened the leads to get rid of some peaking) so no chance of a RMA, even if it were within warranty.

I did a bit of a bodge to improve it by figuring out how the offset circuit worked and padding one of the reference resistors. Can't remember much more detail than that but very annoying to have to do this - given others have the same problem would no longer recommend these even given the cheap price.

[EEVblog]

FYI, just got in new stock of my HVP70 probe.

[boB]

Yep, had the same issue with one of mine, just started doing it when I got it out of the drawer one day. Annoyingly it's the one that I did some hacking on (shortened the leads to get rid of some peaking) so no chance of a RMA, even if it were within warranty.

I did a bit of a bodge to improve it by figuring out how the offset circuit worked and padding one of the reference resistors. Can't remember much more detail than that but very annoying to have to do this - given others have the same problem would no longer recommend these even given the cheap price.

Interesting !   Did you find that the micro added in the anti-offset into that 3rd and final op-amp ?

I was trying to add on to the Dave-Cad schematic but the one that I was just working on appears to be slightly different.  For instance, the main differential amplifiers are in a single chip.  Then the 3rd op-amp  appears to be a dual OPMI labeled TI amp.  AND, I cannot seem to trace the 50 Ohm output to that 3rd op-amp.  Not in the short amount of time I spent on it anyway.

Answer to the question, how much offset...  It varies.  On this latest probe, changing the 500X to 50X does not appear to change the absolute output offset.   The probe is still useful though.  Just have to offset the scope and try to remember where zero is if I am measuring DC.
It also can be worse and maybe even better at different times when letting it sit.   But it is always enough to be annoying.

 I think that it is offset enough where the processor cannot quite put it right again.

Dave, is your  HVP70 probe  the one with 100X that you showed in your video ?    That is a nice compromise.  Hopefully they have gotten the CMR fixed as promised.

boB

[2N3055]

Still nobody mentions any numbers... How much offset do you people see?

[boB]

I will measure it again and give the absolute number.  I can't remember.

The power supply for the op-amps is basically +/- 9.x volts

[Hydron]

Enough to be very annoying for sure, cannot remember though and like I said I've mostly fixed it.

[boB]

I measure it today and the absolute output offset is only  +50 mV

At 500X, this turns out to be + 25V offset so if I could just calibrate this offset out of the scope itself (Rigol) that would
be OK. 

At +/-  9V op-amp supply, this should still work OK for most things.

Still annoying but now that I know there is that little isolated supply powers things with a lot more than just +5V, it's not all that bad.

The schematic for this particular DP10013_V3X  2017.11.10  is somewhat different than the most recent Dave-Cad version though with the single input stage opamps.   For instance, the output R of 50 Ohms is about 1 K Ohm or more away from any of the third dual op-amp output pins so I don't understand that.

Just checked another DP10013 and it HAD +50mV or so but calibrated down to about +2 mV.

PS...   NOW if I could only find the point where the micro normally injects its offset, I could make it work OK.

Actually, I suppose I could just add a trimpot circuit into that 3rd differential op-amp circuit.  I should probably figure out why I am seeing >1K Ohm between main probe output and that op-amp though.

boB

[Hydron]

Had a chance to look inside the probe I fixed and took a pic of the resistor padding I had to install. Can't find the reverse engineered schematic I drew sorry!
Fix seems to work well - am getting a sub-millivolt offset (before multiplication factor), but it looks like I had to use a few resistors to do it, in 2 places.

Attachments are the fix, plus front/back of the board I have (pre-fix).

[2N3055]

I just tested mine.

They generally have arround  100uV of nonscaled offset and arround 500uV RMS of noise (with 20MHz limiter on scope) at 50X.
At 500X they have arround  100uV of nonscaled offset and arround 180uV RMS of noise (with 20MHz limiter on scope)

That is nonscaled for probe factor.

[boB]

Had a chance to look inside the probe I fixed and took a pic of the resistor padding I had to install. Can't find the reverse engineered schematic I drew sorry!
Fix seems to work well - am getting a sub-millivolt offset (before multiplication factor), but it looks like I had to use a few resistors to do it, in 2 places.

Attachments are the fix, plus front/back of the board I have (pre-fix).

Thanks !

So you changed R60 and R64 to fix the offset ?

Not sure what's been done on the back of the PCB but will ZOOM in

boB