EEVblog 1415 - Reverse Engineering the DP10007 Differential Probe

[Just_another_Dave]

Yesterday I had a bit of free time, so I build a prototype of the tool for switching between both photos mentioned in the video on reverse engineering the probe. It doesn’t support marking the components yet and the web version has a bug that impedes loading photos (Godot doesn’t provide access to the native file system and its filedialogue component does not work properly in html5 exports).

Although I’m planing releasing the source code when the application is completed, I’ve uploaded an early prototype to itchio just in case someone is interested in trying it: https://just-another-dave.itch.io/reverse-engineering-helper

Edit: corrected a typo

I’ve just seen that it has been mentioned in the eevblog Twitter account, so I wanted to thank you for giving it visibility. I’m currently working on solving the known bugs, as well as implementing a system to allow adding marks. Thankfully, one of the bugs has been solved in a great open source tool called pixelorama, so the web version will probably work in the near future

[David Hess]

H-bridge driver for a latched relay. One reason for having a latched relay is signal integrity. An energized coil leaks enough magnetic field that his has an impact on the signal.
An ac signal traveling through an magnetic field sees a complex impedance. This throws a spanner in the gain flatness if that relay is used to switch gain settings. The bandwidth will change with energized/non energized state. This behavior is frequency dependent too.
i found this out the hard way when making test equipment for ADSL. even though the signals there are only up to the 1.3 MHz range, traveling through an energized relay was enough to have a few kilobit impact on the line. So i switched to dual coil relays (set-rest type) and the problem went away.
when i left one of the coils energized you could clearly see the drop.

In the 1970s Tektronix was making their own relays, and selling them to others, for use up to at least 100 MHz in low and high impedance circuits which did not have that problem, but later they used latching relays if only for power conservation.  The distinguishing thing about their relays at the time was that the contacts were located at the bottom of the frame limiting wire length and capacitance, which apparently became a design feature in later "telecom" style relays.

Why are they using a relay in the first place? The gain switching happens at a low-impedance node behind the differential stage. An analog CMOS switch would be perfectly fine for the job at less power, size and less cost...

It is difficult to get sufficient frequency and phase flatness out of CMOS switches, which raises the question about how solid state channel switches were designed in the past without relays.  They used current switching instead of voltage switching which works fine with bipolar transistors or diodes.

[oliv3r]

I really like how Marco of https://diystuff.nl/tradfri/tradfri-zigbee-light-link-module/ reverse engineered the IKEA tradfri module (or rather I really like the end-result



I asked him once how he did it, but sadly it's mostly just manual work

Without fancy tools, what I do, I load up various photo's in separate layers (I use the GIMP) and then use the opacity slider in the layer toolbox, pretty much as to what Dave suggested. A snapshot of 50% looks like .

When adding the layers, getting orientation, rotation etc can all be adjusted (requires a beefy CPU too :p) so yes getting everything ideaal is best when taking the photo's, but you can fix quite a lot actually with the '3D transform' I think, as there are anchor points that can be used for alignment (vias for example).

Even adding an X-Ray layer can be useful but then you really need to do the outline thing first (hadn't done that bit yet ...) and again, doing fades on multiple layers in the gimp is easy enough to highlight the things you care about ...

Following that I trace out (either through some image tricks with outline selections n stuff or even manually) the traces in separate colors, to try to get the same result Marco has. Mine where never as pretty My feeble attempt in the attachment ...

Anyway, some food for thought ...

(sorry for the oversized pic there)

[Smokey]

...
edit: good write up here: https://circuitcellar.com/research-design-hub/high-voltage-differential-probe/
they used 1M 0.1%/10pF, and 1GHz opamps. The bandwidth goal was 25MHz but unsure of the end result.

Did anyone actually build this circuit?  I wonder how well it works as written.

[David Hess]

...
edit: good write up here: https://circuitcellar.com/research-design-hub/high-voltage-differential-probe/
they used 1M 0.1%/10pF, and 1GHz opamps. The bandwidth goal was 25MHz but unsure of the end result.

Did anyone actually build this circuit?  I wonder how well it works as written.

That topology will work at lower frequencies.  At higher frequencies common mode rejection of the operational amplifiers will limit performance.  Overload recovery will be horrible.  Unfortunately no measurements of the performance are shown.

I wonder why they did not use a simpler design; it would have been higher performance.