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Floating probe! For $2.50
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JS:
I'll run a sim now to define a full circuit to take to the breadboard. I guess some discrete transistor should be used to take those 6µA up.

The fast response might make it suitable to work inside a feedback loop as well, but the differential approach also makes sense to retrieve higher signal at the output, many things to check now, thanks for the idea!

JS
StillTrying:
"I guess some discrete transistor should be used to take those 6µA up."

I'd just view the +/- 80mV across a 5k1 or 5k6 resistor for now myself. You're going to need at least some resistance from the photo diodes mid point to 0V because a 0.5uA difference in the PD quiescent currents would make that point go to near one of the rails.
It's a good place for a CFA amp, but I've not had a massive amount of luck on a breadboard where every pF at the summing point counts.

"the differential approach also makes sense to retrieve higher signal at the output"

I used 2 optos working in anti-phase to hopefully cancel their non linearity between 2mA-12mA, of course I've no idea how well it might work in practice. :)

If I swap the sim to use the photo transistors rather than the base-collector photo diodes the BW is only 12kHz.
JS:

--- Quote from: StillTrying on September 11, 2018, 09:30:59 pm ---"I guess some discrete transistor should be used to take those 6µA up."

I'd just view the +/- 80mV across a 5k1 or 5k6 resistor for now myself. You're going to need at least some resistance from the photo diodes mid point to 0V because a 0.5uA difference in the PD quiescent currents would make that point go to near one of the rails.
It's a good place for a CFA amp, but I've not had a massive amount of luck on a breadboard where every pF at the summing point counts.

"the differential approach also makes sense to retrieve higher signal at the output"

I used 2 optos working in anti-phase to hopefully cancel their non linearity between 2mA-12mA, of course I've no idea how well it might work in practice. :)

If I swap the sim to use the photo transistors rather than the base-collector photo diodes the BW is only 12kHz.

--- End quote ---

  Well, I made it, I came up with this circuit which simulates nicely, ~600kHz BW, good linearity, decent step response and all that. I leave the schematic here.

  I started with another approach which didn't polarized the output of the optocouplers properly, on the sim seemed all great but it worked only up to 40kHz or so in real life.
  Once on the breadboard I started messing around and came up with the circuit in the schematic, which simulates up to 600kHz but I can only observe a bit over 200kHz in the breadboard. The good thing is that the step response is much better than the one in the previous 200kHz circuit, no ringing at all. Changing R6 and R12 from 10k to 1k takes the BW up to 250kHz in real life, to 2MHz in the simulation. Rise time still at about 1.2µs. I'm observing this response at the output of the optocouplers, so the bandwidth limit is there, didn't improved much using it as diodes, next test could be a dual virtual earth on the output, try with diodes or transistor output connection. I don't know if changing Q4 and Q5 for jfets will make any difference. I also leave a capture of the step response for your joy!

  Thanks for all the help and I listen to any ideas of how to take the BW even further, I'm liking the open loop approach, much cleaner step response and seems it can be pushed a bit more. I'm happy with it as it is so this will end in a PCB inside a box, but the more BW the better!  :-/O

JS
Gyro:
Nice iteration.  :-+

If the bandwidth that you are seeing is still at the outputs of the optocouplers, then you could go ahead and box it with them socketed and then replace them with faster ones when you can obtain them as they are pretty much all pin compatible.

There is a circuit floating around somewhere on the web that splits the LF and HF frequency components, the DC/LF being handled by a linear opto (an IL300 I assume). The HF portion is coupled by a very low value, high isolation voltage capacitor coupled circuit. Both are then re-combined to produce a high fidelity waveform. I though I had seen the circuit on either the EDN or Electronic Design publications' websites but haven't been able to locate it again yet.

Of course this combined approach would reduce the CMRR that your purely opto approach gives you.
JS:

--- Quote from: Gyro on September 12, 2018, 07:00:01 pm ---Nice iteration.  :-+

If the bandwidth that you are seeing is still at the outputs of the optocouplers, then you could go ahead and box it with them socketed and then replace them with faster ones when you can obtain them as they are pretty much all pin compatible.

There is a circuit floating around somewhere on the web that splits the LF and HF frequency components, the DC/LF being handled by a linear opto (an IL300 I assume). The HF portion is coupled by a very low value, high isolation voltage capacitor coupled circuit. Both are then re-combined to produce a high fidelity waveform. I though I had seen the circuit on either the EDN or Electronic Design publications' websites but haven't been able to locate it again yet.

Of course this combined approach would reduce the CMRR that your purely opto approach gives you.

--- End quote ---
The tek uses a transformer for the HF, to get from a few kHz upto 20MHz, I haven't seen a cap approach.

CMRR is a major factor for this amplifiers, the problem for HF is the capacitance between all the input amp and the rest of the world, hence the optic fiber coupling seen in Dave's vid lately. You don't need that separation for 1kV insulation but the higher the capacitance the lower the CMRR and the higher the load for fast common mode signals as they were measuring at the gate driver.

JS

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