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DIY oscilloscope (yet again)

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PMA:
I have been daydreaming about DIY oscilloscope for a while. Not for cost saving or better performance than commercial scopes, but mainly as learning experience.

The end game would be 8ch scope with modest bandwidth (10-50MHZ), but I will start with something very simple.

As a first step I will buy PYNQ-Z2 and design very basic 1ch front end that can be connected to the board. It will probably keep me busy for a while.

I have been reading related articles and forum posts in past days, like:
https://www.eevblog.com/forum/projects/diy-oscilloscope-front-end-queries/
https://reference.digilentinc.com/reference/instrumentation/analog-discovery-2/reference-manual

-What is the best approach for the front end if I want to optimize simplicity/part count, but still keep costs in check? Should I copy the design from the discovery 2 or is there more integrated solutions available with a bit higher price?
-Is there something wrong with my approach in general?

Yansi:
For 10-50Mhz, do the front end the good old fashioned way:  JFET buffer with DC servo.    Such frontend can be pushed even in the hundreds MHz region with good success rate.
Then you place any ADC with integrated VGA (PGA) or an external VGA (PGA) in front of the ADC - whatever suits best.

Principles briefly mentioned for example in this datasheet, starting page 34: http://www.ti.com/lit/ds/symlink/lmh6518.pdf

//EDIT: Typos.

Boscoe:
If you’re happy with a reduced input range, say +/-10V then it makes your life a lot easier. Couple this with a x10 probe and you’ve got yourself some thing that will cover 95% of use cases. I don’t think I’ve ever looked at something more than 100V but I don’t design SMPSs.

The difficulty in front end designs is getting the attenuation at the input with high input impedance with good bandwidth. Remove the need for the attenuator and your done.

PMA:

--- Quote from: Boscoe on November 13, 2019, 09:22:26 pm ---If you’re happy with a reduced input range, say +/-10V then it makes your life a lot easier. Couple this with a x10 probe and you’ve got yourself some thing that will cover 95% of use cases. I don’t think I’ve ever looked at something more than 100V but I don’t design SMPSs.

The difficulty in front end designs is getting the attenuation at the input with high input impedance with good bandwidth. Remove the need for the attenuator and your done.

--- End quote ---

That is definitely something to think about. My use case would be mostly battery operated motor controllers, so supply voltage won't be more than 48V, but it would be nice if somebody else could use my design as well.

I have to take deep dive into front ends, but at the moment it sounds a good idea to keep things simple and go with +/- 10V range and old school design for the front end (+PGA which may not be so old school).

Yansi:
If you go down the discrete frontend route, limiting yourself to 10V is dumb limitation. You just need a simple compensated attenuator. Nothing what couldn't be done with a couple of passives and a small cheap signal relay (for example Omron G6K-2F-Y). Also briefly discussed withing the PDF I have linked.

Look into how cheap DSOs (Riglents and Hanteks) have frontends made. There are numerous reverse engineered schematics floating round the web. Almost always this very same topology or thereof.

Look there for a tip how for example AC coupling mode is accomplished - as this is not discussed within the PDF I have linked. Can be seen well for example from a Rigol DS1054 reverse engineered schematics.  (AC coupling of the DC servo, NOT the input path)

I do not see any issue why it couldn't be possible to design and tune the frontend for 20MHz operation.  Couple years back I have experimented with this out of curiosity. Ugly build of that circuit on a piece of copper clad using just THT components worked well beyond 20MHz without much tuning.  (I just got issues with a bit different gains of the DC and AC paths - but now I know why and could improve it).

Just make a couple of prototype boards. Nothing more needed for debugging other then a decent ARB generator (sine/square) and an oscilloscope.

For the VGA, you could use for example AD8330. (DC to 150 or so MHz VGA). Using a VGA instead of PGA gives a nice ability to self-calibrate gain steps. With PGA, you are stuck with the frontend (and PGA) accuraccy without much possibility to cal it out. (Multiplying the ADC output is not so great idea, but might work as well).

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