I want to play with a custom DSO design:
- sub 1MSPS
- Mostly measuring 0-6V signals
- Some audio signals
- Power line signals
The hardest thing for me is the power line signals. Sometimes I just want to look at the power line itself (full 250V range) and sometimes look at the power line communications signal right after the zero crossing.
I looked at the talk on How does the input circuit on this oscilloscope manage to accept such a large range of voltages? and understand the concept of the voltage divider and using that to start with.
I also get the concept of varying the gain on the opamp stage via software analog multiplexers and resistor networks.
Looking at the design for the Seeedstudio DSO (http://www.seeedstudio.com/wiki/File:DSO_Quad_Schematic.zip) I can see the multirange stage that controls the opamp gain.
What I can't understand is how the input range is 80Vpp (x1 probe). Their design is above my head. I can't see how they are clamping the voltage that goes into the opamp.
If I want the opamp to be protected for up to 250V no matter what range is configured I figured my only option is:
- Put the proper voltage divider in front of the opamp
- Have a configurable gain
- Clamp the output of the opamp to protect the ADC
My concern is that doing it only on the gain may introduce noise or unanticipated signal degradation issues. If possible I would like to also put the input signal through different paths to control its attenuation and filtering, but I need to protect the opamp clamping its input no matter what.
Is there a resource that explains this kind of input stages?