What is the challenge in scope design is that you need to protect the ADC and often user's life!
So the scope has 1Mohm || 15pF input impedance. You need to buffer it. So you first have to attenuate the signal by tapping this input impedance e.g. at 1/10, attenuating the signal. Then you selectively boost it back up for the ADC.
Or you selectively tap it at different ratios.
In any case, you have to protect whatever there is after the tap(s) by diodes that inherently bring parasitic capacitance.
Some scopes avoid expensive buffer ICs and go with split DC path (with gain) using opamp and then AC path (with gain) using e.g. JFET and BJT RF amps and combine those later.
The whole path from input to ADC must have flat frequency response in both magnitude and phase on all gain settings. This is non-trivial, especially with split DC/AC paths.
Sure, HMCAD ADC series help immensely nowadays with their builtin gain, but you still have to give them something flat to digitize as they output 8b streams and thus you won't be able to "fix it up digitally".
And then you also have to be able to inject bias to move the signal up/down.
And some scopes now can toggle between this and just 50 ohm impedance.
So, yeah, it's kinda non-trivial to condition signal somewhere between millivolts and mains to get to the ADC safely.
What is the challenge in scope design is that you need to protect the ADC and often user's life!
So the scope has 1Mohm || 15pF input impedance. You need to buffer it. So you first have to attenuate the signal by tapping this input impedance e.g. at 1/10, attenuating the signal. Then you selectively boost it back up for the ADC.
Or you selectively tap it at different ratios.
In any case, you have to protect whatever there is after the tap(s) by diodes that inherently bring parasitic capacitance.
Some scopes avoid expensive buffer ICs and go with split DC path (with gain) using opamp and then AC path (with gain) using e.g. JFET and BJT RF amps and combine those later.
The whole path from input to ADC must have flat frequency response in both magnitude and phase on all gain settings. This is non-trivial, especially with split DC/AC paths.
Sure, HMCAD ADC series help immensely nowadays with their builtin gain, but you still have to give them something flat to digitize as they output 8b streams and thus you won't be able to "fix it up digitally".
And then you also have to be able to inject bias to move the signal up/down.
And some scopes now can toggle between this and just 50 ohm impedance.
So, yeah, it's kinda non-trivial to condition signal somewhere between millivolts and mains to get to the ADC safely.