Short version: Why is C3 necessary in this circuit?
The schematic for the input buffer/amplifier circuit for xoscope is made up of subsections that are mostly broken down in the descriptions on the site:
- C1: AC coupling, to remove DC component from signals
- In notes, the author suggests moving this to after the impedance stage
- R1, C2: Impedance, to avoid impacting the operation of the circuit under test
- R2, D1, D2: Clamp to +12.7V, -12V, to keep input within spec limits of amp
- R3, D3: -11.3V reference for clamp to avoid dropping below common mode limit of amp
- R5, R4, S1: Switchable 1x/10x feedback divider
- R6: Output amplitude trim
I haven't found an explanation, however, for C3, the 100pF across R2. The circuit this derives from (The Art of Electronics, figure 4.74) does exactly the same thing, once again with no explanation.
Apparently the utility ought to be obvious, but I'm new to earnest analog electronics. I can only assume that it's stabilizing something, but I don't know how the operation would suffer without it.