Guru
Joined: 05/03/2018
Location: NetherlandsPosts: 6025
| Posted: 07:32am 29 Sep 2022 |
I am not sure OA47 is going to build an oscilloscope, so the capacitor may not be needed.
About the capacitor: If you want to measure 50/60Hz or DC it is not needed.
The capacitor is needed to balance against the input capacitance of the pico ADC, and the impedance of the circuit board. The balance (ratio) should be identical to the ratio of the resistor divider.
Since the resistor divider is switching, you will have to switch the capacitance also.
What they do in oscilloscopes is that they add capacitors everwhere. Practical values in your case would be (look at the schematics OA47 provides).
If C1 = 15pF, and the input capacitance of the pico and board layout is 7pF (estimate)
Capacitor parallel to R1 is 22pF (1M R1/1M R2 attenuator)
Capacitor parallel to R3 is 200pF (2x100pF) (1M R1/100k R3 attenuator)
Capacitor parallel to R4 is 2200pF (1M R1/10k R4 attenuator)
But again, for DC and 50Hz this is not needed.