I designed a board to mux 16 inputs into a 4 channel scope using relays. I also included circuitry to replicate a 10x probe on the board. That is, I placed a 9M ohm resistor in parallel with a 1.4pF variable cap. These values came from a 9:1 ratio with the oscilloscopes internal 1M ohm resistor and a 13pF cap. From there, a BNC cable connects the board and the scope.
Here is a schematic:
PV1, PG1, PI1, PV2, PG2, PI2 are all attached to 4 separate normally open relay contacts.
When I went to tune the capacitors for a 1kHz square wave, the wave was completely scewed and no cap value from .9pF to 9pF would fix it. To my surprise I realized that the wave looked the best at the upper value (around 10pF) instead of near 1.4pF like I had expected. I replaced it with a 22pF cap out of curiosity and the square wave looked quite a bit better - not perfect - but also has a slight DC offset.
Any ideas what is going on here? Is the problem with the BNC cable, or possibly inductance / capacitance of the circuit. How should I have designed this in the first place?
FYI: There is no ground plane because each have slightly different grounds. Also, I injected the square wave directly on the cap, with ground connected to the BNC ground. I get the same result when injecting from anywhere else.
Here is some waveforms on the scope. From top to bottom: No cap installed, around 10pF, and the bottom is 22pF.
Update: Thanks for the answers. Someone suggested I use a circuit such as below to create a 100x probe. Would this work better?