I have been reading up a lot on how oscilloscopes work and how to best attenuate signals and figured the best way to learn is to create my own simple oscilloscope. My design goals are quite modest compared to an actual product but I aim for accuracy and quality as well as understanding.
DC - 1MHz bandwidth
1Mohm and <= 25pF input impedance
Settings electronically controlled
+/- 10% variance from input Z (Less is better but absolutely no more!)
I have selected Omron relays High End / Low End as they have great ratings and are somewhat reasonable in price. These will select AC/DC input and choose between the attenuation levels. I chose double throw relays as I will put the attenuation network in series so the fail safe mode is max attenuation and you will opt out attenuation networks by powering a relay and completely isolating the attenuator branch. (DPST on each side of the filter).
The main issue I am having is that I can not get a remotely stable input Z across DC - 1MHz, in fact I have 80.6% variance at some freq which make my setup useless. I have considered using a JFET input buffer instead of Diodes for input voltage protection but even before the protection stage my Z is all over the place.
Can anybody give me a crash course or some pointers on how the heck you attain a stable input impedance across frequencies??