High Input Impedance like an Oscilloscope. (1MOhm)


#1

Hello,
I got the OpenADC and I really enjoy the LNA. I Used the KNJN Flashy knjn.com/Flashy.html before and this is a bit limited. But the Flashy has one big Advantage: The 1MOhm impedance. So it shows the true signals and even weak signals.

I have to digitize sporadic impulses ranging from 0 to +12 Volts, but the Source cant give much current. So with the OpenADC the signals “falls” over time. I mean when a weak source puts a constant voltage, I get the a line parallel to the x-axis on the oscilloscope, but with the OpenADC I get a falling line.

I am using the LNA input and am interested on how to modify the OpenADC. Also on how to modify the OpenADC to accept +12 V as max voltage on the LNA input.

Thank you very much!


#2

Hello,

The best solution here will definitely be to build a simple op-amp to scale the input. This can have a simple voltage divider to scale the input, and would also provide a lower-impedance drive source!

The OpenADC trades off the better low-noise performance for a very limited input range! Anything larger than > 200mV or so will require you to externally use a voltage divider and possibly buffer amp.

Regards,

-Colin