I have a ~5V PMT output that has frequency components ranging from DC-30 kHz. I want to get rid of the frequency components from DC-10 KHz. So I am taking this PMT signal, terminating it with a 10 kOhm resistor and sending it through a 50 Ohm BNC into the ADAU1761-Z DSP to remove the <10 kHz components by a high pass filter. The output from the DSP is sent to a 1 MOhm terminated lock-in amplifier. I am using the lock-in amplifier to see the amount of voltage at frequencies of 1 Hz (DC), 5 kHz, and 25 kHz.
When I turn on the >10 kHz high pass filter in the DSP, I see that the signal (of interest) at 25 kHz is almost all reflected even if the high pass filter in the DSP is bypassed. This makes me think that there is an impedance mismatch coming in from the DSP pins. I am not sure what the input and output impedances are for the DSP.
Could you please confirm if you think I am using the wrong impedances somewhere ? The PMT output has to be terminated at 10 kOhm, and when I send it directly to the lock-in with 1 MOhm termination, things work just fine (althpough we have the DC-10 kHz frequency components which we want to get rid of).
Any comments or suggestions will be extremely helpful.