I'm running an audio signal into an ADC on a teensy 3.1. The teensy then detects the first several harmonics of the signal. I had this project working on a breadboard for a good while and finally got around to putting it on some PCB. Since putting it on the PCB, the signals harmonics are detected properly only from ~1kHz to 2kHz (the program should work from 20hz to 2kHz).
The signal just runs from an audio jack to the ADC pin. I've blindly experimented with different values of termination resistors by touching leads to the exposed solder but haven't come up with anything too good.
The signal is originating from some synthesizer modules - a VCO patched to some modules which attenuate it and add an offset to condition it for the ADC. (I intend to house these signal conditioning things on-board in a future revision.)
When the signal is sourced from a waveform generator its harmonics are detected perfectly so the problem must be something to do with the VCO signal source.
Why is the signal only detectable over the range of 1000-2000Hz?
Is the problem diagnosable from the information given?
Is this a case where I must consider the output impedance of the source? If so, is there a way to design the input to the ADC so that it works with any source, no matter its output impedance? (opamp buffer?)
Are there standard practices for designing ADC input paths?