Hi Jeffrey,
First of all, I would like to thank you and your enginner for the support.
I have done testing the 2 approaches you mentioned.
1) Measure something without IEPE.
The same spikes exist. I have set the IEPE disabled in the python code, and shorted CH0+ and CH0- for the input signal.
2) Add additional capacitance to the 5V net right at the Raspberry Pi.
I still see the same spikes. I tried several configurations below:
C = 47uF, IEPE = disabled, Input = CH0+/- shorted, Power supply = microUSB port
C = 47uF, IEPE = enabled, Input = PCB, Power supply = Direct to RPi GPIO 5V and GND pins
C = 100uF, IEPE = enabled, Input = PCB, Power supply = microUSB port
C = 147uF, IEPE = enabled, Input = PCB, Power supply = microUSB port
C = 194uF, IEPE = enabled, Input = PCB, Power supply = miicroUSB port
C = 194uF, IEPE = enabled, Input = PCB, Power supply = Direct to RPi GPIO 5V and GND pins
I realized that some spikes show the periodic behavior, and found it is about 1 Hz or 1 second cycle.
I have tested with sampling rate 25.6kHz (first fig) and 12.8kHz (second fig), and both show 1 second cycle.
The buffer size is fixed to 100k samles. Does this help you to narrow down the possiblity?
Regards,
Tatsuro