I am using the ADC Pi plus board as a field data logger.
Power supply is a UBEC 5V@3A (from Adafruit) and the raspberry pi (B+) has a USB modem for data transfer.
I recently found that I get lower readings from the ADC Pi plus when the modem connects to internet. It seems that it is because of the increased load on USB-ports and thereby slightly lower ouput voltage from the UBEC.
I thought the onboard voltage reference would make the readings independent of power supply voltage but I must have misunderstood how this works.
Here is a plot showing a simple test:
adc2 - the power supply from UBEC (with external voltage divider)
adc3 - external constant voltage generator
not shown in plot:
adc1 - external power (pre UBEC)
adc4-adc8 - connected to ground
How can I get stable readings from external signals, independently of smaller variations in the power supply?