Content originally posted in LPCWare by bthomas on Fri Apr 08 08:14:47 MST 2016
I have built a board with a LPC824. Amongst other things it utilizes 2 analog inputs, ADC_9 and ADC_10. ADC_9 is connected to a potmeter, ADC_10 is connected to an analog multiplexer (4051). The inputs of the analog multiplexer are 0-5V signals. As these exceed 3.3V, there is a voltage divider of 12k (upper) and 22k (lower) on each input.
The behaviour I observe is that, on the selected channel, a voltage appears when the channel is selected and without anything input to the divider. So to simplify this: ADC_10 has a resistor of 22k to ground and a voltage appears on the input of the ADC. I disabled the internal pull-ups and I am sure this works, as the voltage increased quite a lot when I enable the pull-ups. The voltage I measure is ~180mV. I did notice that the analog inputs have a quite low impedance of 100k, but I assume this 100k is to ground so I don't understand where the current comes from that creates this voltage drop over the 22k resistor.
Even a bit more strange, if I enable ADC_9, connected to the potmeter, the voltage on the resistor on ADC_10 changes with the position of the potmeter, thus with the voltage on ADC_9.
I saw messages here and elsewhere of people describing similar behavior but I have not seen any reply that was confirmed to be the solution.
Anyone has any idea?