I don't understand LPC824 analog input behaviour

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 18, 2016 by lpcware
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?