Negative ADC voltage affecting other ADC channels

Discussion created by lpcware Employee on Jun 15, 2016
Content originally posted in LPCWare by VigneshEP on Mon Aug 25 04:14:44 MST 2014
I'm facing a peculiar issue.

  [*]In my application, channel AD2 (PIO1.1) is driven by a source with 50 kOhms impedance. It will always see a voltage between 0 and 3V3
  [*]AD6 (PIO1.10) can get negative voltage, but not negative than a diode drop below 0 V. This is driven by an impedance of 2 kOhms
  [*]While AD6 sees -100 mV (negative 100 mV), measurement of AD2 is affected probably by same amount

I could see the voltage on AD2 going low whenever AD6 is seeing a negative voltage. If I set A/D bit of IOCON of PIO1.10 as digital (so that analog mux is decoupled from the pin), then there is no 'cross talk'. I haven't checked whether the cross talk exists between any other channel.

Has anyone observed this? It is not mentioned in errata as well. I'm using LPC1114FBD48/323.