problem with ADC...

[lpcware]

Content originally posted in LPCWare by karuna on Sat May 28 02:08:08 MST 2011
I am using lpc1769 controller. While reading ADC value repeatedly in a channel, few times I get "0" or "4095" which is not the actual value. This happens even when the input is not varied. Why this happens?
  How to solve this problem?
  
  using ADC at  2.5MHz..
  
  Thanks in advance

[lpcware]

Content originally posted in LPCWare by Rob65 on Sat May 28 10:52:29 MST 2011
The 0 or 4095 values don't look like an ADC noise problem but more a software issue.

Still, Luis is right in suspecting electrical noise problems, I would almost say he is an expert by experience ... - have a look at his thread and take special attention to the two app.notes referred to byNXP. Have real separate analog and digital power domins with a star grounding if you need proper ADC measurements.
Share your PCB layout and/or projects. Unfortunately I do not yet have a 17xx design to test this (still working on the PCB layout) but others do.

Rob

[lpcware]

Content originally posted in LPCWare by Luis Digital on Sat May 28 06:16:59 MST 2011
Hello,

It seems a classic case of electrical noise. See this thread for tips, and a case study of improvement.

I would like to see a picture of your hardware. Today is Saturday. :)

[lpcware]

Content originally posted in LPCWare by Ex-Zero on Sat May 28 04:11:59 MST 2011
That's easy:[INDENT]Scope your ADC input (and if it's your own board also VREFP/VREFN). If there are no 0 or 4096 signal your software is faulty :mad:


[/INDENT]Note: I hope 2.5MHz is your clock frequency and not your conversion frequency :)