ADC on LPC1768

[lpcware]

Content originally posted in LPCWare by paulantony49 on Tue Dec 30 04:37:11 MST 2014
I am using the 12bit ADC of the LPC1768 for sensor data accusation and naturally require very high reliability. On testing the same using a LPC1768 xplorer board, the results shows that there is a chance of error in every bit. On taking 8000 samples the following probability of error was observed.
err_pb = 0.0135    0.0265    0.0300    0.0289    0.0296    0.0331    0.0441    0.1571    0.1734    0.2404    0.4808    0.4325

The error in the last two bit is understandable (ground noise), but i am puzzled by the error in the higher bits. I tried it on both burst mode and the software controlled mode, with different frequencies of conversion. Still the error in the higher bit remains.

I would be grateful if someone could advice me on the source of error, whether the problem is only for LPC1768 xplorer or LPC1768 or in the programing.

[lpcware]

Content originally posted in LPCWare by LpcWare159 on Thu Jan 08 23:11:19 MST 2015
Did you add a small capacitor (range 10nF) next to the ADC input pin to help the sample&hold circuit?

[lpcware]

Content originally posted in LPCWare by jayudhandha on Thu Jan 08 22:20:47 MST 2015
I think the main reason is that you should make ground of ADC and Microcontroller common.

Is it done by you?

[lpcware]

Content originally posted in LPCWare by R2D2 on Tue Dec 30 05:17:58 MST 2014

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Quote: paulantony49
I would be grateful if someone could advice me on the source of error...

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See: http://www.nxp.com/documents/application_note/AN10974.pdf

IIRC this NGX board isn't isolating VDDA and VREFP...

Keil boards show how this can be done:

http://www.keil.com/mcb1700/mcb1700-schematics.pdf

[lpcware]

Content originally posted in LPCWare by 1234567890 on Tue Dec 30 05:02:26 MST 2014
What does 'err_pb' describe exactly???

There are so many possibilities:

- Stable supply voltage for the chip
- Stable reference voltage. And what source?
- Impedance of the measured voltage source
- Capacitance of the measured circuit
- High frequency switching nearby
- High currents nearby
- ...