ADC Timings

[lpcware]

Content originally posted in LPCWare by peufeu on Sun May 29 00:10:38 MST 2011
Hello !

I'm using a LPCXpresso with LPC1114, I have experience with AVRs but I'm a bit of a newbie with LPC111x...

The microcontroller will control a circuit which sends a pulse of current through a device under test. I would like to use the ADC to sample the voltage when this current pulse is active, allowing for settling time. I would also like the pulse to be as short as possible, and the measurement to be taken as soon as possible (but after settling time of course)

I'll use one of the PWMs to control the pulse and another PWM to synchronize the ADC.

On AVR microcontrollers, the ADC doesn't sample the input when you start the ADC conversion, but a few ADC clock cycles later. Also on AVR it is recommended to wait a little after switching channels (ADMUX) before performing a conversion.

I wonder if it's the same with LPC1114 ? The datasheet (in ADC chapter) doesn't say much...

Anyone knows when the ADC sampling window occurs relative to the ADC trigger ?

Thanks guys :D

[lpcware]

Content originally posted in LPCWare by Rob65 on Mon May 30 00:49:14 MST 2011
Maybe this thread helps, but I am actually still waiting for a solid answer as to what the real truth is.

Keep us informed, I'm anxiously awaiting your results :D

Cheers,

Rob

[lpcware]

Content originally posted in LPCWare by peufeu on Mon May 30 00:33:20 MST 2011
Well, i'd rather avoid extra parts.

The AVR's ADC did a similar job just fine, so the much faster LPC1114 ADC should be able to do it without external parts...

It isn't nanosecond timing ! 1 µs pulse length would be fine. Since the ADC clock is something like 4.5 MHz and the entire conversion takes around 2.5 µs, I guess the sampling window should easily fit in 1 µs...

I'll just experiment and see how that works out.

[lpcware]

Content originally posted in LPCWare by Rob65 on Sun May 29 12:39:45 MST 2011
If it is that important I would suggest to use an external sample and hold.
This gives you exact control over the moment when you capture the voltage to measure.

Rob