[En-Nut-Discussion] AD conversion resolution?

[Bernd Walter]

On Thu, Dec 15, 2005 at 03:43:27PM +0100, Gerwin Voorsluijs wrote:
> Hi Kathy,
> 
> You mean it could be noise? That would not explain why I don't see such 
> jumps on my voltmeter, nor does it explain why I see this behaviour over 
> a number of sweeps. Btw I use the internal reference voltage on the 
> Ethernut.

Of course it would explain that.
Your voltmeter likely has much tighter filters in front of the ADC.
The ethernut board can't have these because ideal filter design require
knowledge about the samples rate, etc..
Unless you stay with a given setup you also have to keep in mind that
your way of using the ADC don't ashure a constant sampling rate, e.g.
an interrupt service routine could stop sampling for a while.

> In the mailing list I saw that you are also using the ADC function of 
> the Ethernut and I was wondering if you could look at the results you 
> are getting to see if maybe I'm unlucky and my Ethernut is broken?

In my expirience the ADC is very linear.
The noise level varies very much with your setup - one or two bit is almost
unavoidable.
The internal Vref is OK in beeing a constant value, but the voltage varies
much between device - I've seen values in the 2-3V range - you can easily
measure it on the Vref pin if enabled.

You also don't explain your hardware setup.
It's even possible to have some kind of capacitive ground loop, which
may influence the ethernut board, but not the battery driven voltmeter.

-- 
B.Walter                   BWCT                http://www.bwct.de
bernd at bwct.de                                  info at bwct.de