Re: Questions about optimizing Sensor outputs to dsp/microcontroller A/D inputs

From: Bob (SkiBoyBob_at_excite.com)
Date: 09/01/04 

Date: Wed, 1 Sep 2004 09:22:49 -0700

"steve" <bungalow_steve@yahoo.com> wrote in message
news:15da8106.0408311537.362a27e0@posting.google.com...
> Ok, when hooking up a sensor output to a/d input we want to minimize
> the ratio of RMS noise to the LSB value. I would think we want a
> sensor with a high voltage output and a A/D with a high voltage input
> so that the LSB is well above the noise floor. Stand alone a/d's
> usually have nice high input ranges (e.g., Maxim 1271 with a +/- 10
> Volt input), but many modern DSP and microcontroller A/D inputs are
> usually +5 volts and the really modern and attractive ones are in the
> +2.5 Volts range (Analog device ADUC7X series, Cypress PSoC ), which
> initially look unattractive as far as the A/D is concerned.
>
> However, when looking at the Maxim data***, although its take +/- 10
> Volt input the internal reference is only 2.5 Volts, which is then
> scaled up interally to 4.096V. Secondly, many sensors with high
> voltage outputs simply have an internal final stage amp that scales
> the output voltage. So my questions are
>
> 1) Are those 2.5 Volt A/D's really at a disadvantage compared to the
> wide range stand alone Maxim type A/D's since they really work off 2.5
> volt references? I think the only purpose of the wide range inputs is
> to eliminate the need to externally rescale large inputs, not to
> decrease the RMS noise to LSB ratio. (which is what I thought at first
> glance)

The wide range parts (+- 10v) trend to be the older parts. Newer parts tend
to target the lower voltages used in battery powered or high speed circuts.
Noise-to-LSB ratio is not a particularly usefull way of looking at these
things. See below.

> 2) When Maxim scales up the 2.5 volts reference to 4.096 volts, what
> the purpose of that? I would think that the amp would amplify the
> noise too (unless its a differential amp). Or maybe they just want 1
> count = 1 mV?

Don't worry about what they do with the reference. Worry about your error
budget.

> 3) Sensors with 5V outputs typically are just scaled up to 5 Volts
> internally, and, with external parts, can be rescaled to 2.5 volts. If
> I do that am I hurting the noise/lsb ratio? I would think that if they
> are scaling up the output they are also scaling up the noise too
> (again unless they are using differential amps).

Your system design should start with a specification of the overall
performance. From there you can begin chosing parts and calculating your
error budget. Broadband noise is only one part of that budget. Sample rates,
gain and offset drift, and linearity are also important (or not, depending
on wht you are trying to do). Sometimes noise > 1LSB is a good thing (google
"dither"). Don't forget to analyze any averaging or other digital filtering
effects on your signal.

>
> If there is a application note somewhere about this let me know
> please!
> thanks
> steve

There are lots of application notes. The key word here is "application". All
of this issues depend on what the *application* is. So, what's your
application?

Bob