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

From: steve (bungalow_steve_at_yahoo.com)
Date: 08/31/04 

Date: 31 Aug 2004 16:37:41 -0700

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)
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?
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).

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

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