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

From: Ban (bansuri_at_web.de)
Date: 09/01/04 

Date: Wed, 01 Sep 2004 11:14:09 GMT

steve wrote:
> 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)

Somehow your opinion seems possible, but it is important to consider the
amount of each individual component in your accuracy Start with the A/D
itself. It has a specified accuracy included the effect of the reference
voltage scaler and other factors (temp. range) which is expressed in for
example +/-1LSB, which in your case would be 2mV.
Now also your sensor has a certain noise level which is amplified by the
signal conditioning. Lets say the voltage noise be 500nV/Hz^-2 at the
0...4.096V output. With a bandwidth of 10kHz this would be 50uVrms, the
peaks might be 6 times higher, which is still only 0.3 LSB . So up- and
downscaling is not of much impact because the converter and sensor noise is
dominant.

> 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

If you do not use standard exchangable sensors with +/-5 or 10V range, you
can better get an unconditioned sensor and build a dedicated amp + filter to
drive the 0...+2.5V analog input. This can be done from a 3.3V supply which
has become standard by now. You can use modern low voltage CMOS parts, needs
some input protection as shown in the data-***. 

-- 
ciao Ban
Bordighera, Italy