[Tinyos-2.0wg] Concerns about ADC in 0.1mV

[David Gay]

I'm not quite convinced by this whole discussion about outputting
millivolts from the HIL. Some questions/comments:

- First and foremost, the meaning of any voltage reading from the ADC
depends on the circuit by which the analog sensor is connected; in
some cases, you may not care about voltages. For instance, a simple
voltage divider powered by the same voltage as the ADC reference is
really returning unitless values.

- Clearly, in many cases, we do care what the voltage reference was;
however, this voltage reference was selected by the code reading the
sensor so I don't see what
advantage you get by pre-converting the A/D result to the
corresponding voltage (presumably, further processing is likely).
Furthermore, the selection of reference voltages is ADC-converter
specific.

- When using a variable voltage as a reference, you need a way to
measure that voltage.

- Do we have any input from "real" sensor designers/users that this is
the right approach?

My larger point may be that I'm unclear that there is a need for
platform-independent analog sensor drivers (*). In there absence, all
this work to convert to platform-independent units seems wasted.
Specifically, for a platform-independent analog sensor driver to make
sense, I would expect we would have to define standard supply and ADC
reference voltages, which I don't think we want to get in to...

David
*: Note that this doesn't preclude from standardising, e.g., a
TemperatureC sensor that returns temperatures in 0.1K units. That one
is yet another can of worms.