On 03-Dec-10 05:03, Forrest W Christian wrote:
> I'm thinking there's a trick I'm missing somewhere.
>
> I'm trying to design a circuit which uses an ADC to measure 0-60V at
> least to the nearest 0.1V.

The easiest way would be to use MCP3421(MCP3425) delta-sigma converter=20
which has internal 2.048V 5ppm reference and two low temperature=20
coefficient resistors for the divider.

You could use standard resistors for the divider and auto-calibrate the=20
division ratio before each measurement but that would add much hardware=20
and software complexity for little money saved.

> The obvious solution is to take a precision reference, and a voltage
> divider with high stability resistors, and a 12bit ADC (although a 10bit
> ADC may be close enough).
>
> But this brute-force method seems to get expensive very quickly,
> especially since I really need this to work from -30 to +50C.

Define expensive?

>
> However, in looking at this closer, it appears I might not need that
> high of stability (or that high of precision resistors).   In a voltage
> divider situation, if both resistors exibit the same 'temperature
> coefficient error' (or whatever the correct term really is) at each
> temperature in the range, then the divider won't change over temperature.
>
> Is it reasonable to expect say metal film resistors from the same
> manufacturer (and type) to be pretty consistent with how many ppm the
> resistance drifts at any given temperature?  For instance if they were
> sitting right next to each other on the board, perhaps with a gob of
> heatsink adhesive encapsulating both of the units?

No, the temperature coefficients between different batches will NOT=20
match, I know this from experience.

It will match in the same batch, some better manufacturers specify this=20
in the datasheet, I remember one specified 5ppm match.

Djula

>
> The other thing that's bothering me is the lack of a precison reference
> with extremely low ppm/C rating.   With the 80*C application range I'm
> talking about, and typical voltage references in the 50ppm/C range, I
> end up with +-0.4% overall temperature drift.   Or 0.24V at full scale
> error - which isn't going to work.
>
> Short of building an oven, is there something else I can do to get a
> decent reference voltage for the ADC?  I haven't seen a
> voltage-reference equivalent of a TXCO, and even if I did I suspect that
> it would be horribly priced.   Plus, I'm trying to keep low power here
> (this product will be used at times in  a solar powered application),
> and burning power to heat up a voltage reference doesn't sound like
> low-power to me.
>
> And what else am I likely to be missing?
>
> -forrest
>
>
--=20
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