At 11:03 PM 12/2/2010, you 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.

Why would you want to do that? Are you sure you don't require measurements
over a 0..60V range that don't differ by more than 0.1V when they are, say,
2V from each other? That's a much easier and MUCH cheaper spec to meet.

>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).

Maybe not good enough if you're mostly looking at voltage differences.

>But this brute-force method seems to get expensive very quickly,
>especially since I really need this to work from -30 to +50C.

>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?

Only what is in the data sheets is guaranteed. If you buy 25ppm/K resistors
then one could be +25 and the other -25. It's 'likely' they will be less,
and 'likely' that one will be relatively similar to the other, but can you
gamble on it? Is your spec real or not?

>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.

You can buy 10ppm/K resistors (or a divider network that has a guaranteed
ratio drift, but they tend to be expensive). Also a 10ppm/K reference,
for less than $10 total. You may also need a precision low-drift op-amp
buffer. 30ppm/K total worst-case leaves you with a 90mV error due to those
causes at -30C (assuming it's calibrated at 20C), probably not quite
good enough to allow for drift and so on. 5ppm/K resistors are several
times more expensive.

Of course, you could add a temperature sensor and a lookup table and
calibrate each unit over temperature in an environmental chamber. That
would be expensive and time consuming, but could be automated. Then
you'd be left with typically smaller errors (initial calibration error,
drift over time, hysteresis etc.)

>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.

Around $80 for 0.6ppm/K and 0.01% initial tolerance. That's the equivalent
of a TXCO, not an OXCO.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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