Forrest W Christian wrote:
> I'm trying to design a circuit which uses an ADC to measure 0-60V at
> least to the nearest 0.1V.

So you need 1 part in 600 accuracy.  That means a minimum of a 10 bit A/D,
so in reality you want a 12 bit A/D.

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

10 bits from -30C to +50C is not trivial.

One possibility is firmware temperature compensation.  We're doing that for
a customer now that wants to control something to .2% over a wide
temperature range.  The feedback sensor doesn't just have a single offset
and gain calibration, but a bunch of offsets and gains in a table indexed b=
y
a temperature measurement.  Individual parts can drift with temperature, bu=
t
that's OK as long as they do it repeatably.  The downside is that each unit
needs to be calibrated at a few different temperatures in production.  The
production fixture will most likely have a Peltier device in it, and there
will be tens of these fixtures to get reasonable bandwidth.

> 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.

Exactly.  That's why you can get "tracking" resistors at various ratios in
the same package.  I haven't done this in a while, but Vishay used to be bi=
g
in that area.  They probably still are.

> 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?

Sortof.  They will likely track, but there is no guarantee.

> 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.

That's another reason we are doing total unit temperature compensation as I
describe earlier.  All that stuff cancels out.


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