[time-nuts] Temperature sensors and bridge amps

[J. Forster]

IC Instrumentation Amplifiers are very good these days both on offset and
bias currents and voltages. They make bridge amps easy. Gain is selectable
with one resistor.

I still prefer IC sensors though.

-John

===============


>
> As I understand it, the problem to be solved is stability to a
> millidegree or some such. That kind of accuracy is not required
> because the flat spot of crystal tempco is not narrow. +/- 1%
> would be accurate accuracy.
>
> For stability, you must remove all sources of variation. Self-
> heating is not a stability problem, it is an accuracy problem.
> Operating at the bridge null reduces the effect of bridge power
> variations.
>
> The challenge is to get enough gain from the low offset error
> amplifier to maintain the required error range without having
> so much gain that the loop is unstable. This usually means
> taking physical design steps to eliminate dead-time or lags
> in the heater control loop. Of course, you can't get to a
> millidegree from ambient with just one oven. And you can't
> eliminate time lags if you have any thermal mass.
>
> Note that you have the same loop stability problems if you use
> a crystal as the sensor and a counter as the detector.
>
> I worked for Rosemount (part of Emerson since 1976), a maker of
> industrial sensors including 100 ohm platinum. The four wire
> Kelvin connection offers the most accuracy, but frugal industry
> finds that a three wire connection is adequate for very long runs
> of cable. This connection puts a lead wire in both legs of the
> bridge.
>
> Bill Hawkins
>
> -----Original Message-----
> From: Bob Camp
> Sent: Friday, November 12, 2010 6:28 AM
>
> The 100 ohm standard for RTD's dates way back. The assumption was that you
> had it on a *long* run of cable (2 pair / sense leads of course). The
> insulation leakage was a bigger issue than anything else in the equation.
>
> On Nov 12, 2010, at 5:36 AM, Poul-Henning Kamp wrote:
>
>> This was one of the things that I wondered about:  How large currents
>> are used ?
>>
>> Can't be too much because that would lead to self-heating...
>>
>
>
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