[volt-nuts] Best reference after LTZ1000

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Aug 14 20:47:03 UTC 2010

Andrea Baldoni wrote:
> On Sat, Aug 14, 2010 at 11:43:38PM +1200, Bruce Griffiths wrote:
>> A ratiometric conversion using the voltage reference to both set the
>> current and the DVM reference will negate slow reference drifts.
>> The stability to a first approximation then depends largely on the
>> precision resistor used to set the current and the RTD.
> Very good idea. The ratio of VRTD vs. Vref
> would directly be the ratio between RTD and the setting current resistor.
> Since I don't really need a precise current source (using<100uA would
> allow me to ignore self heating of the RTD) a simple resistor divider between
> the RTD and a precision 10K resistor would do the same job using only a, say
> 1V source at this point...

Short term stability of the source is important as the voltage across 
the 10k resistor and the RTD aren't measured simultaneously.
>> The internal current sources used for resistance measurement in the
>> 34401A essentially allow such a measurement.
>> The drift of the various opamps used in the internal current source will
>> degrade this somewhat.
> The 34401A has the VDC:VDC ratio function, it has some limits but it's useable
> for this purpose. The internal current source instead (for the resistor value
> of interest) is 1mA, too much, it would cause significative self-heat in the
> RTD.
> I don't understand in which way you mean it is involved in this case.

The internal current source is derived from the internal reference via a 
few opamps and resistors plus some JFETS etc.
To a first approximation its value only depends on the value of a 
resistor and some resistor ratios.
Thus the current tends to track drift in the internal reference.

> Best regards,
> Andrea Baldoni
> ---
> Ermione s.r.l.
> Via Provanone, 9008/D
> 40014 Crevalcore (BO) - ITALY
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