[volt-nuts] Best reference after LTZ1000

[Andrea Baldoni]

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...

> 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.

Best regards,
Andrea Baldoni

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