[volt-nuts] Best reference after LTZ1000

[Bruce Griffiths]

Andrea Baldoni wrote:
> Hello.
> First of all, my greetings to the partecipants of this mailing list.
> I found it very interesting and full of competent people, a rarity nowadays.
> I whish to thank you all for that.
>
>
> Then, let's go to my question.
> I am building a precision current source (10uA/100uA) I want to use with an
> Agilent 34401A and a RTD PT1000 probe to read temperature with good accuracy
> (good is a relative term, actually I would be happy to be limited by the
> precision of the probe, a 1/3 B DIN class).
> Having already the 34401A I thought it was a good (and cheap) solution
> vs. buying a complete thermometer, usually based on less accurate voltmeters
> (apart some very very high price). Also, I like to experiment...
>
> Apart the LTZ1000, that's exceeding the 34401A accuracy (and is probably
> worth to be used for in-house calibration of it, but that's another matter
> I want to pospone for the moment), what would you use, trying to stay in the
> <20$ range?
>
> Actually, I bought a MAX6325 of the best grade, some OP177, some good
> 0.1% 25PPM Vishay resistors, one 10K 0.01% 0.2PPM Z-Foil Vishay resistor for
> calibration purpose of the current source...
>
> I would also like to have your opinions about the trend of all manufacturers
> who are moving away from buried zeners to go versus bandgap and XFETS, but
> looking datasheets I begun to think the reason is more commercial (or
> practical: high inital accuracy, low voltage, low power) than metrological...
> while they publish exceptional data (the MAX6325 is declared 1PPM/C) seems that
> precision instrument makers keep to use (heated) buried zeners... Where is the
> truth?
>
> Best regards,
> (Mr.) Andrea Baldoni
>
> ---
> Ermione s.r.l.
> Via Provanone, 9008/D
> 40014 Crevalcore (BO) - ITALY
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>
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>    
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.
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.

Bruce