[volt-nuts] Best reference after LTZ1000

Andrea Baldoni erm191ba3 at ermione.com
Sat Aug 14 22:03:04 UTC 2010

On Sun, Aug 15, 2010 at 08:47:03AM +1200, Bruce Griffiths wrote:

> 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.

Maybe I start to understand.
I may use the 4W ohm measurement, derive a Vref from the force connection
using a stable resistor, for instance 1V from 1K; use a good operational+MOS to
make a current source from this Vref, say 10uA using 100K; measure the drop over
the RTD with the sense connection and voila', the reading directly in ohm x100.

This way, no need for an external source at all and also the meter could read
at full speed because it has not to multiplex between the two inputs like in
ratio VDC:VDC.
Very nice!
As soon as I receive the OP177s I'll try it. I have just to pay attention to
track the heating of the 1K resistor with the 100K, because 1mW is not to

The same could be done in reverse, to make a microohmeter out of the 34401A.

Best regards,
Andrea Baldoni

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