[volt-nuts] Precision Current Source, was: Best reference after LTZ1000

[Dr. Frank Stellmach]

  For my Ph. D. on High-Tc-Superconductors I had to build several 
precision low-temperature thermometers, i.e. where thermoeletric 
voltages have most influence, compared to current precision.

I used a PT100,  @1mA, for it's stability, useful downto ~ 20K, and a 
precision diode, DT 470, @10µA, 1.5..300K, both from Lake Shore, as far 
as I remember.

The Keithley programmable precision current sources were too expensive, 
as I needed 2-3 fixed currents only, so I built several precision 
current sources for our institute, 1/100th the price, with the following 
BOM:

REF02 (5V reference)
Industrial Chopper OpAmp 7650, low Ibias <10pAe
Low-Ileak JFet, e.g. BS170
ww. precision resistors, T.C.= 1..3ppm/K, 0.1%, 4k99, 49k9 and 499k 
custom specific from German manufacturer Burster
Or standard 5k / 500k ww from alpha electronics, sold by Rhopoint 
Components, U.K.
20 turn trimmers of  20 Ohm, 200Ohm, 2k to trim to 5k/500k exactly, or 
to trim standard resistors over range of +/- 0.2%, ~1ppm resolution.
20 turn trimmer to trim the REF02 to exactly 5.000V or to a value a 
little bit above that, for the 5k / 500k + trim pot standard resistors.
Several 5V Reed relais (steered by parallel port) to switch between 
current ranges and to zero current (open) - most important for 
thermoelectric compensation of PT100, more important  than the precision 
of current.


The 7650 regulates exactly a voltage of 5.000V over the precision 
resistor, connected to ground, by means of an JFet, producing no 
additional cross current.
The load is between the Drain of the JFET and V++ (=15V), giving enough 
compliance to feed the sensor.

I also had an HP3458A to calibrate the current source. Without that, any 
effort for a precise current is worthless, but you should provide the 
metrological means for producing it. (I.e. the idea of buying a standard 
Vishay metal foil is ok, just measure the voltage @ 1mA).
Current is by far the most difficult unit to measure or to produce, as 
it mostly depends on voltage PLUS resistance..


The circuitry delivered about 50ppm stability for 1mA, a little bit less 
for 10µA.

This is very, very good, please compare this to the Keithley current 
sources datasheets.


Much more important for the precision of measuring temperature, using a 
PT100, is the cancellation of thermoelectric voltages.

Therefore, for PT100 you should use 1mA current and a possibility to 
measure the thermoelectric voltage (without current). The HP34401A has 
no possibility to make this compensation automatically (a pity)

So you have to substract the theremoelectric voltage from the resistance 
voltage, produced by the 1mA current source.
4W / Kelvin wiring is mandatory.
I also used a HP34401A just for temperature measurement, just using the 
DCV mode. DCV is quite precise/stable, resistance and current is 
mediocre, but you have the Vishay metal foil for that.

By this method, you may get 10mK precision, provided you have a well 
calibrated sensor.

All additional effort in precision and cost for the currents' precision 
is useless, ie. with those ideas, cost may be below 50$/€ for such a 
high precision current source.

Frank