[volt-nuts] Precision Current Source, was: Best reference after LTZ1000
Dr. Frank Stellmach
drfrank.stellmach at freenet.de
Sun Aug 15 21:05:50 UTC 2010
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
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
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
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
This is very, very good, please compare this to the Keithley current
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
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.
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