[volt-nuts] Precision Current Source

[Dr. Frank Stellmach]

  Andrea,

I have made the design about 20 years ago, so I did not remember 
everything right..

The BS170 MOSFET has a typical leakage current of 10pA only, have a look 
on the onsemi datasheet: 
http://www.onsemi.com/pub_link/Collateral/BS170-D.PDF.
That's similar to what I have found in my old design notes about the 
current source. I think I have tested this parameter somehow, but I 
can`t remember any more, how it's been done.
The MOSFET drives an npn power transistor, perhaps this helps keeping 
its Ibias low.

It's mandatory to keep the FET and the OpAmp bias currents well below 
1nA, better around 10pA each, as those currents degrade the precision 
especially at 10µA.

I extended the current source to 6 ranges, i.e. 1µA - 100mA, and even 
the lowest range works very precisely, measured with an 3458A.
You get several ppm (stability) at 1mA, degrading to around 100ppm at 1µA.
100mA range suffers from thermal drift of its reference resistor @5V, so 
I revised the design later, by dividing the 5V down to 1V and using 1 - 
10 - 100 valued reference resistors.

I also added a manual decade divider by ten precision resistors to get 
0, 0.1, 0.2, .. 1 V, so the smallest current is 100nA.

The MAX 6225 for sure is more stable in temp and time than the old REF 
02, and I also thought about using  the LM399 to reduce drifts to 
negligible values.

But once again, the much simpler design with a REF02 is absolutely 
sufficient, I wouldn't waste too much effort in that aspect, as other 
errors, i.e. leakage currents, 4W connections everywhere, and 
thermoelectric voltages, have a much bigger impact on precision of the 
current source.

---

In my design, I used double pole reed relais for switching of current 
and sense to the reference resistors independently, i.e. a 4W 
connection, on the ground side of the resistors realized by layout.
A normal relais with changeover switch was used for removing current 
from the output. In this state, the other relais contact connects the 
current source over a 100 Ohm to V++, so it will not saturate.

---

I just scanned the actual Lake Shore, Keithley, and Time Electronics 
sites and could not find any programmable current sources being that 
precise like my old design, and costing that little.
--

I can't figure out how to realize the idea of Bruce, as you need a 4W 
connection to the sensor, and also need compensation of thermoelectrics. 
This cannot be accomplished by sort of a ratio bridge.

I realized that you bought a PT1000, so the 4W is in some cases not 
urgently necessary, but as you must apply 100µA max. instead of 1mA, the 
problem with the thermoelectrics is absolutely the same as for a PT100.

..

You may use your 10k Vishay to calibrate the 100µA current source, as 
the 34401 has about 35ppm in DCV / yr. and the resistor is precise to 
0.01% max, which gives about 150ppm current accuracy, compared to 
measuring 100µA in the mediocre 10mA range of the 34401, which is stable 
to 500ppm / yr. only!
Reason for that: Current shunt R121 is a 5 Ohm resistor, used for 10mA 
and also for 100mA.
On latter range, this gives 50mW power dissipation, therefore the 
resistor had to be a TK0 metal film type, obviously. But this thing 
drifts over time like hell. So I replaced it by a TK3 wirewound, giving 
more stability in time. They saved a lot on quality for this instrument..

agilent improved the current ranges on the 34410/11 models by reducing 
the 10m/100m shunt to 2 Ohm, and adding 100µ/1mA with an additional 200 
Ohm shunt.

Well, the 3458A is superior because it has 8 different ranges, each 
having its own, well suited precision shunt.


  --


Resistance of the 34401A drifts 100ppm in one year, even cheap wirewound 
resistors may drift 20ppm / yr. only.

This bad drift behaviour is due to the complicated current source 
design, with the nested reference resistors. 3 resistors in summary 
determine the currents precision: R201/ R202, the 28k57 and the 5k ... 
1M in the fineline array. And those are drifty thin film or thick film 
resistors only.

In comparison, the only component inside the 3458A, which contributes to 
precision of all resistance ranges, is its internal 40k reference.

So you may better use you nice Vishay metal film as your resistance home 
standard (also 20ppm/year?).
You only have to make precise transfers to other ranges. (Just another 
project)

--

Lake Shore offers a 2 point SoftCal for 0.25K precision up to 300K, for  
"just" 214$, including the sensor.
If your current source works fine, this bargain may be interesting..


Regards, Frank