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

Andrea Baldoni erm191ba3 at ermione.com
Mon Aug 16 12:56:42 UTC 2010

On Sun, Aug 15, 2010 at 11:05:50PM +0200, Dr. Frank Stellmach wrote:

> 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.

Your setup is more or less similar of what I have originally thought.

I planned to use the MAX6225/6325 reference divided down to 1V, a non-chopper
(but very low bias current, very low offset voltage and 0.1uV/C offset voltage
drift) OP177 (I was worried of the noise of the chopper), and a common middle
power MOSFET I already have, that actually is the weak ring of the chain with
his 100nA of gate to body leakage...

By the way, I was not able to find a MOS in my component box with gate-body
leakage less than 100nA, even small ones. I have not small signal MOS, just
(even low) power ones... there are still discrete small signal MOS around?

I may buy your BS170 with his 10nA (anyway it's not a JFET, but a MOSFET too).
There is a 2SK1656 with 5nA, but I wonder if, at 10uA, a darlington made of
superbeta BJTs would not do better (using maybe the IT124) or simply a JFET
2N3819 (max -2nA of reverse current at -15V, but in normal working condition it
should stay into the pA range). I already have some of them, I'll try it.

Actually, Bruce suggested me that an independent voltage source, however
stable, may not be a good choice vs. one tracked to the voltmeter internal
one. So my first try would be to replace the MAX6225 with a 1K resistor.
This is assuming that the internal circuitry used inside the 34401A to generate
the ohm current source (plus my external resistor), is less drifty than his
voltage reference. I will try to verify this assumption.

The box of components has arrived this morning, so I'll try soon.

> 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).

I am not so lucky to have a 3458A. I would like to...

Anyway I just measured the 10K Vishay resistor with the 34401A using kelvin
connection and, after warmup and a math null against a short, it measured
10.0000 Kohm stable. Seems promising.
> Current is by far the most difficult unit to measure or to produce, as  
> it mostly depends on voltage PLUS resistance..

And temperature is harder again :)

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

I didn't thought of that, but I see that this matters very much even in
measuring my 10K resistor.
You disconnected power source with the reed, right?

> measure the thermoelectric voltage (without current). The HP34401A has  
> no possibility to make this compensation automatically (a pity)

If you control relays with a computer, you could send the math null command
over the interface port.

> 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.

Yes. If you say that resistance mode is mediocre, probably the internal
current source is driftier than the reference, thus an external reference
would be better. Actually my measures with the 10K resistor are anyway
good... how do you check the (low) performance of the meters?

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

The sensor is my limit. My goal is that will be the only limit, so I could
improve simply by buing a better one.

Best regards,
Andrea Baldoni

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