Re: some questions about high precesion constant current source

eehinjor wrote:
> Thank you,Chris.
> There is a little difference between our circuits,but the theory is the
> same.
>
> I don't know what IC1 is in your first circuit.
>
> Maybe I am confused between precesion and accurary.I only mean the
> result shoule be 0.9999uA~1.0001uA.
>
> The temperature is from 20C to 70C,of cource the wider the better.
>
> The load is resistor,when its value is 5500Kohm and the current is
> 1uA,the voltage will be 5.5V.
>
> waiting for your response.
> best regards.
eehinjor wrote:
> Just now I simulate the Chris's first circuit,found the result is
> worse.
> especially when the load is smaller than 500K,the output is not
> constant.

Hi, eehinjor. Apparently you've cross posted your question over at
sci.electronics.design as well as s.e.b. It's now clear that you are
trying to make a DC current calibrator with switchable currents in
decades down from 1mA to 1uA with 0.01% precision. Here's the key post
from s.e.d.:

eehinjor wrote:
> Thanks all.
>
> For me this task is a chanllege.I have done some experiment during past
> months.
> My goal is 1mA/100uA/10uA/1uA,the accuracy is 0.01%,the result is that
> the output can be switched between them.Then some mux-chips will bring
> error.
>
> I have tried three ways.One,single op-amp & resistor.Second,two
> op-amps,one of them as the feedback.Third,the op-amp & PMOSFET.Before
> experiments,I simulated them by pspice.
>
> The first two ways can acheive 1mA/100uA easily,but they can not
> realize 10uA/1uA.
> So I have to change the way to the third.But until now,I can not
> realize 1mA by the third way.Maybe because I am not familar with
> PMOSFET.
>
> I will buy some chips tommorrow to do some experiments on your advice.I
> am sure I have to study more from all of you.
>
> best regards.

A couple of points:

* I believe the biggest hangup you're having with your P-MOSFET circuit
is getting a 5 Meg resistor to stay stable to 0.01%. Higher resistance
values are a bear. Changes in ambient humidity are particularly
troublesome in making effective resistance values drift all over the
place. I would also be concerned about leakage current in the FET, and
possible oscillations.

* The second circuit I mentioned is only useful if you can put a
resistive load on the circuit board within a couple of inches of the op
amp. The circuit takes advantage of the "virtual ground" at the input
of the op amp. The divider of your voltage reference is set to inject
1uA into the inverting input. The op amp will work to put 1uA through
your load to balance the injected current, and keep the input at 0V.
This doesn't work if the feedback loop goes 2 meters through test probe
wires to your load or a meter, and then back. The inductance of the
leads and any noise pickup on the wires guarantees oscillation. The
simple op amp current null circuit I suggested is not useful for you.

You're discovering the hard way why current source meters with the kind
of accuracy you're specifying usually cost thousands of dollars new
(Keithley in particular is a good source for this kind of instrument).
They produce a dial-in, rock-stable current under varying loads. You
should know, however, that even these instruments are not specified to
the temperature range you specify -- they generally are only spec'ed
over lab temperature range.

If you need a calibrator, you can take hope in the thought that used
instruments are available at a fraction of the price of new. You may
be able to get something close to what you want (if you can lose the
commercial temp range) for hundreds rather than thousands of dollars.
An 0.01% dial-in or switchable current source is not a trivial project,
and not suitable for newbies.

If you still want to pursue this as a project, reread all the responses
to your posts in both newsgroups, particularly those of Mr. Popelish,
Ban, and Winfield Hill. They've given you a free helping of really
good practical advice.

Then go to instrument manufacturer websites, and take a look at what's
out there commercially, what it can do, and what it costs. These
instruments are generally a real bargain, and give you good value for
your money. You will see, if nothing else, that what you're proposing
isn't exactly a trivial newbie project. If somebody could slap
together a little perfboard circuit that does what you propose, they
wouldn't be spending thousands of dollars for one of these instruments.

Also, get the operation/service manuals on one of the older Keithley
current sources, and take the time to examine the circuits in detail.
There's a real education in doing that, and may help you in getting the
knowledge you need to approach your job. But you might want to shoot
for 0.1% at best, especially considering your temp range.

Good luck
Chris

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