some questions about high precesion constant current source

added:

1,the voltage of the load is between 0V~5.5V.

2,the output of voltage reference is 5V.

3,the sources of op-amp are +12V and -12V.

thanks

Hi, eehinjor. Welcome to the newsgroup. For things like this, typically

• 1% is easy -- just cobble together a cookbook circuit

• 0.1% requires quite a bit of attention to detail, but is usually not too tough and somewhat more expensive

• 0.01% starts getting into lab quality -- really tough and much higher cost for precision components

or M$ Notepad): ` ` ` VCC VCC ` | + ` .-. | -----> + ` | | '----------o ` R1| | 1uA ` '-' .----------o ` |5.000V VCC | --------' ` .-. .-. .---|+/ ` | | | | | |/| ` | | | | | - ` '-' '-' =3D=3D=3D Vee ` | | GND ` - =3D=3D=3D ` Vee GND created by Andy=B4s ASCII-Circuit v1.24.140803 Beta

By using a pA input low drift op amp and high precision low temperature coefficient resistors, you might be able to get close to 0.01% precision without too much difficulty within a lab temperature range if your reference is up to the task. Getting it accurate to 1.00000uA, though, would pretty much be luck, or would require careful nulling of the op amp offset and an adjustable reference, which has its own set of problems (like potentiometer drift with temperature, &c).

Of course, this basic circuit won't help you if you've got those hundreds of feet of wire, a reactive load, or you have to be able to disconnect the load hot. It also is very susceptible to ESD damage, as you've got an op amp input floating in the breeze.

A better response awaits more information from you.

Good luck Chris

Thank you very much. I know this is very difficult,I have done some experiment,but all the result is so bad.

The resistor is 5000K.In my circuit,the op-amp is OPA602.

So you must have a current sense resistor that drops 5 volts when 1 uA passes through it. That would be a 200k resistor, right? In order for the closed loop to have .01% accuracy, not only do you need sufficient loop gain, but the opamp input impedance would have to be more than 10,000 times this 200k, or greater than 2 giga ohms. What opamp are you using?

Damn, I have to get some caffeine in me before posting. 5 volts reference with 1 uA current implys a 5 mega ohm current sense resistor and an opamp impedance of 5 gig ohms for .01% accuracy. Of course, the sense resistor also has to be accurate and stable to better than .01% and so does the reference. The opamp offset voltage must also be less than a half millivolt. The frequency response of such a source will also be accurate only for very low frequencies where capacitive current is less than .01% of the source current.

...

This

^^^^^^^^^^^^^ inverting ...

--
-- 
John Fields
Professional Circuit Designer

The opamp is pretty good (bias current within minimum requirements and offset voltage probably also). But what is the gate leakage current of your PMOS transistor, and how do you build this thing with surface leakage current to the sense node much lower than the .1 nA error budget?

the type of my p-mosfet is VP0610L.its gate leakage current is only

10nA(max.).

Unfouturnately VP0610L is obselete from last year,I don't know which one can replace it or better than it.

I used a intrument of Agilient,the type is 3458A.it can measure current as low as 1nA.

Thank you very much,John.

But when I try to use this circuit to realize 1mA constant current source,the result is also unsatisfied.

I don't know the reason,so I am afraid the circuit is wrong.

At what level of accuracy do you have to start worrying about thermoelectric effects of the interconnect? mike

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Thanks all.The circuit is shown as below. Q1:VP0610L; Q2:2N4402; RL:the load(resistor); IC1:OPA602; R1:the resistor(5000K ohm),it is composed of 4990Kohm fixed resistor(0.1%,15ppm/c) and a 20Kohm variable resistor;

+12V,-12V:power; +7V:it is produce by INA105 and AD586(+5V voltage reference); +5.5V:to avoid the voltage of RL over this value;

In fact,my goal is 1mA/100uA/10uA/1uA when R1 is 5K/50K/500K/5000K.

By the way,the VP0610L has been obseleted from last year,would you like recommand one?

result is so bad.I don't know how to modify it.

+12V | .-. | | R1| | '-' | |-----------------------o | +12V | | + | | IC1 | | | |\\| ||-+ o-------------|-\\ ||-> Q1 | >---||-+ +7V--------------|+/ | |/| | | | - | -12V |-------------------o | | |< .-. +5.5V----| Q2 | |RL |\\ | | | '-' | | o------------------ o | === GND

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.

Yikes. That is 100 times your leakage current budget of 0.1 nA.

I don't know what a better one would be, but it must have lower gate leakage and probably would be a much smaller die, maybe rated for a higher voltage.

I haven't seen your circuit, including the test setup that you are using to get this unsatisfactory result.

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.

That not only depends on the level of accuracy, but on the reference voltage. This design, supposedly is based on a 5 volt reference. That is pretty huge compared to thermoelectric effects.

eeh> Just now I simulate the Chris's first circuit,found the result is

Hi, eeh> Thanks all.

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

Thank you Chris. This is my first time to google's group.This question is postd at sci.electronics.design.

During these two days,I have learn more.There are little description about MOSFET in our book,so my fault is some knowledge about it.

On your advice,I will do more expeiment step by step until the goal.

best regards.

One thought you might want to consider: I have seen super-accurate *voltage* calibrators that work via digital counters. The basic idea is that you use a counter with some logic to go high for the proportion of time you want to divide the master reference by. For example, to produce

1.0 V from a 10.0 V master, you generate a pulse that is high for one count out of 10, then use that to switch the master. You have to filter the result to get DC. That's easier at higher clock rates, but higher clock rates have more time spent in switching which needs to be compensated. Other than that, the clock frequency is not critical since the whole trick relies upon duty cycle, which being digital is rock-solid.

Application of the above to *current* sources is "left as an excercise for the student". Just giving you an alternative approach.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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