hi,everybody.
I have some questions about hight precesion constant current source.My
goal is 1uA whose error is blow 0.01%.
In my circuit I used a op-amp,p-mosfet and 5V voltage reference.
But the result is so bad,the error is almost 1%.
Could somebody give me some hints?
Thanks very much.
eehinjor
eehinjor said:
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
From your description, you have something like this (view in fixed font
or M$ Notepad):
`
`
` VCC VCC
` | +
` .-. | -----> +
` | | '----------o
` R1| | 1uA
` '-' .----------o
` |5.000V VCC | <---- -
` o-----. + |
` | | IC2 | |
` IC1/-/ | |\| ||-+
` ^ '-------|+\ ||-> Q1
` | | >---||-+
` | .--|-/ |
` | | |/| |
` === | === |
` GND | GND |
` '------------o
` |
` .-.
` R2 | |
` | |
` '-'
` |
` ===
` GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
(You might want to download Andy's ASCII Circuit -- it's a lot easier
to use a picture to describe what you're doing.)
If you would, a few questions:
* If the above circuit is correct, could you please specify what you're
using for IC1, IC2, R2 (wirewound/metal film/metal oxide/carbon film,
with tolerance), and Q1? Each one of these components could easily be
the culprit. If the above isn't correct, what's different?
* What's your budget? If you actually had to pay much more than a
couple of bucks for 0.01%, would you still want it as badly? Can you
live with less?
* Do you need accuracy as well as precision? In other words, do you
need an 0.01% DC current source at somewhere around 1uA, or does it
have to be exactly 1.00000uA? That's going to be more difficult.
* What's the temperature range for this circuit?
* You've said you've got a burden which will cause a load voltage
between 0 and 5.5V. But you didn't describe the load, or what you're
doing with it. Is this an instrument which has banana jacks, and you
want 200 feet of wire going out to the load and back? Is the load
purely resistive? Are you planning on unplugging it while the circuit
is on? Or only off? Please describe.
* Have you put a scope on the output, looking for oscillations? This
is basically a unity gain non-inverting amplifier, and is prone to
misbehaving. Depending on your components, your circuit may actually
be operating correctly as a high speed oscillator.
* While we're on the subject, what are you using to measure current and
check your circuit? Even measuring DC current to that accuracy
requires a bit more than a handheld DVM.
If this is a dedicated circuit with a purely resistive load in very
close proximity to your circuit, it might be easier to lose the FET and
do something like this:
` .---. R(load)
` | | + ___ -
` === | .-o--|___|--o---.
` GND | | |
` | | |
` /-/ | VCC |
` ^ | + |
` | ___ ___ | |\| |
` -5.000Vo--|___|-o-|___|---o---|-\ |
` | | | >--------'
` .-. .-. .---|+/
` | | | | | |/|
` | | | | | -
` '-' '-' === Vee
` | | GND
` - ===
` Vee GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
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