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Help! - Constant Current Sources

Help! - Constant Current Sources

Help! - Constant Current Sources

(OP)
Hi All,

I'm currently working on a signal conditioning/isolation design for our products.  Part of the design utilizes a constant-current source so that I can deal with bi-polar voltages.  I am using a modified Howland, which incorporates a JFET and a LM313 instead of all the resistors.  This circuit outputs 100uA and I need stablilty to as close to perfect as I can get (preferably less than +/-5%).  The problem is that the LM313 seems extremely sensitive to temperature and does not seem to hold my calibration settings.

Does anyone know how to build an ultra-precise current source?  Do any pre-manufactured ones exist for the PCB integration level?

RE: Help! - Constant Current Sources

Well, sorry, but a Howland isn't going to do it for you if you need high stability or accuracy.  Way too many sensitivities to try to account for.

A Widlar topology has much better stability.  And there are other topologies better than that, I just can't remember which ones at the moment.  Senior moment!  Argh!!!!

RE: Help! - Constant Current Sources

(OP)
Thanks!

I think the Burr-Brown part will work much better than what I have now.  I guess I'll also have to go through my small "library" and look up the Widlar sources too.  If you can think of anything else I should know, please feel free to beat me over the head with it. ;)

Sorry for the dumb question, but 90% of my work now is Microprocessor/FPGA/Digital logic.  This is the first chance I've had to mess around in the world of current sources...

RE: Help! - Constant Current Sources

To start with
What kind of voltage reference do you have ?
Or which Vreference would you use to make a CC source ?

RE: Help! - Constant Current Sources

(OP)
Thanks again for the info!  I've had a chance this week to implement the Burr-Brown REF102a part.  Even with a cheap op-amp, this circuit was far better than what I had.  Before I was seeing 50-100mV of drift, now it is almost not measurable with our Tek scope!!

<The original circuit was a modified Howland that had the anode of the LM313 tied to the op-amp (OPA177G) output and the cathode tied to the negative input.  A 12k resistor was then connected between the output and the positive input.  100uA of current then flows out of the positive input connection.>

You all have managed to save my job, thanks!

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