[volt-nuts] Precision current source

Dick Moore richiem at hughes.net
Tue Aug 17 17:48:25 UTC 2010

On Aug 17, 2010, at 5:00 AM, volt-nuts-request at febo.com wrote:

> Message: 5
> Date: Tue, 17 Aug 2010 01:29:18 +0200
> From: "Andrea Baldoni" <erm191ba3 at ermione.com>
> Subject: Re: [volt-nuts] Best reference after LTZ1000
> To: Discussion of precise voltage measurement <volt-nuts at febo.com>
> Message-ID: <20100816232918.GA13815 at sol.ermione.com>
> Content-Type: text/plain; charset=us-ascii
> On Sun, Aug 15, 2010 at 01:56:03PM -0700, Dick Moore wrote:
>> The 34401 uses a reference from the National LM199 family -- I suspect
>> they're selected in some first-order way to keep costs down. So using a few
>> LM399's, each with a little series resistance in parallel driving a
>> stable op-amp like an LT1150 chopper amp might be the best way to go. In my
>> experience, the LM399 aging curve tends to flatten out significantly after
>> about 10 hours of run time, so it doesn't take too long to get them pretty
>> stable, and then using two or three or four is cost effective and tends to
>> make a more stable system.
> Do you think that controlling the temperature of LM399 by sensing his case
> and using a PWM (or linear) control over the heater might improve his stability?
> After all (ok, not quite) it's like to have an LTZ1000 that way...
> The cost would be _very_ different hovever and you may make batteries of 4
> as you wrote.
> Another thing to try at home. 
> Best regards,
> Andrea Baldoni

I suspect (which means "I don't know") that trying to regulate the 399 heaters beyond what is incorporated in their design will be unproductive, and that averaging the outputs of several units in tandem will be better. Providing some thermal isolation for the 399s by protecting them from stray air currents and using thin wires or PCB traces is a good idea.

Dick Moore

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