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From: Winfield Hill
Subject: Re: Adjustable current source
Date: 18 Dec 2002 09:11:08 -0800
Organization: Rowland Institute
References: <3DFFF5AD.1D45@sneakemail.com> <3E00C854.7CC5@sneakemail.com>
X-Newsreader: Direct Read News 4.20
Mike Monett wrote...
> The next trick wil be to find a very low noise current source for a
> different application. Filtronic offers a low-cost YIG oscillator that
> covers 2 to 6 GHz for US$500 in single quantities that might be ideal:
> Their app note shows the maximum noise current to obtain -100 dBc/Hz at
> 10 KHz offset is 10 picoamperes/sqrt(Hz). This is while delivering
> currents in the mA range. ...
10pA? No problem.
Let's say you make a conventional quiet 10mA current source using
a 1.0k sense resistor. The Johnson current-noise density is
(4kT/R)^1/2 = 4.0pA/Hz^1/2, so the resistor isn't a problem. We'd
like the voltage noise of the 10V reference and controlling opamp
to be under say 4pA * 1k = 4nV/Hz^1/2, which isn't a problem either**
using the right components. This would yield a 0 to 10mA source with
under 5.6pA of current noise. BTW, the shot-noise density of 10mA
current is (2 q I)^1/2 = 56pA, but film resistors don't have shot
noise (see AoE page 432), so that's no problem. :>)
** At least it's not a problem at frequencies above where the
voltage reference's low-pass filter (you did use one didn't you?)
becomes effective. :>) But I imagine that at low frequencies
voltage reference "noise" will create a messy 1/f noise / drift
in your otherwise-quiet current. :>(
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