Message-ID: <3E0111D8.2D1D@sneakemail.com>
From: Mike Monett
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Newsgroups: sci.electronics.basics,sci.electronics.misc,sci.electronics.design
Subject: Re: Adjustable current source
References: <3DFFF5AD.1D45@sneakemail.com> <3E00C854.7CC5@sneakemail.com>
Date: Wed, 18 Dec 2002 16:24:56 -0800
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NNTP-Posting-Date: Wed, 18 Dec 2002 16:24:58 EST
Organization: Bell Sympatico
Winfield Hill wrote:
[...]
> 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. :>(
>
> Thanks,
> - Win
Actually, the current may be closer to 200 mA. According to their spec,
the main coil sensitivity is 20 MHz/mA. Assuming they use a permanent
magnet to supply a bias field, the range from 2 to 6 GHz can be covered
with a 0 to 200 mA source. The coil resistance is 20 ohms, so the voltage
across the coil is 4V at 6 GHz. Another 30 ohms could be added in series
to adjust the maximum voltage to 10V.
If I have the calculation right, the op amp would need to have 10e-12*50,
or 0.5nV/Hz^1/2, which is a bit lower than the best I can find.
The reference is another problem:)
Mike