Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DE69C97.email@example.com> <3DE76E53.firstname.lastname@example.org> <3DE90887.A854256A@rica.net> <3DE9798E.22051DE1@SpamMeSenseless.us.ibm.com>
Subject: Re: photodetector circuit, high speed with good ambient light rejection
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Date: Sun, 1 Dec 2002 09:37:58 -0000
NNTP-Posting-Date: Sun, 01 Dec 2002 09:38:05 GMT
Phil Hobbs wrote:
> Kevin Aylward wrote:
>> The simplest improvement/simplification is simply to replace the 2
>> bipolar with a low noise jet. Your then left with basically
>> 1nv/sqrthz (e.g U309) voltage noise and essentially zero current
>> noise. This results in a much simpler circuit with equivalent or
>> better performance, and the analysis is also much simpler to boot!
> Jerry Graeme's book on transimpedance amps has a lot of circuits using
> discrete front ends on ordinary op amps. However, getting 1
> nV/sqrt(Hz) from a JFET requires a *big* JFET, for which read
> "high-capacitance". U309s are nowhere near 1 nV/sqrt(Hz)--the data
> sheet I'm looking at says 10 nV, typical, and that's at 10 mA drain
> current. At 1 MHz, that will contribute 6 pA/sqrt(Hz), which is 17
> dB above the shot noise of 2 microamps.
I done a little search on the web just to refresh my memory on the exact
figures. I did find one quote of 10nv, however on the Vishay site there
is a full data sheet,
http://www.vishay.com/products/mosfets/SSFsgnchjamp.html, its quoted at
6nv at 100Hz @10ma, Ciss=4pf, with Crss=1.9p. The graph of noise goes
from 12nv/100Hz to ~1.5nv/100khz, the noise is still heading down at
100khz, so 1nv at 1MHz is a reasonable guess. They also quote a NF of
1.5db at 100Mhz, of course, without the source resistance, its difficult
to know what this really means.
I assume your refering the voltage noise to 100p @ 1Mhz @ 10nv = 6.2pa.
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