Reply-To: "fred bartoli"
From: "fred bartoli"
References: <3D760B64.CC303206@scs.uiuc.edu> <3D763949.4B6D795E@scs.uiuc.edu>
Subject: Re: High impedance low noise
Date: Tue, 10 Sep 2002 16:57:45 +0200
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NNTP-Posting-Date: 10 Sep 2002 16:56:22 MEST
Win Hill a écrit dans le message :
> my email is broken this morning, so I'm writing to Jim Wentz here...
> Hi Jim. On Wednesday, September 4, 2002, you wrote:
> > Win wrote,
> >> In one opamp, yes, but that's not necessary a good approach. When
> >> measuring voltage, the reason for considering an FET opamp operated
> >> as unity-gain follower is to drive a second high-bandwidth low-Zin
> >> amplifier stage that uses conventional components, such as current-
> >> feedback opamps that have low noise compared to the FET, obviating
> >> the need for the FET stage to have any gain. Well, a gain of 2x in
> >> the FET stage would be nice, and has over 1MHz bandwidth with any of
> >> the parts below. The follow-on G = 50 low-noise 1MHz stage is easy.
> > Well, this was exactly my thinking too, but I keep hearing from others
> > that "all (or most of) the gain should be in the first stage, for max
> > sig/noise. We were pondering the trick of multiple fet op amp inputs
> > all paralleled, then summed. I read somewhere that this is a trick for
> > cancelling random noise. I think another engineer here tried that a
> > while back and it did not work well. We are all having a meeting
> > tomorrow, and I should know more then. I just don't have enough
> > details yet.
> Hi Jim, best wishes in your meeting. To explain the issue of one stage
> vs. several stages. The situation is accurately understood by evaluating
> the effect of each stage on the effective input noise. It's useful to
> use input-noise-density squared, for a simple easy-to-understand formula.
> The formula below is for a three-stage amplifier. Please rewrite it on
> paper, to see it easier without the poor ASCII-character limitations.
> v_n(in)^2 = v_n1^2 + (v_n2/G1)^2 + (v_n3/G1G2)^2
> We can see that if the gain G1 of the 1st-stage in a 2-stage amplifier
> is 1, and both stages have the same noise level v_n, the effective input
> noise will be v_n(in)^2 = v_n1^2 + v_n2^2 = 2 * v_n^2, which means that
> V_n(in) is sqrt 2 = 1.4 times the noise of the 1st stage alone. That's
> where the common wisdom comes from, assuming that the 1st stage uses the
> lowest-noise amplifier, and saying that G1 = 1 is a bad thing.
> However, we know that high-Z JFET amplifiers are much noisier than good
> low-Z wideband amplifiers. For example, an AD8011 has 2nV of noise, as
> compared to a JFET opamp with say 6nV of noise. Using the formula, we
> see that the overall noise is sqrt(6^2 + 2^2) = sqrt 40 = 6.32nV. This
> is so close to 6nV we can ignore the extra noise from the 2nd stage.
> If the 1st stage has a gain of 2x, the effect is more striking, 6nV vs
> 6.08nV. Clearly the old rubric is wrong under these conditions.
> Best regards,
> - Win
> Winfield Hill
> Rowland Institute
> 100 Land Blvd
> Cambridge, MA 02142
> . f_T e_n Cin max Vcc
> . (MHz) (nV) (pF) (+/-V)
> . ----- ----- ---- ------
> AD743 4.5 2.9 18 18
> LT1792 5.6 4.2 14 20
> AD711 4 16 5.5 18
> LT1057 5 13 4 20
> OPA655 240 6 1.0 5.5
> AD8011 * 2 low-Z 6.3 current-feedback, 57MHz at G = 10.
> - Win
Also don't forget the TI winners
f_T e_n Cin max Vcc
(MHz) (nV) (pF) (+/-V)
----- ----- ---- ------
THS4021 350 1.5 1.5 +/-16 stable G>10
THS4031 100 1.6 1.5 +/-16 stable G>2
THS4011 290 7.5 1.2 +/-16