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/iframe></center><br><br> <br><br> </center> </td></tr> </table> <center> <table> <tr> <td width="600"> <center><hr width="400"><font size="2"><b>The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Usenet Archive Feeds Directly<br>From The Open And Publicly Available Newsgroup<br>Sci.Electronics.Design<br><br>This Group And Thousands Of Others Are Available<br>On Most IS NNTP News Servers On Port 119.<br><br><a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Is NOT Responsible For Any Topic,<br>Opinions Or Content Posted To This Or Any Other<br>Newsgroup. This Web Archive Of The Newsgroup And<br>Posts Are For Informational Purposes Only.<br><br></b></font><hr width="400"></center> <center><font size="2"><pre><b> From: dd <doug@ddwyer.demon.co.uk> Newsgroups: sci.electronics.design Subject: Re: High impedance low noise Date: Wed, 11 Sep 2002 19:25:47 +0100 Message-ID: <Kpsp8UArq4f9Ew2Q@ddwyer.demon.co.uk> References: <3D760B64.CC303206@scs.uiuc.edu> <al5bh30gmo@drn.newsguy.com> <3D763949.4B6D795E@scs.uiuc.edu> <al5ir901f6b@drn.newsguy.com> <al7dm20qj@drn.newsguy.com> <3d7e0815$0$244$626a54ce@news.free.fr> <allt4p02e5e@drn.newsguy.com> <3d7f3fa8$0$474$626a54ce@news.free.fr> NNTP-Posting-Host: ddwyer.demon.co.uk Mime-Version: 1.0 NNTP-Posting-Date: Wed, 11 Sep 2002 18:27:20 +0000 (UTC) X-Newsreader: Turnpike Integrated Version 5.01 U <iq8rvUYxwmrVWYgB1gsh5A7Rzo> In article <3d7f3fa8$0$474$626a54ce@news.free.fr>, fred bartoli <toto@hotmail.toto> writes > >Win Hill <whill@picovolt.com> a écrit dans le message : >allt4p02e5e@drn.newsguy.com... >> "fred wrote... >> > >> > >> >Win Hill <whill@picovolt.com> wrote... >> >> >> >> 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. >> >> >> >> . 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. >> >> >> >> Thanks, >> >> - 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 >> >> Indeed those are nice amplifers, but they aren't JFET input >> oamps, and they have an input current of up to 6uA, which >> would be a killer for Jim, I suspect!! >> >Oops.... >I just found those amps for my VNA IF amplifier and their nice perfs made me >forget this point. > >One not so nice point is that the lowest noise amps aren't unity gain >stable, and that seems to be a constant, at least for the TI amplifiers. > >Do you have any idea about the reason(s) ? >That's a too bad point for me because this prevent me from using them in the >Fliege filter. > >Fred. > >> Thanks, >> - Win >> >> Thanks, >> - Win >> > > I have an on-going minimum noise design task at 100kHz where the source series cap is 0.7pF and a shunt cap following of 10pF. Currently employing an LT1793 10MEGfeedback // with 6.8pF. The gbw of this marginal for the design. I have recently got the data sheet of the AD8065 145MHZ gbw 7nVroot Hz and 0.6fA / root Hz employing Fastfet (jfet?) Have requested samples so well see how it goes, in the past Ive had a lot of work re-designing circuits to employ alternatives for AD opamps that have been discontinued so there is my prejudice to overcome. A new LT .9nVroot Hz op amp from linear has an application circuit employing a Phillips Jfet front end. Sure it works but the Jfet has no appropriate leakage or current noise specifications in the data sheet. -- dd </b></pre></font><hr width="650"><a href="http://www.cyber-spy.com/electo-index.html"><font size="3">Go Back To The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a><br>Usenet Web Archive Index Of<br>The sci.electronics.design Newsgroup</font></a><br><br></center></td><img src="http://cyber-spy.com/cgi-bin/livecntr.pl?0+1+unetsci" width="1" height="1" alt="The Cyber-Spy Toolkit - powerful internet spy software - live police scanners - usenet fbi secrets archive - newsgroup spy electronics archive - search engine spy tools - download TCST today!"> </tr> </table> </center> <center> <br><center><hr size="3"></center> <table width="100%" cellpadding="0" border="0" cellspacing="0"> <tr> <td align="left" valign="top"><small><b><i><a href="http://www.cyber-spy.com/index.html">Cyber-Spy.Com Live Resources</a></i></b></small></td> <td align="right" valign="top"><small> Copyright Alpha Omega Delta,<br> All rights reserved. </small></td> </tr> </table> <center><hr size="3"></center> <br><br> <br><br></center> </body></html>