Return-Path: <owner-rsgb_lf_group@blacksheep.org> Received: (qmail 2378 invoked from network); 15 May 2004 16:51:15 -0000 Received: from unknown (HELO ptb-mxscan01.plus.net) (212.159.14.235) by ptb-mailstore02.plus.net with SMTP; 15 May 2004 16:51:15 -0000 Received: (qmail 66196 invoked from network); 15 May 2004 16:51:15 -0000 X-Filtered-by: Plusnet (hmail v1.01) X-Spam-detection-level: 11 Received: from ptb-mxcore01.plus.net (212.159.14.215) by ptb-mxscan01.plus.net with SMTP; 15 May 2004 16:51:14 -0000 Received: from post.thorcom.com ([193.82.116.20]) by ptb-mxcore01.plus.net with esmtp (Exim 4.30; FreeBSD) id 1BP2NV-000Gsz-S1 for dave@picks.force9.co.uk; Sat, 15 May 2004 16:51:14 +0000 X-Fake-Domain: majordom Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1BP2NI-0004in-Ob for rs_out_1@blacksheep.org; Sat, 15 May 2004 17:51:00 +0100 Received: from [213.232.95.59] (helo=relay.salmark.net) by post.thorcom.com with esmtp (Exim 4.14) id 1BP2NI-0004ie-26 for rsgb_lf_group@blacksheep.org; Sat, 15 May 2004 17:51:00 +0100 Received: from cmsrelay03.mx.net ([165.212.11.112]) by relay.salmark.net with smtp (Exim 4.24) id 1BP8vq-0002kD-Br for rsgb_lf_group@blacksheep.org; Sun, 16 May 2004 00:51:06 +0100 Received: from cmsapps01.cms.usa.net (165.212.11.136) by cmsoutbound.mx.net with SMTP; 15 May 2004 16:50:44 -0000 Received: from usa.net [151.41.154.107] by cmsapps01.cms.usa.net (ASMTP/dibene@usa.net) via mtad (C8.MAIN.3.13N) with ESMTP id 860ieoqYp0050M36; Sat, 15 May 2004 16:50:41 GMT X-USANET-Auth: 151.41.154.107 AUTH dibene@usa.net usa.net Message-ID: <40A64A60.9030308@usa.net> Date: Sat, 15 May 2004 18:50:40 +0200 From: Alberto di Bene <dibene@usa.net> User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.4) Gecko/20030624 Netscape/7.1 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org References: <DC9AE469E761D611BD420002A5EC855C7726F4@cerberus.herts.ac.uk> In-Reply-To: <DC9AE469E761D611BD420002A5EC855C7726F4@cerberus.herts.ac.uk> X-Spam-Score: 2.8 (++) X-Spam-Report: autolearn=no,FAKE_HELO_USA_NET=2.8 Subject: LF: RE: VDC source with microvolt resolution Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: ** X-Spam-Status: No, hits=2.8 required=5.0 tests=FAKE_HELO_USA_NET autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SPAMFiltered: yes X-Spam-Rating: 1 Dear Jim, thanks for your further considerations. I will have a look at those opamps. And yes, working at the uV level is a bit of challenge. I wonder if reducing the Hz/V sensitivity of the EFC with a passive attenuator could be a good strategy... after all, I don't need an adjustment range of plus/minus 8 Hz. A tenth of this should be enough... 73 Alberto I2PHD ---------------------------------- James Moritz wrote: >Dear Alberto, LF Group, > >I think Johan's "PWM-ed PWM" is a better idea, since you should be more or >less guaranteed a monotonic Vout vs. input code transfer function for the >DAC. > >I don't think op-amp noise in the filter will be a major problem - the noise >voltage is proportional to sqrt(bandwidth), and the bandwidth is only a >small fraction of a Hz. The high resistor values needed for the filter and >their associated thermal noise mean that an op-amp with a low input noise >voltage is pointless (1Meg has about 130nV/rootHz)- it is more important to >minimize the input current noise, ie. use a FET input device. The active >filter op-amp driving the EFC will have wide-band noise in its output, but >most of this could be filtered out with a simple RC section (say >1kohm/100uF) between the op-amp and the EFC pin. > >In some work on an electrometer circuit I did some years back, I found the >newer types of CMOS op-amp, like the LMC660/LMC662 were cheap and good in >this sort of application. Usually, the noise and drift of the op-amp was >less significant than thermal effects, leakage, vibration, cross-talk from >the other parts of the circuit and all sorts of peculiar effects that plague >DC circuits at the uV level. > >Cheers, Jim Moritz >73 de M0BMU > > > >. > > >