Technical question about good low power rail to rail opamp
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From: TheM on 13 Apr 2010 08:31 "Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message news:hq1mqk$bo3$1(a)news.albasani.net... > On a sunny day (Mon, 12 Apr 2010 17:11:10 -0800) it happened Robert Baer > <robertbaer(a)localnet.com> wrote in > <sbudnULSLNOZKl7WnZ2dnUVZ_qSdnZ2d(a)posted.localnet>: > >> !!MAXIM parts??? >> They actually _exist_??? >> Gag me with a spoon! > > How many MAX232 do you want? > :-) Sometimes it looks as if this is their only actual product. Maybe they should be renamed into MAX232 from MAXIM. :) M
From: Jan Panteltje on 13 Apr 2010 08:44 On a sunny day (Tue, 13 Apr 2010 14:31:46 +0200) it happened "TheM" <DontNeedSpam(a)test.com> wrote in <hq1ogf$f4$1(a)news.siol.net>: >"Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message news:hq1mqk$bo3$1(a)news.albasani.net... >> On a sunny day (Mon, 12 Apr 2010 17:11:10 -0800) it happened Robert Baer >> <robertbaer(a)localnet.com> wrote in >> <sbudnULSLNOZKl7WnZ2dnUVZ_qSdnZ2d(a)posted.localnet>: >> >>> !!MAXIM parts??? >>> They actually _exist_??? >>> Gag me with a spoon! >> >> How many MAX232 do you want? >> :-) > >Sometimes it looks as if this is their only actual product. >Maybe they should be renamed into MAX232 from MAXIM. > >:) > >M For me it *is* their only product :-)
From: Spehro Pefhany on 13 Apr 2010 09:41 On Tue, 13 Apr 2010 14:31:46 +0200, "TheM" <DontNeedSpam(a)test.com> wrote: >"Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message news:hq1mqk$bo3$1(a)news.albasani.net... >> On a sunny day (Mon, 12 Apr 2010 17:11:10 -0800) it happened Robert Baer >> <robertbaer(a)localnet.com> wrote in >> <sbudnULSLNOZKl7WnZ2dnUVZ_qSdnZ2d(a)posted.localnet>: >> >>> !!MAXIM parts??? >>> They actually _exist_??? >>> Gag me with a spoon! >> >> How many MAX232 do you want? >> :-) > >Sometimes it looks as if this is their only actual product. >Maybe they should be renamed into MAX232 from MAXIM. > >:) > >M Would that justify the higher price compared to the TI equivalent?
From: MooseFET on 13 Apr 2010 09:51 On Apr 13, 5:27 am, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > On a sunny day (Mon, 12 Apr 2010 19:39:21 -0700 (PDT)) it happened MooseFET > <kensm...(a)rahul.net> wrote in > <ce35884c-01fe-47e8-921d-83da65f06...(a)12g2000yqi.googlegroups.com>: > > > > >On Apr 12, 4:33 am, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > >> Technical question about good low power rail to rail opamp. > >> I published 'scope_pic' > >>http://panteltje.com/panteltje/pic/scope_pic/ > >> and am getting questions as 'to where to connect the input etc..'. > > >> The question is how to make a 1 MOhm, few pF, input with a bandwidth of a= > >bout 1MHz, > >> with as few components as possible, that also creates a +2V DC offset > >> (to get the ADC midrange and allow negative signals), > > >Op-amp idea: > > >The LT1352 gives a darn good performance for very little supply > >current. > > >It is not rail to rail but you could power it from the +/- 9V of an > >LT1081 > > >A simple voltage follower and then an inverting amplifier with the > >noninverting pin at 1V seems like a good way to go. > > >PNP circuit: > >If you don't mind some drift and can afford to lose gain, you could > >use a > >PNP emitter follower with the emitter resistor being a voltage divider > >up to > >the +5V supply. > > >I very bad idea: > >You could use a flying capacitor switching circuit screaming along at > >10 or more > >MHz. > > >> Now My first idea was to make a JFET input, tried something in spice like= > > this: > >>ftp://panteltje.com/pub/fet.gif > > >> There is a lot of spread in individual jfets, and the only negative volta= > >ge > >> available is a tap of the -8 V from a MAX232. > > >The LT1081 is a little more robust than the MAX232. > > Ok, got it, but I already have a whole bunch of MAX232. > > My other idea of an input was this: > > +5 +5 > | | > e e > PNP b-------- b PNP > c | c > +5 | |______| > -2 to | | | sets > +2 V c | R3 DC shift > ---- b NPN e--------1 k ---> 0 to 4V | > e ---------- b PNP R2 /// > | c > R1 | > | -8 > -8 > How about: +5 ! +5 [R] ! ! [R] +---------- Out ! !/c +-------! NPN ! !\e !/e ---[R]-- ----! PNP e\! !\c !----+---!<----[LT1004-2.5]---GND ! /! ! -8V ! ----[R]--- -8 -8 It inverts but it also shifts For a JFET circuit a dual like the SST440 may be the way to go. You could use the second one to compensate for the Vgs of the first.
From: Jim Thompson on 13 Apr 2010 11:01
On Mon, 12 Apr 2010 11:33:36 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >Technical question about good low power rail to rail opamp. > I published 'scope_pic' > http://panteltje.com/panteltje/pic/scope_pic/ >and am getting questions as 'to where to connect the input etc..'. > >The question is how to make a 1 MOhm, few pF, input with a bandwidth of about 1MHz, >with as few components as possible, that also creates a +2V DC offset >(to get the ADC midrange and allow negative signals), > >Now My first idea was to make a JFET input, tried something in spice like this: > ftp://panteltje.com/pub/fet.gif What do you call that configuration, "do nothing" ?:-) > >There is a lot of spread in individual jfets, and the only negative voltage >available is a tap of the -8 V from a MAX232. > >So that leaves opamps, and the ones I found (5V rail to rail) >have either low bandwidth, bad slew rates, >incredible distortion, or all of those at the same time. > >So I am looking for a decent quad 5V to 12 supply opamp with low current draw, >only needs to drive a PIC input, say 10k in 25 pF, with low distortion, high >gain, slew rate > 10V us, voltage swing to very near the positive supply, >input impedance 00, input cap < 10 pF. > >I looked at LMC6036, some others, but no good. >Any ideas? >So it is for the scope input amp. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy |