Instrumentation op-amp for DC-coupling to audio input?
in [Design]
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From: George Herold on 14 Jul 2010 10:33 On Jul 13, 4:53 pm, Lostgallifreyan <no-...(a)nowhere.net> wrote: > George Herold <gher...(a)teachspin.com> wrote in news:734e6be6-17bd-465c-9aaf- > ec3d7423f...(a)j4g2000yqh.googlegroups.com: > > >> (CS5335) with a 2.2nF cap with two 150R resistors to remove anything abov > > e a > >> few hundred KHz. > > > 300 ohms inline with the signal and 2.2nF to ground? Something like > > 240kHz if I punched the numbers correctly. You could scale the above > > noise numbers up a factor of sqrt (2.4). If you want to be real fussy > > there is something called the equivalent noise bandwidth. (ENBW) > > (Your low pass filter is not a brick wall and higher frequencies add > > more noise.) For a simple RC the ENBW is 3.1415/2 times bigger than > > the 3dB corner frequency. > > Hmm, I didn't describe that filter fully, it's got one 150R in series with > each new signal line (one original plus one from an inverting buffer amp that > copied it at unity gain), and each line then feeds one of two differential > inputs on the ADC, and the 2.2nF cap goes directly across those. By > 1/(2*PI*R*C) I assumed that either 240 KHz or 480 KHz would be the relevant > frequency, and as both easily leave 100 KHz of bandwidth I didn't inquire > further. :) I doubt that any input I'll put into this system will have much > signal above audio band. Oh, your opamps and resistors will have noise that goes out to MHz and beyond. And will only be fall off when you reach the gain BW limit of the opamp. > > I'll rework any other calculations I did for power consumed, resistance for > voltage reference etc, based on passive adders with 47K resistors. That > should allow the extra scaling needed, or close enough. I don't want to go > lower than that. If the OPA2277 will work ok it will also be good for power > consumption, as it's more efficient that the LF412 I originally planned > before I learned how limited it would be. > > I won't go any deeper than this now, I'll try it to see how it can illustrate > what I've seen here. If my original question-and-answer post about slew rates > is ok I've probably got the op-amp I'll stay with. > > BTW, I saw mention of OPA134 (and OPA2134 I think, a dual one) but I haven't > looked into that in much detail yet. Might not have to, I hope to use what I > have first. I drew a blank on OPA164 though, that one apparently doesn't > exist. :) Like LT, they have so many amps that it can be very hard to choose, > so I'll look more for contexts described by people who are working with > similar situations, and see what they're using (is how I bumped into the > OPA134 last night..). The TI website is now working for me. The 'new' audio opamp is the OPA1641, 1642 and 1644. The 'audio' specs on these look pretty good. (You should at least give them a glance.) The only down side is the DC performance. The offset voltage is 0.5mV or 1 mV. George H.
From: Lostgallifreyan on 14 Jul 2010 10:59 George Herold <gherold(a)teachspin.com> wrote in news:11271e1e-5ded-4bee-9852- 9152ec537b42(a)j13g2000yqj.googlegroups.com: >> further. :) I doubt that any input I'll put into this system will have mu > ch >> signal above audio band. > > Oh, your opamps and resistors will have noise that goes out to MHz and > beyond. And will only be fall off when you reach the gain BW limit of > the opamp. > Yes, but I won't be adding to it in any obvious or fixable way, and the systems I'm adapting a DC coupler to will already have filtering for their signal converters unless they weren't designed well. > The TI website is now working for me. The 'new' audio opamp is the > OPA1641, 1642 and 1644. The 'audio' specs on these look pretty good. > (You should at least give them a glance.) The only down side is the > DC performance. The offset voltage is 0.5mV or 1 mV. > > George H. > Thanks. I'll look at them, and if I can afford to, buy some at some point to get used to them. I'm staying with OPA2277 for now though, as DC is what I want, I just wanted to be sure it didn't let me down for AC, and my slew rate and sample rate calculations suggest it won't. I'm not building high gain mic preamps with them..
From: Lostgallifreyan on 14 Jul 2010 11:11 George Herold <gherold(a)teachspin.com> wrote in news:a02bdb85-1047-4ba3-aaec- b24eee25f070(a)s9g2000yqd.googlegroups.com: > What about the Johnson noise of your 100 kohm resistor? > (forget the 1Meg!) it's got 40 nV/rtHz. or about 14uV of noise... > that's starting to have an impact... I decided to reduce to 10K. :) It means I have to stiffen the bias voltage I'm making for it with a voltage follower but it's easier that way, especially given that it might have up to ten inputs tapping it.
From: Jamie on 14 Jul 2010 11:37 Lostgallifreyan wrote: > George Herold <gherold(a)teachspin.com> wrote in news:a02bdb85-1047-4ba3-aaec- > b24eee25f070(a)s9g2000yqd.googlegroups.com: > > >>What about the Johnson noise of your 100 kohm resistor? >>(forget the 1Meg!) it's got 40 nV/rtHz. or about 14uV of noise... >>that's starting to have an impact... > > > I decided to reduce to 10K. :) It means I have to stiffen the bias voltage > I'm making for it with a voltage follower but it's easier that way, > especially given that it might have up to ten inputs tapping it. There's that Johnson guy again! Why not Steve, Larry or Moo?
From: tony sayer on 14 Jul 2010 11:41
In article <Xns9DB5A2B3F13DDzoodlewurdle(a)216.196.109.145>, Lostgallifreyan <no-one(a)nowhere.net> scribeth thus >George Herold <gherold(a)teachspin.com> wrote in news:11271e1e-5ded-4bee-9852- >9152ec537b42(a)j13g2000yqj.googlegroups.com: > >>> further. :) I doubt that any input I'll put into this system will have mu >> ch >>> signal above audio band. >> >> Oh, your opamps and resistors will have noise that goes out to MHz and >> beyond. And will only be fall off when you reach the gain BW limit of >> the opamp. >> > >Yes, but I won't be adding to it in any obvious or fixable way, and the >systems I'm adapting a DC coupler to will already have filtering for their >signal converters unless they weren't designed well. > >> The TI website is now working for me. The 'new' audio opamp is the >> OPA1641, 1642 and 1644. The 'audio' specs on these look pretty good. >> (You should at least give them a glance.) The only down side is the >> DC performance. The offset voltage is 0.5mV or 1 mV. >> >> George H. >> > >Thanks. I'll look at them, and if I can afford to, buy some at some point to >get used to them. I'm staying with OPA2277 for now though, as DC is what I >want, I just wanted to be sure it didn't let me down for AC, and my slew rate >and sample rate calculations suggest it won't. I'm not building high gain mic >preamps with them.. Is it possible to say more precisely what you are doing with this application at all JOOI?... -- Tony Sayer |