Instrumentation op-amp for DC-coupling to audio input?
in [Design]
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From: Lostgallifreyan on 13 Jul 2010 10:18 Lostgallifreyan <no-one(a)nowhere.net> wrote in news:Xns9DB497D3D3EBAzoodlewurdle(a)216.196.109.145: >> What's the source impedance driving the opamp? >> > > Good question, and one I've yet to follow up, my understanding of these > things has only just reached that bit.. I learned that two input noise > figures can be divided one by the other to find out the ideal source > impedance to feed a given input with, but I only read that last night, > these things take time to explore... So far I've always used the basic > logic that is usually applied to avoid precise impedance matching: make > sure the source is very low, and the input very high. This is apparently > fine for readio reception and most audio couplings, so I assumed I could > do it too. I took the idea further, I assumed that if I keep the adapter > as simple as possible I can reduce noise more than it will rise due to > thermal noise in large resistor values, hence I used a passive adder > with 1Meg resistors. I can change this to 100K perhaps, at risk of > drawing more power. This method already works fine in my power meter > design so I guess it's ok here too. > I just found this via Google: "Typical low-noise work with bipolar input transistors depends upon having a low-impedance signal source to shunt and thus reduce noise developed in the input transistor. The transistor noise flows out the preamp input into the signal source output. That means the impedance between the signal source and the preamp op amp must be low at the frequency of interest, say, a tenth of the target impedance." I guess that means my idea falls foul of ideal practise because the OPA2277 is a bipolar input device, and I usually do that passive adder trick with high values and a JFET input amp. The laser power meter showed no apparent noise problems despit using a differentiator in its design that might amplify any noise problem that did exist, but even so, that can't be expected to hold true for audio up to 24 KHz I guess. I still think it will work ok though, I'll just not be soldering any IC's in till I've tested stuff. There's an upgrade to the LF412 called AD712 that I liked in principle, but its only ideally aimed at 12 bit systems, so while it will be fine for my logging intents, it won't preservew the audio specs of ther device I'm modifying.. I'll look up the OPA134 and OPA164 amps, but I think knowing more about the limits imposed by context will tell me more about what I need to know that looking up yet more op-amps. :)
From: George Herold on 13 Jul 2010 10:32 On Jul 13, 9:55 am, Lostgallifreyan <no-...(a)nowhere.net> wrote: > George Herold <gher...(a)teachspin.com> wrote in news:c9ea2d68-021e-4387-8389- > a53e590bf...(a)b35g2000yqi.googlegroups.com: > > > Hi, I usually reserve the name 'instrumentation amp' for those > > differential input three opamp things that I use for bridge circuits. > > That said, I do like the OPA277 for DC measurments. Do you need the > > 10uV offset? The OPA227 is a bit faster, but has a bit more DC offset > > voltage. > > It makes life easier if I have that low offset. :) I'd also strongly prefer > to use what I have, it means I can buy more cheaply and try to encourage > makers to persist in making and selling certain devices by staying with > those. I also like dual-amp IC's a lot, I find them very practical to lay out > compact boards for them. > > I see what you mean about the three-amp devices, with no compromise on input > resistance between the two inputs of a single amp. I guess I use the term > 'instrumentation' fairly loosely, based on intended purpose rather than the > device itself. > > > Have you looked at the OPA134? There is also a newer version... I > > think the number is OPA164??, (I couldn't get the TI website to work) > > smaller voltage noise than the 134 but more cuurent noise and large > > capacitance on the input. What's the source impedance driving the > > opamp? > > Good question, and one I've yet to follow up, my understanding of these > things has only just reached that bit.. I learned that two input noise > figures can be divided one by the other to find out the ideal source > impedance to feed a given input with, but I only read that last night, these > things take time to explore... So far I've always used the basic logic that > is usually applied to avoid precise impedance matching: make sure the source > is very low, and the input very high. This is apparently fine for readio > reception and most audio couplings, so I assumed I could do it too. I took > the idea further, I assumed that if I keep the adapter as simple as possible > I can reduce noise more than it will rise due to thermal noise in large > resistor values, hence I used a passive adder with 1Meg resistors. I can > change this to 100K perhaps, at risk of drawing more power. This method > already works fine in my power meter design so I guess it's ok here too. Oh I've been doing all sorts of noise stuff lately. I don't do much A- D, so if I make a mistake there I hope someone will correct me. So 20 bits is about 10^6, if you have 10 volts full scale that means 10 uV is your LSB. So if your noise is much greater than 10uV you are losing resolution. Lets do the voltage noise first and use the opa277. The voltage noise is 8nV/rtHz. Your band width is maybe 100kHz? (Do you have any filtering before the A-D) So the rms noise from the opamp will be about 8nV *sqrt (100k Hz) ~ 2.5 uV. That looks fine. 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... And now the current noise. The current noise ofthe opa277 is 0.2pA/rtHz. (Hey that's pretty good for a BJT front end) times 100kohm is 20nV/rtHz of voltage noise or about 7uV rms. (assuming I guessed you bandwidth correctly.) To get the total noise you have to add in quadrature. (sqrt of the sum of squares) Which means it's only the big one that matters. The 14 uVrms from the 100k resistor. Reduce the resistance a bit more and this looks fine. (unless I've made some hugh blunder.) George H.
From: Lostgallifreyan on 13 Jul 2010 11:02 George Herold <gherold(a)teachspin.com> wrote in news:a02bdb85-1047-4ba3-aaec- b24eee25f070(a)s9g2000yqd.googlegroups.com: > On Jul 13, 9:55�am, Lostgallifreyan <no-...(a)nowhere.net> wrote: >> George Herold <gher...(a)teachspin.com> wrote in news:c9ea2d68-021e-4387-83 > 89- >> a53e590bf...(a)b35g2000yqi.googlegroups.com: >> >> > Hi, �I usually reserve the name 'instrumentation amp' for those >> > differential input three opamp things that I use for bridge circuits. >> > That said, I do like the OPA277 for DC measurments. �Do you need the >> > 10uV offset? �The OPA227 is a bit faster, but has a bit more DC offse > t >> > voltage. >> >> It makes life easier if I have that low offset. :) I'd also strongly pref > er >> to use what I have, it means I can buy more cheaply and try to encourage >> makers to persist in making and selling certain devices by staying with >> those. I also like dual-amp IC's a lot, I find them very practical to lay > out >> compact boards for them. >> >> I see what you mean about the three-amp devices, with no compromise on in > put >> resistance between the two inputs of a single amp. I guess I use the term >> 'instrumentation' fairly loosely, based on intended purpose rather than t > he >> device itself. >> >> > Have you looked at the OPA134? �There is also a newer version... I >> > think the number is OPA164??, (I couldn't get the TI website to work) >> > smaller voltage noise than the 134 but more cuurent noise and large >> > capacitance on the input. �What's the source impedance driving the >> > opamp? >> >> Good question, and one I've yet to follow up, my understanding of these >> things has only just reached that bit.. I learned that two input noise >> figures can be divided one by the other to find out the ideal source >> impedance to feed a given input with, but I only read that last night, th > ese >> things take time to explore... So far I've always used the basic logic th > at >> is usually applied to avoid precise impedance matching: make sure the sou > rce >> is very low, and the input very high. This is apparently fine for readio >> reception and most audio couplings, so I assumed I could do it too. I too > k >> the idea further, I assumed that if I keep the adapter as simple as possi > ble >> I can reduce noise more than it will rise due to thermal noise in large >> resistor values, hence I used a passive adder with 1Meg resistors. I can >> change this to 100K perhaps, at risk of drawing more power. This method >> already works fine in my power meter design so I guess it's ok here too. > > Oh I've been doing all sorts of noise stuff lately. I don't do much A- > D, so if I make a mistake there I hope someone will correct me. > > So 20 bits is about 10^6, if you have 10 volts full scale that means > 10 uV is your LSB. So if your noise is much greater than 10uV you are > losing resolution. Lets do the voltage noise first and use the > opa277. The voltage noise is 8nV/rtHz. Your band width is maybe > 100kHz? (Do you have any filtering before the A-D) So the rms noise > from the opamp will be about 8nV *sqrt (100k Hz) ~ 2.5 uV. That looks > fine. 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... And now the current noise. The > current noise ofthe opa277 is 0.2pA/rtHz. (Hey that's pretty good for > a BJT front end) times 100kohm is 20nV/rtHz of voltage noise or > about 7uV rms. (assuming I guessed you bandwidth correctly.) To get > the total noise you have to add in quadrature. (sqrt of the sum of > squares) Which means it's only the big one that matters. The 14 > uVrms from the 100k resistor. Reduce the resistance a bit more and > this looks fine. (unless I've made some hugh blunder.) > > George H. > Thanks a lot, it's a good example for me to learn from. I don't know enough to be sure but I worked it through and it looks right to me. I can certainly lower the passive adder resistances, the only penalty is increased current draw (there might be 8 or more channels of this), and I'm hoping to hitch a ride on existing PSU's of anything I adapt this to, but I don't think they'll begrudge me a couple of tens of mA. BTW, I think the device I'm adapting already has noise enough to reduce its claim to useful 20 bit depth but I appreciate the rigour, I wouldn't want to be adding to the problem.
From: Lostgallifreyan on 13 Jul 2010 11:26 George Herold <gherold(a)teachspin.com> wrote in news:a02bdb85-1047-4ba3-aaec- b24eee25f070(a)s9g2000yqd.googlegroups.com: > Your band width is maybe > 100kHz? (Do you have any filtering before the A-D) Sorry, I didn't remember to directly answer that... They just have a differential input stage that adds the two differential signals (doubles) then quarters the output in the gain of the second of two amps they do this with, so halving the total signal. After that it goes through a digitally controlled analog variable resistor (CS3310-KS) followed by a DC blocking capacitor and then an op-amp stage to make a differential signal for the ADC (CS5335) with a 2.2nF cap with two 150R resistors to remove anything above a few hundred KHz. Apart from using that variable resistor IC they kept it as simple as they could. Total dynamic range through the system is 98 dB, so only a tad more than 16 bits worth, but aiming for better in my DC coupler board means I can adapt other devices reliably.
From: John Devereux on 13 Jul 2010 11:47
Lostgallifreyan <no-one(a)nowhere.net> writes: > I'm considering an op-amp for making a DC coupling adapter to a soundcard to > convert it to signal logging purposes while retaining its audio performance. > It uses a passive adder and a gain of 2 to add a bias voltage to the signal > before an ADC input. > > The sound card is one with external analog circuitry in a rack unit, it has > 20 bit signal conversion, so this op-amp will have to be good to maintain > that and the other specs this unit has. Audio ADCs usually have bad DC specifications - why wouldn't they. You may want to verify this before you try too hard to find the perfect opamp. [...] -- John Devereux |