Instrumentation op-amp for DC-coupling to audio input?

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.

I looked first at a few audio amps and noticed that their claims for CMRR and open-loop gain often fall well short of the claims made for the equipment they go into, but never mind, that's another issue for another day.. :)

Then I looked at a DC instrumentation amp (OPA2277) I'm using in a laser power meter design. If I can use it, it saves me buying varieties of expensive chips in small quantities. Audio boffs high, wide and plentiful will say don't do it, slew rate is slow, etc, but is it?? 0.8V/µS. It doesn't sound a lot when people are saying I need 16V/µS or whatever, but I calculated it, and it looks fine to me. The sound unit I'm adapting to is considerably better than CD quality, sampling with 20 bits at up to 48 KHz, and I calculated that this means a sample at intervals of a tad over 20 µS. As 20 µS of 0.8V/µS is 16V, and as the device I'm adapting to has a ±15V supply and a differential input design that halves the input, the largest possible voltage change will occur, and fully settle, in the time between samples at highest sample rate available.

As all the other figures for dynamic range and noise are so good that they will allow the original specs for the entire unit to remain intact, is there any reason I should not use this op-amp? It's a lot cheaper than any audio amp that looks like it will do as well as this. And as I'm after DC as well as AC capability, it seems that this is the right decision, but I'm interested in other views before I decide anything. (I could just use sockets, but for a low profile board I'll be soldering it in, and don't want to have to mess with that later. :)

Reply to
Lostgallifreyan
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Hi,

So did you think about noise?

are you shure? What's about your supply?

Marte

Reply to
Marte Schwarz

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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. 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?

George H.

Reply to
George Herold

"Marte Schwarz" wrote in news: snipped-for-privacy@mid.uni- berlin.de:

Yes, several times. This amp has THD+N of 0.00005% and is going into a device specificied as 0.002%. Good enough? I think so.

I described that already, with regard to slew rate and maximum possible voltage change in a given time at maximum sample rate, so yes, I think I considered that too.

Reply to
Lostgallifreyan

George Herold wrote in news:c9ea2d68-021e-4387-8389- snipped-for-privacy@b35g2000yqi.googlegroups.com:

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.

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.

Reply to
Lostgallifreyan

Lostgallifreyan wrote in news:Xns9DB497D3D3EBAzoodlewurdle@216.196.109.145:

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. :)

Reply to
Lostgallifreyan

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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.

Reply to
George Herold

George Herold wrote in news:a02bdb85-1047-4ba3-aaec- snipped-for-privacy@s9g2000yqd.googlegroups.com:

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.

Reply to
Lostgallifreyan

George Herold wrote in news:a02bdb85-1047-4ba3-aaec- snipped-for-privacy@s9g2000yqd.googlegroups.com:

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.

Reply to
Lostgallifreyan

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.

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John Devereux
Reply to
John Devereux

John Devereux wrote in news: snipped-for-privacy@devereux.me.uk:

Possibly so, but I have already done somethign similar with a DAC and found it be be excellent at outputting any abitrary DC or slowly changing voltage and holding it where it's meant to be, and the ADC's and DAC's in this equipment were chosen from the same maker. I know I'm making an assumption that it will work, but I do think it will. Besides, I have to make some decent effort to get this op-amp decision right, partly to help me learn, and not least, to make sure than when I do test that assumption, it isn't my choice of ap-amp or adapter design method that is screwing it up. :)

Reply to
Lostgallifreyan

Hi,

Oh I didn't talk about distortion, I mean noise SNR or ENOB in total system.

Marte

Reply to
Marte Schwarz

"Marte Schwarz" wrote in news: snipped-for-privacy@mid.uni-berlin.de:

Not my job. I'm adding an op-amp and four resistors per channel. Not much I can do about the rest of the system, except to note that it claims 20 bits while having a 98dB dynamic range that barely scrapes past 16 bits worth of resolution (though this does apply to the DAC too, the whole chain..). All I'm concerned with is that I do not add to the existing reduction significantly. George Herold already helped me with that, and I'd already suspected I could reduce noise to acceptable levels for real 20 bit (120 dB) dynamic range by lowering the value of the resistors in the passive adder (which George Herold confirms, but with useful detail I didn't yet have). Apart from choosing the op-amp there isn't anything else I can do. I'm not trying to improve the system, just to make effective DC coupling for it without harming it by adding complex circuitry where simple stuff will do it better. If I didn't have to handle the bias voltage as I do, I wouldn't be looking for an op-amp at all.

Some systems are nice, they have no DC bias on either side of the DC blocking cap, but when they do, I can't think of a way to satisfy all conditions without an op-amp to mix the bias voltage into its output without it appearing on the output of the previous stage, so that op-amp must be there if the capacitor isn't.

If there is a simpler and quieter way to mix two signals without output inversion than a passive adder based on an op-amp (gain of 2) and 4 resistors I'd like to know what it is, because in many years I have never found one.

Reply to
Lostgallifreyan

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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.
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OK that's much less of a constraint then. Sounds like you've got more than 100uV of 'noise' in the system already.

George H.

Reply to
George Herold

George Herold wrote in news:734e6be6-17bd-465c-9aaf- snipped-for-privacy@j4g2000yqh.googlegroups.com:

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.

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..).

Reply to
Lostgallifreyan

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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.

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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.

Reply to
George Herold

George Herold wrote in news:11271e1e-5ded-4bee-9852- snipped-for-privacy@j13g2000yqj.googlegroups.com:

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.

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..

Reply to
Lostgallifreyan

George Herold wrote in news:a02bdb85-1047-4ba3-aaec- snipped-for-privacy@s9g2000yqd.googlegroups.com:

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.

Reply to
Lostgallifreyan

There's that Johnson guy again! Why not Steve, Larry or Moo?

Reply to
Jamie

In article , Lostgallifreyan scribeth thus

Is it possible to say more precisely what you are doing with this application at all JOOI?...

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Tony Sayer
Reply to
tony sayer

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