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I plan on building this buffer circuit to use my sound card as an oscilloscope, but I’m not sure on the best/acceptable way to get the -12V.

  • Can I use a 24V DC power supply and then use a voltage divider (maybe with a zenner diode) to create a virtual ground?
  • Can I use something like an LM7812 to step the voltage down from 24V to 12V and use that as a virtual ground?
  • Or is there a more preferred way to accomplish this that someone can point me to?
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  • \$\begingroup\$ You can use a 24vdc with a voltage divider, just add a non inverting voltage follower after it to stiffen your ground a bit. I suspect your 24vdc is a switching PSU, if this is the case you will probably need a couple of 10V LDOs (if +-10V is enough) to filter hhff noise. \$\endgroup\$ – Vladimir Cravero May 23 '17 at 11:50
  • \$\begingroup\$ Since you're planning to use your PC's sound card, a practical option would be using +12V and -12V outputs of ATX supply. \$\endgroup\$ – Rohat Kılıç May 23 '17 at 11:52
  • \$\begingroup\$ +/-12V is rather over the top for this requirement - the line input of your PC only needs about 1V peak to peak and it's almost certainly AC coupled anyway. Just use a single 12V supply and create a virtual ground reference in the middle. \$\endgroup\$ – Finbarr May 23 '17 at 11:55
  • \$\begingroup\$ @RohatK I plan to use this with a laptop so an ATX power supply is not available. I was thinking about scavenging one from an old computer but they are kinda big and bulky for this project. \$\endgroup\$ – jayveesea May 23 '17 at 15:45
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In general a virtual ground would do the trick, and then you should use something like this:
enter image description here

In this specific case however it may not be a good idea, since the ground is also connected to your oscilloscope, and to the ground of the device you are measuring on. So I would advice you to create the -12V using a simple DC/DC converter. The 'MULTICOMP MCE12S05S Isolated Board Mount DC/DC Converter' from Farnell is only €4,- and would work fine in your application.

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  • \$\begingroup\$ Only if you can afford the noise produced. PSRR isn't infinite for a TL082, particularly up at switching regulator frequencies. \$\endgroup\$ – Warren Young May 23 '17 at 12:16
  • \$\begingroup\$ @Cees Do you mean to use the DC/DC converter with a 24V power supply and step the 24V to 12V to be used as ground? Or can that DC/DC converter convert 12V to -12V??? \$\endgroup\$ – jayveesea May 23 '17 at 15:58
  • \$\begingroup\$ @coppolaij : First, I posted the wrong model number. You will need 'XP POWER IK2412SA' which is a 24VDC to 12VDC converter. So this will indeed convert your 24 V to 12VDC, and since the output is isolated you can just connect the + output to gnd and get a -12V on the other pin. I would add some capacitors on the output to filter noise (100 nF parallel to a 10uF would probably be fine) \$\endgroup\$ – Cees Meijer May 24 '17 at 9:25
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Crazy answer. Don't.

TL082 will easily run off 9V. You'll be using the sound card's line input which is approximated 10k, which is easily driven by this op amp. So use two 9V batteries to create a +/- 9V supply. The power will be as smooth as possible without major shielding. It will certainly be quieter than the analogue and digital chaos inside your PC. Just use a double throw switch to control both sides of the supply simultaneously. Don't forget that you only need about 3Vp-p to max. out your line input, so 12V isn't strictly necessary to use it as an oscilloscope.

I don't know your use case, but I suspect that the number of hours you'll be using this will easily fall into the useful operational life of a good branded alkaline PP3. You'll only be consuming ~30 mA. And it's less hassle than building an active power supply. I've built a very similar buffer for my sound card and it works very well. However, I'm sure that I don't need to remind you of the very serious bandwidth limitations you'll face. You'll achieve 20kHz nominally, but might be able to push that to 96kHz if your sound card / software supports it.

Most importantly though I would share a personal philosophy. I never build test equipment. I buy the best I can and if I can't, I save up till I can. The reason is that if you build your own (and you're an amateur -like me), you'll never have full confidence in it's readings. You'll always be wondering if that 50Hz signal dominating all your results is from the device under test, or mains feeding through your circuit's voltage regulators. At least eliminating your PSU goes some way to meeting this philosophy.

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    \$\begingroup\$ "TL082 will easily run off 9V." Only when run into a high impedance input, and it'll only give you about 1 Vac RMS when doing so, due to the ±3V input-to-rail limit. 1 Vac RMS is 2.828 Vp-p. Throw in battery voltage droop, and, well, let's just say "not recommended." \$\endgroup\$ – Warren Young May 23 '17 at 12:19
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    \$\begingroup\$ I agree with Warren in that +-9 V is not a good match for a TL082. However, plenty of opamps can run nicely from +-9 V. This could still be a valid answer just by recommending a different opamp. \$\endgroup\$ – Olin Lathrop May 23 '17 at 13:12
  • \$\begingroup\$ @OlinLathrop I just went with the OP's choice as that's what my buffer used, and it works well. The TL08x is my go to amp. You're right that there will be more technically appropriate devices. Care to edit? \$\endgroup\$ – Paul Uszak May 23 '17 at 14:03
  • \$\begingroup\$ This for sure would be the easiest and I could always add something else later if it did not work out well. Any advice on an op-amp that would work for this situation that's more appropriate for +/- 9V? \$\endgroup\$ – jayveesea May 23 '17 at 15:52
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The circuit is AC-coupled, so it doesn't technically require a dual-voltage power supply. If you replace the -12 V references with the ground side of a single-ended power supply, you'd just need to work out some way to get a virtual ground in order to bias the input signal up between the I/O limitations of the buffer/amplifier circuit.

The reason people use dual-ended power supplies with op-amp circuits like this is specifically in order to avoid the hassle and problems of virtual ground circuits. There is no difficulty-free solution, only a choice of which bag of problems you want to accept.

Can I use a 24V DC power supply and then use a voltage divider (maybe with a zenner diode) to create a virtual ground?

That's one method, but as you'll see by reading the article linked above and the articles linked from it at the end, it would be facile to describe this as an easy solution.

Your idea is closest to that of the CMoy virtual ground criticized at the top of the article, only without mention of the rail caps, which mitigate a lot of its problems, but not all. You can do a lot better, which is the subject of the rest of that linked article.

Can I use something like an LM7812 to step the voltage down from 24V to 12V and use that as a virtual ground?

Yes, but TANSTAAFL:

  • It is only an exact ½ rail virtual ground to the extent that the 24 V supply and the LM7812 are accurate. If the input supply is 5% high and the half-rail regulator output is 5% low, the virtual ground point will be off by roughly 10% of ideal. Depending on conditions, the virtual ground circuits in the linked article can be more accurate than that.

  • It burns half the rail voltage to achieve this. It'll throw off a fair bit of heat. Probably not enough to require a heat sink, and certainly not enough to require forced-air cooling, but...well, it isn't very engineer-y. Sloppy. Wasteful.

Or is there a more preferred way to accomplish this that someone can point me to?

Lots. Even if you collect all the ideas in my article and those I've linked to, it is still only a subset of the ways people have invented to tackle this very problem.

The most important thing to keep in mind when pursuing all of this is that voltages are relative, not absolute. There is no "12 V", only "12 V with respect to X", where X could be any other voltage potential. Any. Most likely it's 12 V above some ground point, but it could just as well be 12 V above the output of a 500000 V electrical generation plant. Even if you discard wild possibilities like that, you've got many mundane possibilities, like which ground; all "grounds" are not equal.

Also keep in mind that the only reason this circuit you're referring to recommends ±12 V is to make enough room between this op-amp's rails for the input signal to get through and be amplified. If the input signal could be as high as 0.1 V peak-to-peak and the output is therefore up to 1 Vp-p, you could probably push this circuit down as far as 12 V single-ended, depending on the load characteristics. That is an informed guess based on testing.

Keep the load in mind: as those same tests show, if this is supposed to drive 50 Ω input circuitry, you may indeed need 24 V single-ended or ±12 V.

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  • \$\begingroup\$ thanks for the virtual ground link, I was searching for some info like that. \$\endgroup\$ – jayveesea May 23 '17 at 16:11
  • \$\begingroup\$ Reading through the link it seems like the TLE2426 “rail splitter” would do exactly what I need. It looks like the TL082's max input current is 2.8mA and it looks like TLE2426 can output 80mA. \$\endgroup\$ – jayveesea May 24 '17 at 11:27
  • \$\begingroup\$ @coppolajj: 80 mA is the dead-short current, and is meant to tell you the point where the device is expected to melt. You want to stay well under the short-circuit curves given on page 15 in practice. Also, the 2.8 mA spec is only the idle current for the op-amp, not its running current. If driving a substantial load, the current will go out the op-amp's OUT terminal, through the load, and some of it will come back into the buffer's virtual ground. How much comes back depends on the ground paths inside the load circuit. You may need one of the more substantial options in my article. \$\endgroup\$ – Warren Young May 24 '17 at 12:09
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For a one-off project, a really simple way is to get two 12 V wall warts. Connect the - end of wall wart A to the + end of wall wart B and call that ground. The + end of A is +12, and the - end of B is -12.

Again, this isn't for volume productions, but a quick and relatively cheap way to address the problem in a hobby context.

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    \$\begingroup\$ Two isolated wall-warts. Many wall warts are switching-regulated with one of the outputs tied to one of the inputs for safety and noise control, so they'll fight when arranged that way. \$\endgroup\$ – Warren Young May 23 '17 at 12:15
  • \$\begingroup\$ I would substitute 'one of the inputs' with 'earth'. One could almost think it is possible to have the how wire on one of the outputs :D \$\endgroup\$ – Vladimir Cravero May 23 '17 at 13:02
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    \$\begingroup\$ @Warren: Anything called a "wall wart" will be isolated. You'll have a hard time finding one that isn't. I'm torn between mentioning it and causing unnecessary confusion, and not mentioning it and the very rare resulting short. I think I'll just leave it to your comment unless you can show me a consumer wall wart that ties one of its outputs to ground. Note that such a thing would need to have a three prong plug, which is already quite rare. \$\endgroup\$ – Olin Lathrop May 23 '17 at 13:09

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