Audio circuits using Opamps with 5V single supply. How to: virtual ground? - Page 1
Forum section: electronics / beginners

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 14, 2015, 02:21:44 pm
content: Hey eevblog forum.I'm new here and I've been browsing the forum search results but couldn't really find what I was looking for.Before I go into the details, here is the basic question:When using OpAmps with a 5V single supply in audio circuits, how do I establish a virtual ground that is stable? Is it as easy as using a voltage divider and a buffer?My knowledge level: I studied physics at university for two years and then switched to studying music instead. I know the all the basic physics, but the practical electronics part is a bit new to me.I am working on small audio circuits using opamps. For my current project I decided to go with a 5V single supply configuration. That meant using rail-to-rail OpAmps to not lose so much dynamic range. It also means that I have to establish a virtual ground at 2.5V.The device is supposed to take ADAT data, convert that into 8 analog channels, mix them together and send them to a small headphone amp.ADAT Converter -> DAC -> mixer section -> low pass filter -> headphone amp.for the whole analog signal flow, I need a stable 2.5V virtual ground to reference my signals to, right?The DAC I am using has a dedicated output pin that provides these 2.5V (or Vcc/2). In the datasheet it is shown that it needs an opamp as a buffer. They use some expensive precision opamp for that. What kind of speccs does this op amp have to have? Can I go with a cheaper alternative?What I am basicly looking for are instructions on how to do it right. How to work with a virtual ground at Vcc/2.

author: con-f-use
author_rank: Unknown
timestamp: June 14, 2015, 02:39:49 pm
content: Quote from: Chaos_Klaus on June 14, 2015, 02:21:44 pmYes. But watch how much your buffer sink/source. There should be number of suitable, cheap opamps. I would recommend to post a datasheet for the DAC you already have.

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 14, 2015, 02:48:39 pm
content: This is the DAC I have:http://www.ti.com/lit/ds/symlink/pcm1680.pdfOn page 28 there is an example circuit using an OPA337 for the buffer.

author: LA7SJA
author_rank: Regular Contributor
timestamp: June 14, 2015, 02:49:51 pm
content: I use this devicehttp://www.ti.com/lit/ds/symlink/tle2426.pdfit should do the trick in your cirquit.Johan-Fredrik

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 15, 2015, 08:07:32 am
content: Well thatisa rather expensive solution.What is the advatage over using just any opamp?

author: LA7SJA
author_rank: Regular Contributor
timestamp: June 15, 2015, 10:59:32 am
content: Board space (3-terminal TO-226AA), power consumption (170 uA Typ), it can source and drain 20mA, Excellent Output Regulation-45 uV Typ at IO = 0 to -10 mA +15 uV Typ at IO = 0 to +10 mA and Low-Impedance Output . . . 0.0075 Ohm Typ. I bought quite a few when RoHS was implemented and they sold out the parts with lead so I forgot to check the price, sorry.Johan-Fredrik

author: Farley
author_rank: Regular Contributor
timestamp: June 15, 2015, 02:13:47 pm
content: Here is Analog Devices' application note AN-581, Biasing and Decoupling Op Amps in Single Supply Applications.http://www.analog.com/media/en/technical-documentation/application-notes/AN-581.pdf

author: Delta
author_rank: Super Contributor
timestamp: June 15, 2015, 06:05:00 pm
content: I would strongly recommend using at least 12v, this will give you a much wider range (no pun intended) of opamps to use.Just whack in a 7805 5v regulator to power your digital stuff. 😀

author: calin
author_rank: Regular Contributor
timestamp: June 15, 2015, 09:15:49 pm
content: This is a nice one -- I keep it bookmarkedhttp://tangentsoft.net/elec/vgrounds.htmlSincerely when I need a "weak" negative supply is use one these - ICL7660. It is a "voltage converter" - aka charge pump which will give you a negative rail, not able to handle a lot of power but good enough for opamp stuff, plus as long as you stay relatively far by the rails these are quiet enough. The app note Intersil has on their site has the application - all you need is the chip and two caps and you are set. They are pretty cheap .. I got 50 for like 5$ off ebay.

author: edavid
author_rank: Super Contributor
timestamp: June 15, 2015, 09:29:58 pm
content: Quote from: LA7SJA on June 14, 2015, 02:49:51 pmNo, it's completely wrong for the job, since the DAC in question generates its own VCOM.

author: Delta
author_rank: Super Contributor
timestamp: June 15, 2015, 10:30:20 pm
content: That ICL7660 looks SUPERB! Just ordered 20 on ebay!Using split supplies for audio opamp stuff makes things soooo much easier! Cheers for the info!Ps. Sorry to hijack your thread OP...

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 16, 2015, 08:06:33 am
content: QuoteYes.Well, I think I'll just go with the OPA337 as a buffer for the Vcom pin. Just as described in the application example given in the datasheet of the DAC.http://www.ti.com/product/opa337QuoteWell, the problem is not what opamp to use. It's rather what wall wart to use. 9V ones are pretty cheap. Also, I just want to experiment with these low voltages. I know that higher voltages give me more headroom.

author: mikerj
author_rank: Super Contributor
timestamp: June 16, 2015, 08:30:30 am
content: Quote from: Delta on June 15, 2015, 10:30:20 pmNote that "flying capacitor" switchers such as the 7660 are very noisy electrically. You need a good layout with a solid ground plane and plenty of decoupling.