Audio circuits using Opamps with 5V single supply. How to: virtual ground?

[Chaos_Klaus]

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.

[con-f-use]

Is it as easy as using a voltage divider and a buffer?

Yes. 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.

[Chaos_Klaus]

This is the DAC I have:

http://www.ti.com/lit/ds/symlink/pcm1680.pdf

On page 28 there is an example circuit using an OPA337 for the buffer. 

[LA7SJA]

I use this device http://www.ti.com/lit/ds/symlink/tle2426.pdf it should do the trick in your cirquit.

Johan-Fredrik

[Chaos_Klaus]

Well that is a rather expensive solution. 

What is the advatage over using just any opamp?

[LA7SJA]

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

[Farley]

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

[Delta]

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. 😀

[calin]

This is a nice one -- I keep it bookmarked  http://tangentsoft.net/elec/vgrounds.html

Sincerely 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.

[edavid]

I use this device http://www.ti.com/lit/ds/symlink/tle2426.pdf it should do the trick in your cirquit.

No, it's completely wrong for the job, since the DAC in question generates its own V_COM.

[Delta]

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...

[Chaos_Klaus]

the DAC in question generates its own VCOM

Yes.

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/opa337

I would strongly recommend using at least 12v, this will give you a much wider range (no pun intended) of opamps to use.

Well, 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.

[mikerj]

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...

Note 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.