Negative swing with a DAC

[The Master]

Hi, I have an 8-bit DAC which is currently outputting about 0-3V. After some filtering etc this goes into the line-in of an audio amplifier.

Line level is supposed to swing from about -1.5 to 1.5V. I've seen online that this can be achieved by "biasing" the amplifier's ground to 1.5V.

The issue I've got is that the amplifier is powered from the same power supply as the rest of the circuit. The reference ground on the amplifier is the same as the power ground so there's no way for me to bias it.

Is there another way to get the required swing? I feel that I need to connect my DAC to -3V instead of ground but I don't have a negative supply.

[pedja089]

Just add high pass filter
http://www.electronics-tutorials.ws/filter/fil11.gif

[The Master]

Sorry, I should mention that I only have 24V DC available from a switching PSU and the amplifier is single supply (not split).

I could potentially move the entire amplifier ground up by 1.5V as 22.5V would still be enough to power it.
A potential divider won't be suitable to hold the ground of a 100W amp though.

[pedja089]

Did you see my post?
That is common way to remove dc offset and center signal around 0V.

[Dave]

If it were me, I would vary the signal between 1 and 4 volts with the PIC, 2.5 volts being your artificial ground (bias) and then cap couple it to the amplifier. I have done this before with our radio repeater for the courtesy tone's.

[HenrikOlsson]

I'm not sure why you're ignoring the idea of simply AC-coupling the signal into the amplifier but if that doesn't work for whatever reason it might be possible to use the PIC to generate a negative supply rail for the DAC using a chargepump circuit and the CCP module in PWM mode. It'll add circuitry though.

[The Master]

pedja089:-
I did see your post. Are you suggesting to use that circuit on the output from the DAC? I thought the sine wave was representing an AC supply.

Dave/Henrik:-
I've just done a bit of reading and it looks like "cap coupling", "AC-coupling" and pedja089's schematic are all the same thing. I will give that a go and see what happens.

Henrik:-
I have plenty of board space for once so if I do need to add extra stuff it won't be a problem.

[pedja089]

Yes, that circuit should be on output of DAC. And it is just first image from WEB.