PT2258 volume controller distorts signal. What am I doing wrong?

[jeroen94704]

I'm trying to build a volume controller for a line level signal using the PT2258, which is a dedicated 6-channel volume controller IC. This is the reference circuit from the datasheet:

I built this circuit on a breadboard, and while it's functional there is a weird distortion of the signal. Here's a screenshot of what a sinewave (~300Hz) looks like coming in (yellow) and going out (blue), with the attenuation set to 0 dB:

Note that both signals are DC coupled, so it's clear the outgoing signal gets a DC offset from the PT2258. It also looks like the outgoing signal gets clipped at the bottom somehow.

In any case, I don't understand what I'm doing wrong here. I've rechecked the circuit I built several times and checked all the connections are good. So if anyone has any ideas what's going on here I'd love to hear about it.

Thanks!

 

[rsavas]

Is it clipping?
Datasheet states maximum input is typ. 2.8Vrms using a 9V supply
Looks like you have ~3Vp-p input? What is your supply V?
Reduce your input sine wave V and see what the output does
Good luck

 

[jeroen94704]

Ah, so the supply voltage seems to be the issue. Although Vdd min is specified as 5V, at that voltage an input signal over ~700 mVrms gets clipped, and there are weird transient effects when switching attenuation between 0dB and -79dB. When I increase Vdd to 9 volts there is no more distortion of the signal up to the 2.1 Vrms maximum amplitude my DAC can output.

So this turned out to be much easier to fix than I feared.

Thanks for the help!