Avoiding IIR instability in DAC

Uri Choresh

Other Parts Discussed in Thread: PCM4104

We use 16bit 16ksps CODECs and have some problem when our central DSP inserts a "zero-signal" when squelching an audio input that have no sufficiant audio energy: that squelch is randomly causing our DAC to produce self oscillations in frequencies higher then the 16khz sampling of our system. We think it is the IIR instability for no-signal at the DAC, as the DAC has decimating post filter that may not designed to handle sero-signal conditions. Do anybody knows a work-around for this fenomenom, or a better CODEC DAC that is better behaved in this situation? Is gradual squelch helps? Is a "non-audio" tone injection may help?

over 15 years ago

Dominik Hartl11 over 15 years ago

TI__Guru 51346 points

What you experience is most likely the idle tone problem that is inherent to basic delta-sigma DACs. It is produced by the modulator for DC and very low frequency input data. TI has high performance delta sigma DACs that address this problem and have reduced or no idle tones.

Please take a look at the PCM17xx family or the PCM4104.

Uri Choresh over 15 years ago in reply to Dominik Hartl11

Prodigy 40 points

Can this sensitivity to low-frequency input can cause a persistant self-oscillations of 16 khz & up while the digital input to the DAC is hel at zero TDM?

Dominik Hartl11 over 15 years ago in reply to Uri Choresh

TI__Guru 51346 points

Yes. Idle tone (digital input held at 0) can (and will) produce self-oscillations (in your case at 16kHz+) in sigma-delta DACs that don't have an idle-tone reduction mechanism (like internal dithering, which essentially always presents a non-DC signal to the DAC internally).

Uri Choresh over 15 years ago in reply to Dominik Hartl11

Prodigy 40 points

Thanks. Can you recommend some codcs that do not have this instability?

Dominik Hartl11 over 15 years ago in reply to Uri Choresh

TI__Guru 51346 points

The PCM17xx family and the PCM4104 are examples that don't have this problem with idle tones.

Audio

Audio forum

Avoiding IIR instability in DAC