Lately I've started designing an audio amplifier with usb input. After searching I found about T.I. and their usb audio interface-codecs.

I am going to talk specifically about the PCM2900C, even thought all of them seem to have a similar dac.

In the datasheet it is stated that the output voltage of this dac is 0.6*Vcccl Vpp with 0.5*Vcccl V center voltage. So how these values affect the outputted ac signal? For example if I were to use 3.3V for Vcccl, then with respect to AGNDC what will the Vout voltage be:

  1. If the connected computer is not outputting any audio.

  2. The min and max voltages when the computer is outputting audio.

Also I am wondering how and if these voltages are affected if I were to add coupling capacitors in series with the Vout pin and the load/speaker/amp.

(I am reffering to the "typical circuit connection" design as demonstrated in the datasheet of the PCM2900C)


1 Answer 1

  1. \$0.5\cdot Vccci\$, or \$0.5 \times 3.3\$ or \$1.65V\$
  2. \$0.6\cdot Vccci\$ p-p around 1.65V or \$1.65-1.1\$ to \$1.65+1.1\$ or \$0.55V\$ to \$2.75V\$

Adding capacitors will remove the DC offset of \$1.65V\$ making the output \$\pm 1.1V\$

  • \$\begingroup\$ With some of these single rail DACs, in addition to the cap, you might need an external declicking circuit to mask off the transient when the chip powers up and the output goes from zero to 1.65 volts. I've used an analog switch and a 100Kohm resistor in parallel to smoothly charge the coupling capacitor before switching on the analog switch, thus removing the power-up pop. I'm not sure if it's needed with the PCM2900; I don't know how gracefully the PCM2900 handles the power up. \$\endgroup\$
    – PkP
    Commented Dec 16, 2016 at 19:00
  • \$\begingroup\$ @PkP All the TI DACs I have worked with have had a "soft start" facility to avoid that power-on click. \$\endgroup\$
    – Majenko
    Commented Dec 16, 2016 at 19:01
  • \$\begingroup\$ Good to know the TI DACs soft start! Thanks, Majenko! \$\endgroup\$
    – PkP
    Commented Dec 16, 2016 at 19:02
  • \$\begingroup\$ @Pkp The datasheet on that one doesn't mention it, however the sample schematics all have a directly connected capacitor and not much else. \$\endgroup\$
    – Majenko
    Commented Dec 16, 2016 at 19:08
  • \$\begingroup\$ @PkP also you would generally have a soft-start in the amplifier (which is maybe what I am thinking, actually - maybe it's the TI amplifiers I have used that soft-start). \$\endgroup\$
    – Majenko
    Commented Dec 16, 2016 at 19:09

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