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I am designing a DAC and have a Vin of 5V.

For the analog stage I have a couple of options :

  1. Use an Op amp that has +5V in (low audio quality)
  2. Use an Opamp that has +-5V in (better audio quality)

    So I will use option 2 and now I have the choice betwen DC/DC convertor or a charge pump. (to make the negative rail from positive one) (both options cleaned with LDOs and PI filters)

I would like my audio to be as clean as possible , what would you recommend between those choices and why.

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  • \$\begingroup\$ What limits you to bad audio quality with a single supply? \$\endgroup\$ – Colin Nov 7 '17 at 12:17
  • \$\begingroup\$ Vin or Vsupply? \$\endgroup\$ – Andy aka Nov 7 '17 at 13:02
  • \$\begingroup\$ V supply , to be clear. I cannot change it. \$\endgroup\$ – Johan B. Nov 7 '17 at 13:39
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Increasing the headroom of Opamps usually allows you to increase the signal to noise ratio by increasing the gain and thus signal amplitude. Also it helps avoiding clipping due to output limitations of the operational amplifiers.

The +-5V Dual supply will increase the headroom compared to a 5V single supply, but so will a +10V single supply. With dual supplies you don't need virtual grounds and less/no coupling capacitors between stages.

The supplies and the Opamps have to match. Not every Opamp is designed for dual rails or a total supply voltage of +10V.

For low current requirements it's easier and likely more efficient to use a charge pump. Check the current consumption of your output stage and whatever load/receiver you want to connect it to. If you only use a couple of Milliamps a charge pump would be a good choice.

However the switching frequency of many charge pumps ICs are within the audio frequency band or close to it. If the switched capacitor is a multilayer ceramic capacitor (MLCC), it can start to vibrate mechanically and audibly due to the switching.

Using a higher switching frequency or a different capacitor type avoids this. A higher switching frequency should also reduce the ripple of your supply. But power supply rejection ratio (PSRR) goes down with increasing frequency, thus decreasing the former effect. You can also put a low-dropout linear voltage regulator after your switching inverting stage to get a cleaner negative supply voltage.

If you want to learn more about Audio quality in analog stages I recommend reading "Small Signal Audio Design" by Douglas Self. The chapter about power supplies only uses line voltage transformers as first stage though, no switching supplies.

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  • \$\begingroup\$ In this instance I don't think the OP is talking about powering the DAC with the charge pump, it's for an analogue stage afterwards. \$\endgroup\$ – Colin Nov 7 '17 at 13:08
  • \$\begingroup\$ You are right, I misread that. Will update, the effects are similar anyway. \$\endgroup\$ – Grebu Nov 7 '17 at 13:14
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Learn about PI filtering for VDD generation of clean clean clean rails.

There are several answers on VDD filtering.

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