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The Vref pin on an ADC/DAC is one of the most critical nodes in a circuit, and is often over looked. Internally, this pin is normally used to supply charge to a Switch capacitor network (in a 1 Bit or Low Bit DAC). The charge is drawn at the internal switching rate – but with an Audio modulation component. Therefore, this Node needs to be well de-coupled at both the Audio Band and “RF” frequencies.

If you where to perform an Impendence plot of any capacitor, you will see a wildly varying curve, with the De-Coupling properties of the capacitor rapidly reducing at LF. This increasing impendence at LF has a large impact on Bass quality – resulting in “1 Bit or Low Bit” DAC’s sounding “light” in the Bass.

The RF quality of the 100nF ceramic is critical, try placing it directly across the DAC Vref Pin and Ground

With the external VGen circuit, Bass becomes very tight – controlled with no “Bloom” and the sound stage opens up!

With the Vref pin - noise is not the only important issue – the foremost reason for using the external VGen circuit is to reduce the VREF impedance across the entire audio band including LF.