All audio data converters require a DC reference voltage. The PCM3120-Q1 achieves low-noise performance by internally generating a low-noise reference voltage. This reference voltage is generated using a band-gap circuit with high PSRR performance. This audio converter reference voltage must be filtered externally using a minimum 1-µF capacitor connected from the VREF pin to analog ground (AVSS).

The value of this reference voltage can be configured using the P0_R59_D[1:0] register bits and must be set to an appropriate value based on the desired full-scale input for the device and the AVDD supply voltage available in the system. The default VREF value is set to 2.75 V, which in turn supports a 2-V_RMS differential full-scale input to the device. The required minimum AVDD voltage for this mode is 3 V. Table 8-11 lists the various VREF settings supported along with required AVDD range and the supported full-scale input signal for that configuration.

To achieve low-power consumption, this audio reference block is powered down as described in the Section 8.4.1 section. When exiting sleep mode, the audio reference block is powered up using the internal fast-charge scheme and the VREF pin settles to its steady-state voltage after the settling time (a function of the decoupling capacitor on the VREF pin). This time is approximately equal to 3.5 ms when using a 1-μF decoupling capacitor. If a higher-value decoupling capacitor is used on the VREF pin, the fast-charge setting must be reconfigured using the VREF_QCHG (P0_R2_D[4:3]) register bits, which support options of 3.5 ms (default), 10 ms, 50 ms, or 100 ms.