As far as I know, the 1.8V supply is for VDDD, text below from the Datasheet, the max for VDDD is 2.0V-
The SGTL5000 requires 2 external power supplies: VDDA and VDDIO. An optional third external power supply VDDD may be provided externally to achieve lower power. This external VDDD power supply is required for new designs. A description for the different power supplies is as follows:
• VDDA: This external power supply is used for the internal analog circuitry including ADC, DAC, LINE inputs, MIC inputs, headphone outputs and reference voltages. VDDA supply ranges are shown in Maximum Ratings. A decoupling cap should be used on VDDA, as shown in the typical application diagrams in Typical Applications.
• VDDIO: This external power supply controls the digital I/O levels as well as the output level of LINE outputs. VDDIO supply ranges are shown in Maximum Ratings. A decoupling cap should be used on VDDIO as shown in the typical application diagrams in Typical Applications.
Note that if VDDA and VDDIO are derived from the same voltage, a single decoupling capacitor can be used to minimize cost. This capacitor should be placed closest to VDDA.
• VDDD: This is a digital power supply that is used for internal digital circuitry. An external VDDD power supply is required for new designs. For lowest power, this supply can be driven at the lowest specified voltage given in Maximum Ratings. If an external supply is used for VDDD, a decoupling capacitor is recommended, as shown in the typical applications diagram. VDDD supply ranges are shown in Maximum Ratings for when externally driven. If the system drives VDDD externally, an efficient switching supply should be used or no system power savings is realized.