The ADS8681W incorporates a high precision 4.096V internal voltage reference. Alternatively, the REF5040 (U2), an onboard external 4.096V precision voltage reference, is selectable for evaluation purposes if the external voltage reference is necessary for a system application. The reference voltage source is determined by bit 6 of the RANGE_SEL_REG register of the ADS8681W. Configure the reference settings on the ADS8681WEVM-PDK by navigating to the Register Map Config page on the GUI as described in Section 4.1.2. By default, the internal reference is enabled after the ADC is powered up, so the jumper on J3 must not be connected when powering up. If the ADS8681W must be configured with the external reference, make sure to disable the internal voltage reference by setting bit 6 of the RANGE_SEL_REG register before using a jumper on J3 to connect the external reference. The schematic for the reference path is shown in Figure 2-3.

Figure 2-3 Onboard Reference Signal Path