The ADS131A0x offers an integrated low-drift, 2.442-V or 4.0-V reference option. For applications that require a different reference voltage, the device offers a reference input option for use with an external reference voltage.
The reference source is selected by the INT_REFEN bit in the A_SYS_CFG register. By default, the external reference is selected (INT_REFEN = 0). The internal voltage reference requires 0.2 ms to settle to 1% and 250 ms to fully settle to 0.01% when switching from an external reference source to the internal reference (using the recommended bypass capacitor values). The external reference input is internally buffered to increase input impedance. Therefore, additional reference buffers are usually not required when using an external reference. Connect the reference voltage to the REFEXT pin when using an external reference.
External band-limiting capacitors determine the amount of reference noise contribution. For high-end systems, choose capacitor values such that the bandwidth is limited to less than 10 Hz so that the reference noise does not dominate the system noise. In systems with strict ADC power-on requirements, using a large capacitor on the reference increases the time for the voltage to meet the desired value, thus increasing system power-on time. Figure 43 illustrates a typical external reference drive circuitry with recommended filtering options.
Set the INTREF_EN bit to 1 in the A_SYS_CFG register to use the internal reference. When the internal reference is selected, use the VREF_4V bit to select between a 2.442-V or 4.0-V reference. By default, the device is set to use the 2.442-V reference. The VREF_4V bit has no function when set to use the external reference. When enabling the negative charge pump with a 3.0-V to 3.45-V analog supply, the internal reference must be set to 2.442 V. Figure 44 shows a simplified block diagram of the internal ADS131A0x reference. The reference voltage is generated with respect to AVSS requiring a direct connection between REFN and AVSS.