The REF33xx series of precision low-power voltage references pair well with the low power consumption of the MSP430, while achieving the target accuracy goals. The 16-bit converter in the MSP430F2013 accepts an external reference voltage from 1 V to 1.5 V with a typical reference input of 1.25 V, as shown in Table 2.
|VREF(I)||Input voltage range||VCC = 3 V, SD16REFON = 0||1||1.25||1.5||V|
|IREF(I)||Input current||VCC = 3 V, SD16REFON = 0||50||nA|
The REF3312 provides the desired 1.25-V reference voltage for the MSP430 ADC. The accuracy of the REF3312 output, shown in the Electrical Characteristics, directly affects the accuracy of the entire system and must be less than the desired unadjusted error goals. The REF3312 maximum ±0.15% initial accuracy specification is equal to the unadjusted error design goal of 0.15%, indicating that most of the error budget in this design must be devoted to the reference accuracy.
The 3.3-V system supply voltage that powers the MSP430 can also supply other devices, and therefore may have regulation and noise issues. The REF3330 creates an accurate and stable 3.0 V output used by the op amp, REF3312, and other low-power analog circuitry. The REF33xx series has a drop-output voltage of VOUT + 200 mV; therefore, as long as the input supply remains above 3.2 V, the REF3330 produces a regulated 3.0 V output. The output current for the REF33xx series is specified at ±5 mA, as shown in Figure 9, and is sufficient for the REF3312 and a low-power op amp.