Texas Instruments MSP430x1xx manual Voltage Reference Generator, Auto Power-Down

ADC12 Operation

17.2.3 Voltage Reference Generator

The ADC12 module contains a built-in voltage reference with two selectable voltage levels, 1.5 V and 2.5 V. Either of these reference voltages may be used internally and externally on pin VREF+.

Setting REFON=1 enables the internal reference. When REF2_5V = 1, the internal reference is 2.5 V, the reference is 1.5 V when REF2_5V = 0. The reference can be turned off to save power when not in use.

For proper operation the internal voltage reference generator must be supplied with storage capacitance across VREF+ and AVSS. The recommended storage capacitance is a parallel combination of 10-∝F and 0.1-∝F capacitors. From turn-on, a maximum of 17 ms must be allowed for the voltage reference generator to bias the recommended storage capacitors. If the internal reference generator is not used for the conversion, the storage capacitors are not required.

Note: Reference Decoupling

Approximately 200 ∝A is required from any reference used by the ADC12 while the two LSBs are being resolved during a conversion. A parallel combination of 10-∝F and 0.1-∝F capacitors is recommended for any reference used as shown in Figure 17−11.

External references may be supplied for VR+ and VR− through pins VeREF+ and VREF− /VeREF− respectively.

17.2.4 Auto Power-Down

The ADC12 is designed for low power applications. When the ADC12 is not actively converting, the core is automatically disabled and automatically re-enabled when needed. The ADC12OSC is also automatically enabled when needed and disabled when not needed. The reference is not automatically disabled, but can be disabled by setting REFON = 0. When the core, oscillator, or reference are disabled, they consume no current.

17-6 ADC12