Is an external capacitor needed on the VeREF+, VREF+ or 
VREF-/VeREF- pins of the ADC12 module?

If you are using an external reference or one of the internal 
references for the positive reference, then  a 5-10uF capacitor
is required on pin VeREF+ or VREF+. If you are using an external 
reference for the negative reference, then a 5-10uF capacitor is 
required on pin VREF-/VeREF-. However, if you use AVCC as the 
positive reference, then no capacitor is required on pin VREF+ 
or VeREF+ and if you use AVSS as the negative reference, then no 
capacitor is required on pin VREF-/VeREF-. When using the AVcc 
and AVss as references the power supply decoupling capacitor 
provides the necessary low impedance path. Please refer the ADC12 
Grounding and Noise Considerations section under the ADC12 chapter 
in the MSP430xxxx User`s guide for more details.