I use the MC9S08LL16 device and the ADC. There are few pins associated with ADC: Vdda, Vssa, Vrefh, Vrefl and other. The Vrefl is usually tied to Vssa. I would like to know, what is it good for to have separated the Vrefl pin. Can its voltage level be externally 'shifted'? For example with voltage reference to 1.2V?