ADS8166: range for external reference

Hi Hans,

The voltage reference at REFIO pin (internal or external) is buffered by the internal reference buffer in the ADS8166. The reference buffer output is REFP pin where an external decap is required for supplying transient current to the ADC during conversion.

if an external voltage is forced on REFP pin, it will conflict with the internal reference buffer output and result in significant current draw that can potentially damage either the external voltage source or the ADS8166 itself. The short circuit current limit of the internal reference buffer driving the REFP pin is 30mA.

Importantly, the ADC core is specified for reference voltage >= 2.5V. It does not support lower reference voltage.