The ADC integrates an optional level-shift voltage on the AINCOM pin. As shown in Figure 9-26, the level-shift voltage is the mid-voltage of the analog power supply. The level-shift voltage shifts floating sensors (that is, sensors isolated from the ADC ground) to within the ADC specified input range. Thermocouple and 4-mA to 20-mA transmitters (isolated supply) are examples of floating signals.
When operating the ADC with ±2.5-V analog supplies, either ground the AINCOM pin or use the level-shift voltage. Level shift other inputs by connecting the input pins to the REFOUT pin (2.5 V). The turn-on time of the level-shift voltage depends on the pin load capacitance. The total capacitance includes those connected to AVDD, AVSS and ground. Table 9-7 lists the level-shift voltage settling times for various external load capacitances. Be certain the level-shift voltage is fully settled before starting a conversion.
|LOAD CAPACITANCE||LEVEL-SHIFT VOLTAGE SETTLING TIME|
|0.1 µF||0.22 ms|
|1 µF||2.2 ms|
|10 µF||22 ms|