8.3.7 Voltage Reference Inputs
(VREFP, VREFN)

The voltage reference for the ADS1278-SP ADC is the differential voltage between VREFP and VREFN: V_REF = (VREFP – VREFN). The voltage reference is common to all channels. The reference inputs use a structure similar to that of the analog inputs with the equivalent circuitry on the reference inputs shown in Figure 66. As with the analog inputs, the load presented by the switched capacitor can be modeled with an effective impedance, as shown in Figure 67. However, the reference input impedance depends on the number of active (enabled) channels in addition to f_MOD. As a result of the change of reference input impedance caused by enabling and disabling channels, the regulation and setting time of the external reference should be noted, so as not to affect the readings.

Figure 66. Equivalent Reference Input Circuitry

Figure 67. Effective Reference Impedance

ESD diodes protect the reference inputs. To keep these diodes from turning on, make sure the voltages on the reference pins do not go below AGND by more than 0.4 V, and likewise do not exceed AVDD by 0.4 V. If these conditions are possible, external Schottky clamp diodes or series resistors may be required to limit the input current to safe values (see the Absolute Maximum Ratings table).

Note that the valid operating range of the reference inputs is limited to the following parameters:

–0.1 V ≤ VREFN ≤ +0.1 V

VREFN + 0.5 V ≤ VREFP ≤ AVDD + 0.1 V