8.3.3.1.5 Voltage Reference Inputs (VREFP, VREFN)

The voltage reference of the ADS1284 is the differential voltage applied between pins VREFP and VREFN:

The reference inputs use a structure similar to that of the analog inputs with the circuitry of the reference inputs shown in Figure 42. The average load presented by the switched-capacitor reference input can be modeled with an effective differential impedance of:

Note that the effective impedance of the reference inputs loads the external reference.

R_EFF shown for high-resolution mode operation. R_EFF for low-power mode operation is 170 kΩ

Figure 42. Simplified Reference Input Circuit

Place a 0.1-µF ceramic capacitor directly between the ADC VREFP and VREFN pins. Multiple ADC applications can share a single voltage reference, but must have individual capacitors placed at each ADC.

The ADS1284 reference inputs are protected by ESD diodes. In order to prevent these diodes from turning on, the voltage on either input must stay within the range shown in Equation 8:

Equation 8.

The minimum operational input range for VREFN is AVSS – 0.1 V, and the maximum operational range for VREFP is AVDD + 0.1 V.

To achieve the best ADC performance, use a low-noise 5-V voltage reference. A 4.096-V or 4.5-V reference voltage can be used; however, these lower reference voltages reduce the signal input range and corresponding decrease SNR. Noise and drift on the reference degrade overall system performance. To achieve optimum performance, give attention to the circuitry providing the reference voltage including possible use of noise filtering. See the Application Information section for reference recommendations.