Reference Inputs (V REFP and V REFN )
These high-impedance inputs are used to deliver a differential reference voltage for the analog-to-digital conversion process. ADCs are only as accurate as the reference provided. Any reference-voltage error results in a gain error versus the ideal ADC transfer function (see Chapter 2, Analog-to-Digital Converter ). Errors in the reference voltage affect the accuracy of absolute measurements for both on-chip sensors and external channels. Noise on the reference voltage also adds noise to the ADC conversion and results in more code transition noise or poorer than expected SNR.
For typical usage, the reference voltage between V REFP and V REFN should be maintained at 1.25V ± 0.2% using an external reference IC. Reference voltage ICs that deliver 1.25V are widely available from several vendors. Many vendors offer reference voltage ICs in small packages (SOT-23 and SC70). The 1.25V reference should be placed as close as possible to the reference pins and connected directly to the V REFP input, using the decoupling capacitors recommended in the reference IC data sheet. The recommended reference connections are illustrated in Figure 6-1 .
The XADC also has an on-chip reference option which is selected by connecting V REFP and V REFN to ADCGND as shown in Figure 6-1 . Due to reduced accuracy, the on-chip reference does impact the measurement performance of the XADC as explained previously.