An optional negative charge pump is available to power AVSS with an operating voltage of –1.95 V. Enabling the negative charge pump allows for input signals below analog ground when using a unipolar analog supply (for example, AVDD = 3.3 V, AVSS = 0 V). The VNCPEN bit in the A_SYS_CFG register must be set high by the user to enable the negative charge pump. The VNCP pin outputs the nominal –1.95-V negative charge pump output and requires a capacitor to AVSS in the range of 220 pF to 470 pF. The charge pump operates at a switching frequency of 2fMOD. The minimum ADC absolute input voltage range is –1.5 V with the negative charge pump enabled. The maximum analog supply limit (AVDD – AVSS) is restricted to 3.6 V maximum. Exceeding this limit can permanently damage the device.
The negative charge pump is internally activated when the VNCPEN bit is set to 1 and the device is in wake-up mode with all ADC channels enabled (ADC_ENA = 0Fh).
Connect VNCP directly to AVSS when not using the negative charge pump.