C8051F850-C-GM Datasheet, PDF (105/328 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

14.8. Voltage and Ground Reference Options
The voltage reference multiplexer is configurable to use an externally connected voltage reference, the
internal voltage reference, or one of two power supply voltages. The ground reference mux allows the
ground reference for ADC0 to be selected between the ground pin (GND) or a port pin dedicated to analog
ground (AGND).
The voltage and ground reference options are configured using the REF0CN register.
Important Note About the VREF and AGND Inputs: Port pins are used as the external VREF and AGND
inputs. When using an external voltage reference, VREF should be configured as an analog input and
skipped by the digital crossbar. When using AGND as the ground reference to ADC0, AGND should be
configured as an analog input and skipped by the Digital Crossbar.
14.8.1. External Voltage Reference
To use an external voltage reference, REFSL should be set to 00. Bypass capacitors should be added as
recommended by the manufacturer of the external voltage reference. If the manufacturer does not provide
recommendations, a 4.7uF in parallel with a 0.1uF capacitor is recommended.
14.8.2. Internal Voltage Reference
For applications requiring the maximum number of port I/O pins, or very short VREF turn-on time, the high-
speed reference will be the best internal reference option to choose. The internal reference is selected by
setting REFSL to 11. When selected, the internal reference will be automatically enabled/disabled on an
as-needed basis by the ADC. The reference can be set to one of two voltage values: 1.65 V or 2.4 V,
depending on the value of the IREFLVL bit.
For applications with a non-varying power supply voltage, using the power supply as the voltage reference
can provide the ADC with added dynamic range at the cost of reduced power supply noise rejection. To
use the external supply pin (VDD) or the 1.8 V regulated digital supply voltage as the reference source,
REFSL should be set to 01 or 10, respectively.
Internal reference sources are not routed to the VREF pin, and do not require external capacitors. The
electrical specifications tables detail SAR clock and throughput limitations for each reference source.
14.8.3. Analog Ground Reference
To prevent ground noise generated by switching digital logic from affecting sensitive analog
measurements, a separate analog ground reference option is available. When enabled, the ground
reference for the ADC during both the tracking/sampling and the conversion periods is taken from the
AGND pin. Any external sensors sampled by the ADC should be referenced to the AGND pin. If an
external voltage reference is used, the AGND pin should be connected to the ground of the external
reference and its associated decoupling capacitor. The separate analog ground reference option is
enabled by setting GNDSL to 1. Note that when sampling the internal temperature sensor, the internal chip
ground is always used for the sampling operation, regardless of the setting of the GNDSL bit. Similarly,
whenever the internal 1.65 V high-speed reference is selected, the internal chip ground is always used
during the conversion period, regardless of the setting of the GNDSL bit.
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