C8051F85X86X Datasheet, PDF (86/290 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

C8051F85x/86x
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.
Preliminary Rev 0.6
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