C8051F02X Datasheet, PDF (93/272 Pages) Silicon Laboratories – 8K ISP FLASH MCU Family

C8051F020/1/2/3
10. VOLTAGE REFERENCE (C8051F021/3)
The internal voltage reference circuit consists of a 1.2 V, 15 ppm/°C (typical) bandgap voltage reference generator
and a gain-of-two output buffer amplifier. The internal reference may be routed via the VREF pin to external system
components or to the VREFA input pin shown in Figure 10.1. Bypass capacitors of 0.1 µF and 4.7 µF are recom-
mended from the VREF pin to AGND, as shown in Figure 10.1. See Table 10.1 for voltage reference specifications.
The VREFA pin provides a voltage reference input for ADC0 and ADC1. ADC0 may also reference the DAC0 out-
put internally, and ADC1 may reference the analog power supply voltage, via the VREF multiplexers shown in
Figure 10.1.
The Reference Control Register, REF0CN (defined in Figure 10.2) enables/disables the internal reference generator
and selects the reference inputs for ADC0 and ADC1. The BIASE bit in REF0CN enables the on-board reference
generator while the REFBE bit enables the gain-of-two buffer amplifier which drives the VREF pin. When disabled,
the supply current drawn by the bandgap and buffer amplifier falls to less than 1 µA (typical) and the output of the
buffer amplifier enters a high impedance state. If the internal bandgap is used as the reference voltage generator,
BIASE and REFBE must both be set to 1 (this includes any time a DAC is used). If the internal reference is not used,
REFBE may be set to logic 0. Note that the BIASE bit must be set to logic 1 if either ADC is used, regardless of
whether the voltage reference is derived from the on-chip reference or supplied by an off-chip source. If neither the
ADC nor the DAC are being used, both of these bits can be set to logic 0 to conserve power. Bits AD0VRS and
AD1VRS select the ADC0 and ADC1 voltage reference sources, respectively. The electrical specifications for the
Voltage Reference are given in Table 10.1.
Figure 10.1. Voltage Reference Functional Block Diagram
REF0CN
External
Voltage
Reference
Circuit
VDD
R1
DGND
VREFA
4.7µF
VREF
0.1µF
Recommended Bypass
Capacitors
AV+
1
0
ADC1
Ref
DAC0
Ref
DAC1
x2
REFBE
ADC0
Ref
0
1
BIASE
EN
1.2V
Band-Gap
Bias to
ADCs,
DACs
Rev. 1.4
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