ST72324_05 Datasheet, PDF (144/163 Pages) STMicroelectronics – 5V RANGE 8-BIT MCU WITH 8 TO 32K FLASH, 10-BIT ADC, 4 TIMERS, SPI, SCI INTERFACE

ST72324
ADC CHARACTERISTICS (Cont’d)
12.13.1 Analog Power Supply and Reference
Pins
Depending on the MCU pin count, the package
may feature separate V_AREF and V_SSA analog
power supply pins. These pins supply power to the
A/D converter cell and function as the high and low
reference voltages for the conversion. In some
packages, V_AREF and V_SSA pins are not available
(refer to Section 2 on page 8). In this case the an-
alog supply and reference pads are internally
bonded to the V_DD and V_SS pins.
Separation of the digital and analog power pins al-
low board designers to improve A/D performance.
Conversion accuracy can be impacted by voltage
drops and noise in the event of heavily loaded or
badly decoupled power supply lines (see Section
12.13.2 General PCB Design Guidelines).
12.13.2 General PCB Design Guidelines
To obtain best results, some general design and
layout rules should be followed when designing
the application PCB to shield the noise-sensitive,
analog physical interface from noise-generating
CMOS logic signals.
– Use separate digital and analog planes. The an-
alog ground plane should be connected to the
Figure 86. Power Supply Filtering
1 to 10µF
V_DD
ST7
DIGITAL NOISE
FILTERING
digital ground plane via a single point on the
PCB.
– Filter power to the analog power planes. It is rec-
ommended to connect capacitors, with good high
frequency characteristics, between the power
and ground lines, placing 0.1µF and optionally, if
needed 10pF capacitors as close as possible to
the ST7 power supply pins and a 1 to 10µF ca-
pacitor close to the power source (see Figure
86).
– The analog and digital power supplies should be
connected in a star network. Do not use a resis-
tor, as V_AREF is used as a reference voltage by
the A/D converter and any resistance would
cause a voltage drop and a loss of accuracy.
– Properly place components and route the signal
traces on the PCB to shield the analog inputs.
Analog signals paths should run over the analog
ground plane and be as short as possible. Isolate
analog signals from digital signals that may
switch while the analog inputs are being sampled
by the A/D converter. Do not toggle digital out-
puts on the same I/O port as the A/D input being
converted.
0.1µF V_SS
ST72XXX
V_DD
POWER
SUPPLY
SOURCE
EXTERNAL
NOISE
FILTERING
0.1µF V_AREF
V_SSA
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