ST72321B STMICROELECTRONICS [STMicroelectronics], ST72321B Datasheet - Page 168

• Manufacturer Part Number: ST72321B
• Description: 64/44-pin 8-bit MCU with 32 to 60K Flash/ROM, ADC, five timers, SPI, SCI, I2C interface
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

ADC CHARACTERISTICS (Cont’d)
12.12.1 Analog Power Supply and Reference
Pins
Depending on the MCU pin count, the package
may feature separate V
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.
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
12.12.2 General PCB Design
12.12.2 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-
Figure 96. Power Supply Filtering
168/187
alog ground plane should be connected to the
digital ground plane via a single point on the
PCB.
POWER
SUPPLY
SOURCE
V
DD
AREF
1 to 10µF
Guidelines).
and V
ST7
DIGITAL NOISE
FILTERING
SSA
EXTERNAL
NOISE
FILTERING
Section
analog
0.1µF
0.1µF
– Filter power to the analog power planes. It is rec-
– The analog and digital power supplies should be
– Properly place components and route the signal
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
96).
connected in a star network. Do not use a resis-
tor, as V
the A/D converter and any resistance would
cause a voltage drop and a loss of accuracy.
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.
V
V
V
V
SS
DD
AREF
SSA
AREF
ST72XXX
is used as a reference voltage by
Figure