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参数资料
型号： ST72621L4M1
厂商： STMICROELECTRONICS
元件分类： 微控制器/微处理器
英文描述： 8-BIT, FLASH, 8 MHz, MICROCONTROLLER, PDSO34
封装： 0.300 INCH, LEAD FREE, SHRINK, PLASTIC, SOP-34
文件页数： 26/139页
文件大小： 1264K
代理商： ST72621L4M1
121/139
Doc ID 6996 Rev 5
ADC CHARACTERISTICS (Cont’d)
12.12.0.1 Analog Power Supply and Reference
Pins
Depending on the MCU pin count, the package
may feature separate VDDA and VSSA analog pow-
er supply pins. These pins supply power to the A/D
converter cell and function as the high and low ref-
erence voltages for the conversion. In some pack-
ages VDDA and VSSA pins are not available (refer
to Table 1, “Device Pin Description,” on page 8). In
this case the analog supply and reference pads
are internally bonded to the VDD and VSS 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
10.7.3.2 PCB Design Guidelines).
12.12.0.2 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
digital ground plane via a single point on the
PCB.
– Filter power to the analog power planes. The
best solution is to connect capacitors, with good
high frequency characteristics, between the
power and ground lines, placing 0.1F and 10pF
capacitors as close as possible to the ST7 power
supply pins and a 10F capacitor close to the
power source (see Figure 84).
– The analog and digital power supplies should be
connected in a star network. Do not use a resis-
tor, as VDDA 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.
12.12.0.3
Specific
Application
Design
Guidelines
– When a USB transmission is taking place during
A/D conversion, the noise caused on the analog
power supply by the USB transmission may re-
sult in a loss of ADC accuracy.
– If the USB is used to supply power to the appli-
cation, this causes noise which may result in a
loss of ADC accuracy.
Figure 84. Power Supply Filtering
VSS
VDD
0.1
μF
10pF
VDD
ST72XXX
VDDA
VSSA
POWER
SUPPLY
SOURCE
ST7
DIGITAL NOISE
FILTERING
EXTERNAL
NOISE
FILTERING
10
μF
0.1
μF
10pF
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