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ADC CHARACTERISTICS (Cont’d)
12.13.0.1 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 smaller
packages VDDA and VSSA pins are not available
and the analog supply and reference pads are in-
ternally 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
Section12.13.0.2 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. It is rec-
ommended to connect capacitors, with good high
frequency characteristics, between the power
and ground lines, placing 0.1F and optionally, if
needed 10pF capacitors as close as possible to
the ST7 power supply pins and a 1 to 10F ca-
pacitor close to the power source (see
Figure– 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.
12.13.0.3
Software
Filtering
of
Spurious
Conversion Results
For EMC performance reasons, it is recommend-
ed to filter A/D conversion outliers using software
filtering techniques.
Figure 122. Power Supply Filtering
VSS
VDD
0.1
μF
VDD
ST72XXX
VDDA
VSSA
POWER
SUPPLY
SOURCE
ST7
DIGITAL NOISE
FILTERING
EXTERNAL
NOISE
FILTERING
1 to 10
μF
0.1
μF