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Electrical characteristics
ST72324B-Auto
12.13.1
Analog power supply and reference pins
Depending on the MCU pin count, the package may feature separate VAREF and VSSA
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, VAREF and
and reference pads are internally bonded to the VDD and VSS pins.
Separation of the digital and analog power pins allow board designers to improve A/D
performance. Conversion accuracy can be impacted by voltage drops and noise in the event
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 analog 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 recommended 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 capacitor close to the power source (see
Figure 85).
●
The analog and digital power supplies should be connected in a star network. Do not
use a resistor, as VAREF 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 outputs on the
same I/O port as the A/D input being converted.
Figure 85.
Power supply filtering
VSS
VDD
ST72XXX
VAREF
VSSA
Power
supply
source
ST7
digital noise
filtering
External
noise
filtering
1 to 10F
0.1F