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MSM66573 Family User's Manual
Chapter 13 A/D Converter Functions
13
13.5.2 Noise-Suppression Measures
Based on the voltage difference between the analog reference voltage (VREF) pin and the
analog ground (AGND), the A/D converter in the MSM66573 family converts an analog
voltage at the analog input pin into digital data. Because this type of A/D converter does
not have a reference voltage source inside the microcomputer, "stability" and "noise-
suppression measures" for VREF and AGND are important.
As noise-suppression measures, insert a bypass capacitor between the analog reference
voltage (VREF) pin and the analog ground (AGND) pin. Also, connect the analog ground
(AGND) to a stable GND on the circuit board.
If the digital and analog layouts can be separated on the circuit board, separate the circuit
into a digital system (VDD/GND) and an analog system (VREF/AGND). Connect bypass
capacitors to each system to reduce the circulation of GND noise in the digital system.
Divide the circuit board into separate GND planes for the digital and analog systems, and
then connect each GND plane to a common location where there is a stable GND supply.
In addition to inserting a bypass capacitor of 10 mF to 47 mF or larger between VREF and
AGND, the stability of VREF can be maintained by connecting a 0.01 mF to 0.1 mF high-pass
capacitor in parallel. Because the VREF voltage supply is used to avoid the effect of digital
noise on the comparator used in A/D conversion, adding a high-pass capacitor is effective
in reducing VREF fluctuations.
Figure 13-9 shows an example of noise-suppression measures.
Analog
input
Main clock
oscillator side
Oscillation
circuit
Analog system GND plane
Digital system GND plane
C0: 0.01 mF to 0.1 mF
C1: 10 mF to 47 mF or larger
TOP VIEW
GND
C1
C0
C1
C0
VDD
GND
VREF
AGND
V
DD
GND
1-PIN
+
Figure 13-9 Example of Noise-Suppression Measures