
ML66517 Family User’s Manual
Chapter 13
A/D Converter Functions
13 – 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 ML66517 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 0.01
F to 0.1 F between V
REF and AGND, the stability of VREF
can be maintained by connecting a filter capacitor of 10
F to 47 F or larger in parallel. Because the V
REF
voltage supply is used to avoid the effect of digital noise on the comparator used in A/D conversion, adding a
filter capacitor is effective in reducing VREF fluctuations.
Figure 13-9 shows an example of noise-suppression measures.
Figure 13-9
Example of Noise-Suppression Measures
Analog
input
Main clock
oscillator side
Oscillation
circuit
Analog system GND plane
Digital system GND plane
C0: 0.01
F to 0.1 F
(bypass capacitor)
C1: 10
F to 47 F or larger
(filter capacitor)
TOP VIEW
C1
C0
C1
C0
VDD
GND
VREF
AGND
GND
1-PIN
+
VDD GND
GND