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8XC196MC, MD, MH USER’S MANUAL
10-10
10.5.4 Generating Analog Outputs
PWM modules can generate a rectangular pulse train that varies in duty cycle and period. Filter-
ing this output will create a smooth analog signal. To make a signal swing over the desired analog
range, first buffer the signal and then filter it with either a simple RC network or an active filter.
Figure 10-7 is a block diagram of the type of circuit needed to create the smooth analog signal.
Figure 10-7. D/A Buffer Block Diagram
Figure 10-8 shows a sample circuit used for low output currents (less than 100 µA). Consider
temperature and power-supply drift when selecting components for the external D/A circuitry.
With proper components, a highly accurate 8-bit D/A converter can be made using the PWM.
Figure 10-8. PWM to Analog Conversion Circuitry
Buffer
to Make
Output Swing
Rail
to
Rail
MCS
®
96
Microcontroller
PWM
Filter
(Passive
or
Active)
(Optional)
Power
Amp
(Optional)
Analog
Output
A2391-01
74ACxxx
Buffer
PWM
Analog
Output
Op Amp
R
C
-
+
A2390-02
Consider both ripple and response time requirements when selecting R and C.
MCS
®
96
Microcontroller