Book Analog Interfacing to Embedded Microprocessors
one system to another. The ground and power connections for the system
may be completely separate. Even in a single system where the grounds
are nominally the same, an optoisolator may be used to prevent current
from flowing from one ground to the other. If one ground is particularly
noisy, such as the ground for a PWM (pulse-width modulated) motor
system, an optoisolator may be used to keep the motor noise out of the logic
ground.
Some optoisolators are available with logic outputs instead of phototran-
sistor outputs. These devices typically place a logic gate inside the IC to
convert the analog output into a digital level.
Optoisolators have the same speed and gain issues that optical sensors do.
The CTR of an optoisolator can be higher, typically in the 20% to 100% range,
because the LED is closer to the phototransistor base.
68 Analog Interfacing to Embedded Microprocessors
Figure 3.13
6N136.
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