AN729
REPLACING TRADITIONAL OPTOCOUPLERS WITH Si87XX
DIGITAL ISOLATORS
1. Introduction
Opto-couplers are a decades-old technology widely used for signal isolation, typically providing safety isolation,
signal level shifting, and ground loop mitigation. They are commonly used in a wide range of end applications,
including data communication circuits, switch mode power systems, measurement and test systems, and isolated
data acquisition systems. Optocouplers have several weaknesses, including parametric instability with temperature
and device aging, significant internal parasitic couplings, long propagation delay times, narrow operating
temperature ranges, and relatively low reliability. Optocouplers have been the “go-to” isolation device of choice for
the past 30+ years because they were the only integrated solutions to the problem of signal isolation. Today's
advanced CMOS signal isolation products offer better timing performance, higher reliability, and lower power
consumption compared to optocouplers and are capturing sockets traditionally held by optocouplers. However,
converting to CMOS isolation devices has, most often, required circuit changes and PCB modifications that cost
money and create design risks, until now.
The Si87xx family can be used as a pin-compatible optocoupler upgrade in existing systems or as a design-in
component for new product designs. Package and pin compatibility allow the Si87xx to replace optocouplers
without PCB modifications, with substantial gains in device performance and reliability. Device operation is simple:
the Si87xx output is held low when anode current is above the turn-on current threshold and pulled high by an
internal or external pull-up resistor when anode current is below the turn-off current threshold. (For more
information about current thresholds and hysteresis values, see the Si87xx data sheet.)
2. Si87xx Operation
Figure 1 shows an Si87xx block diagram in which the input-side circuit contains a diode emulator, high-frequency
transmitter, and galvanic isolator, all of which are powered by the voltage present on the anode pin. The diode
emulator mimics the behavior of an optocoupler LED to ensure compatibility with existing optocoupler input circuits.
Device operation is straightforward: the diode emulator enables the transmitter when anode current is above its
turn-on threshold. This action causes the transmitter to propagate a high-frequency carrier across the isolation
barrier to the receiver, which, in turn, forces the output driver low. Conversely, an anode current below the turn-off
threshold disables the transmitter, causing the receiver to release the output pin to be pulled high by the pull-up
resistor.
VBIAS
VDD
NC
OUTPUT DIE
INPUT DIE
V
IF
RF
ANODE
NC
V
DDI
DD2
VF
ENABLE
e
XMIT
RECV BUFFER
CATHODE
VO
GNDI
GND2
GND
NC
Si8710 Digital Isolator
Figure 1. Si8710 Digital Isolator Block Diagram
Rev. 0.1 8/12
Copyright © 2012 by Silicon Laboratories
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