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Application Brief

How to Replace Optocouplers with Digital Isolators inStandard Interface Circuits

Sadia Khan
Tony Calabria

Systems Engineer, Isolation, Interface Group
Systems Manager, Interface Group

V1Competitor A1

RL

EM

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TX

ANRi
CATH

GND1

GND2

ISO6721
V2

Competitor A2RXAN

CATH

GND1

ANOD
E

COL

V2

V1Cdcpl
GND1RLRi

Only two channels need to be isolated for UART
and the low speeds make a straightforward and lowcost solution most desirable. Figure 1 shows a UARTinterface isolated using an optocoupler circuit and anequivalent circuit using ISO6721, TI's digital isolator
solution targeted for cost-sensitive applications.

ANOD
E

COL

TX

RX

VCC1TXRX

OUTAINB
GND1EM

GND2

VCC2ISOLATION

Universal Asynchronous Receiver and Transmitter(UART) is a very simple two-wire serial interface
that allows for low-speed communication between twodevices. The two push-pull, unidirectional lines are
Receive (RX) and Transmit (TX). The interface works
without a clock, so the receiver determines the bitrate automatically by sampling the period of positive
voltage pulses. The automatic bit rate detection limits
the max bit rate achieved with UART, making data
rates over 100 kbps extremely rare.

The open collector output requires a pull-up resistor,
RL, to create the logic high state. The value of RL is
selected to minimize power consumption while also
maintaining desired timing specs. Selecting a high RL
value is important to ensure that the output of the
optocoupler has the maximum voltage swing to meeta proper logic high state and maintain robust signal
integrity. Conversely, increasing this value reduces thespeed at which the optocoupler can respond to fast
voltage changes due to a high resistance load and thehigh junction capacitance of the phototransistor. Thisultimately limits the overall speeds that a designer
could implement for UART.

ISOLATIONUARTA UART interface is commonly configured witha standard diode input and open collector
output optocoupler. Additional circuitry is neededwith optocouplers, which adds board space andcontributes to additional power consumption. On theinput side of the optocoupler, a series resistor, Ri,is required to be connected to the anode to limit
the current that flows across the diode. Due toaging, the current transfer ratio (CTR) of optocouplers
degrades. To maintain the same level of performancethroughout the lifetime of the system, designers haveto overdesign by selecting a high Ri, which burns
extra current.

ISOLATION

Serial communication interfaces are commonlyused to transmit and receive data between
two devices in industrial or automotive systems.
Common interface types used in short distance,
intra-board communication are UART, SPI, andI2C. Isolating these types of digital interfaces has
historically been achieved using optocouplers, also
known as optoisolators, despite their weak isolation
performance, poor timing specifications, and limited
temperature range. Optocouplers only remain a
popular option when isolating digital interfaces dueto their low cost. However, digital isolators can
now provide a cost-competitive, easy-to-implement,
and higher-performance solution, all in a small form
factor. This article compares common optocoupler
circuits to digital isolator circuits used to isolate
common digital interfaces and demonstrate the overall
benefits of a digital isolator solution. For more about
the advantages of digital isolators compared tooptocouplers, see Improve Your System Performanceby Replacing Optocouplers with Digital Isolators.

INA
OUTB

Cdcpl
TX GND2
RX

GND2

GND1

GND2

Figure 1. UART Optocoupler Circuit vs ISO6721
Digital Isolator Circuit

How to Replace Optocouplers with Digital Isolators in Standard Interface
Circuits
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Tech Note

The ISO6721 digital isolator consumes five to 10times less current than an optocoupler solution.
The difference in total current consumption between
the two circuits is shown in Table 1. Additionally,
ISO6721 provides very tight channel-to-channel
timing specifications and a smaller solution size due
to its lack of additional passive components. The
optocoupler circuit footprint is roughly two times thesize (7 mm × 7.4 mm) as a single digital isolator(6 mm x 4.9 mm). The ease-of-use of the ISO6721
digital isolator circuit makes it a better solution to awidely used and simple digital interface.
Table 1. Current Consumption of Competitor A vsISO6721Competitor AISO6721

Primary Side
Current

10 mA

1.45 mA

Secondary SideCurrent

2 mA

1.45 mA

Total Current

12 mA/ch1.45 mA/ch

To mitigate this effect, a capacitor, Cpeak, is placed
across the resistor, Ri, to sharpen the edges of thebitstream. Optocouplers that have an open collector
output will require a pull-up resistor, RL, to create
the logic high state. The value RL poses a challenge
since a high resistor value limits achievable bit rate
but lowers power consumed by the circuit stage.
Along with high-speed optocouplers with open
collector output, there are also offerings with pushpull outputs. For the high-speed totem-pole output
optocoupler, the input is configured the same way
as the open collector, but the output is a pushpull configuration and the pull-up resistor is not
required. An output inversion is accomplished inside
the optocoupler for this topology.

SPI
The Serial Peripheral Interface (SPI) is a CMOS logic
serial interface that requires four unidirectional datalines. The benefit of SPI is its higher speed where bit
rates commonly exceed 1 Mbps and in some cases,reach as high as 30 Mbps. SPI is able to meet highspeeds due to its push-pull output implementation
allowing for fast switching, separate transmit andreceive lines enabling full duplex communication,and a shared interface clock that automaticallysynchronizes bit boundaries between the parties. SPI
is a “main-secondary” interface, meaning only one
device on the bus controls when communication isallowed to occur.
Four channels are required to isolate the SPIinterface. The four typical SPI interface lines areSCLK, CS, MOSI, and MISO. There are two typesof high-speed optocouplers that can be used with
SPI: open collector output and totem-pole output.
These types of optocouplers only integrate up to twochannels, which means two to four optocouplers haveto be used together to isolate all four SPI channels.
Using a high-speed open collector output, as shownin Figure 2, also requires a series resistor, Ri, and,in this case, the combination of the resistor and theinput shunt capacitance of the optocoupler creates a
low-pass filter. The low-pass filter limits the maximum
bit rate and overall SPI throughput.

2Figure 2. SPI Footprint of Optocoupler Circuit vsISO6741 Digital Isolator Circuit
Given that an SPI interface is targeted for fast datatransfer, it is critical that the isolation system supports
20 Mbps or 10 MHz frequency to meet industry speed
requirements. It is a common misinterpretation thatthe data speed of the SPI interface is limited by theisolator speed, when in fact it is the propagation delaythat limits the speed. The round trip delay of the clock
to data path will set the limit of the maximum speed ofthe interface.

How to Replace Optocouplers with Digital Isolators in Standard Interface
Circuits
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Tech Note

Figure 3 shows the relationship of a SCLK signal
going through an isolator and the MISO responding
and receiving data back. In the figure, the blue
clock signal from the SOC gets delayed by theisolator propagation delay at the MCU. When theMCU responds and sends data back to the SOC, thereceive data experiences a second delay through theisolator. Therefore, the total delay between the initialclock edge and the receive data is approximately
twice the isolator propagation delay: 2 × tPROP < ½tSCLK.
Clock signal (blue)

Clock with 1x prop delay (green)

V2

V1
VCC1

VCC2

GND1

GND2

Cdcpl

Cdcpl
GND2

GND1
INA

CS

INB

MOSI

INC

MISO

OUTD

GND100SLCKOUTAISOLATION

SOC

SLCK

OUTB

CS

OUTC

DIN

IND

DOUT

EN1

EN2

GND1

GND2

MCU

GND21

Data with 2x clock prop delay (red)001

Data with 1x clock prop delay (yellow)

Figure 3. Digital Isolator Propagation Delay for SPI
Loop Timing
To meet 10 MHz, a maximum prop delay of 25 ns
needs to be met. Optocouplers typically require asmall RL of 350-Ω to meet fast timing specs andare commonly limited to a SPI clock rate of 7 MHz,
without taking into account additional margin. The
small RL causes more power to be consumed on
the output of the optocoupler. There are a varietyof optocoupler solutions in the market but there is astrong tradeoff between high power consumption andtighter timing.
Figure 2 shows how SPI can be isolated with
only one ISO6741 device versus two high-speed
optocouplers with additional passive components. The
board footprint of the optocoupler circuit is 12 mm ×21.5 mm versus the ISO6741 digital isolator footprint,
which is only 16 mm x 12 mm. The ISO6741 has
CMOS inputs and push-pull outputs that eliminates
the need for all of the passive components in thedata path. The power consumption comparison is also
noted in Table 2. Selecting an isolator that has robust
timing specs to meet fast data rates while consuming
low power is key for a more sophisticated digitalinterface solution like SPI.

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Table 2. Current Consumption of Competitor A,Competitor B, and ISO6741
Competitor A(open-collector)

Competitor B(totem-pole)

ISO6741

Primary SideCurrent

10 mA

2 mA

1.27 mA

SecondarySide Current

14 mA

2 mA

1.12 mA

Total Current

24 mA/ch

3 mA/ch

2.4 mA/ch

I2C
The Inter-Integrated Circuit (I2C) interface is a twowire serial interface that efficiently allows multiple
devices to be connected on the same bus. The I2C
bus consists of two lines: a bidirectional serial dataline (SDA) and a serial clock line (SCL) that caneither be unidirectional or bidirectional depending on
the design. The output structure of the bus lines
are open collector to allow for easy sharing of thebus. The I2C bidirectional channels provide a uniquechallenge for isolation. The isolation channel must
support bidirectional data transfer and the circuit mustensure that it does not drive its own line while it isbeing driven by the transmitter.
Historically, isolating I2C was accomplished withoptocouplers and discrete logic to control the datadirection on the lines. The discrete components arerequired to avoid bus glitches and any latch up
conditions on the bidirectional lines. Without thediscrete components, the design runs the risk of
having a condition on the line where the left side isdriving the signal line low to the right and then, inturn, the right side drives the signal low back to theleft. This would create a latch up condition where thebidirectional lines could never be released to returnhigh. Figure 4 shows an I2C isolation circuit using
optocouplers capable of reaching speeds up to 400kbps. For the sake of simplicity, this I2C diagramshows a “main-secondary” implementation where
SCL is a unidirectional signal only. Two optocoupler
devices are required: one to transmit SCL and SDA,
and one to receive SDA.

How to Replace Optocouplers with Digital Isolators in Standard Interface
Circuits
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Competitor A1V1

100 nF

VCC

GND110 kO

V2

V1

VE

10 kO

VoV2

V2330 OISOLATION

V1330 O 330 OAN

D2CATH

SDA

GND

GND1

GND2V2Competitor A2SDA

CATH330 OQ1CATH

GND1
AN

GND1

VO1SCLVO2
GND

GND2V110 kO

GND2VCC

ISOLATION

AND1100 nF

Q3SCL

Figure 4. I2C Optocoupler Circuit
In addition to the pull-up resistors required for the I2C
interface, the circuit requires transistors, Q1 and Q2,and diodes, D1 and D2, to create an open collector
output and to avoid bus contention on the bidirectional
line. The 330-Ω pull-up resistor values are used toprovide higher input current to the optocoupler input tomeet required turn on times.
More modern designs with digital isolators utilize
integration of the external components to provide asmaller and simpler solution. Figure 5 shows thelayout of a solution using optocouplers compared
to TI's single chip ISO1641 with unidirectional clockand bidirectional data. The ISO1641 has integrated
the internal circuitry required to prevent SDA frombeing simultaneously driven by both sides of theisolator. The ISO1640 and ISO1641 have integrated
the internal circuitry on both the SDA and SCLline to enable bidirectional control on the SCL linefor I2C clock stretching if the design requires. Thesolution is very simple with SDA and SCL being
the only single bidirectional data pins on either sideof the isolation boundary. Furthermore, the inputs
are buffered, reducing the current consumption ofthe solution and eliminating the need for the LED
current limiting resistors. The ISO1640 and ISO1641
significantly reduces the size and complexity of thecircuit compared to the optocoupler solution.

4Figure 5. I2C Footprint of Optocoupler Circuit vsISO1641 Isolator Circuit
While existing optocoupler based designs may haverequired as many as 16 discrete components, TI'sI2C isolator solution can be used with as few as six
components in a very small footprint while simplifying
signal routing. The only discrete components requiredalong with the single ISO1640 and ISO1641 are
pullup resistors, for the open collector I2C bus lines,
and decoupling capacitors for the digital isolator
power supply pins. Reducing this component count toa single digital isolator with a few passive components
allows a design to save as much as three times
the circuit board space. As Figure 5 shows, theoptocoupler circuit is 22 mm x 15.5 mm versus theISO1641 isolator footprint, which is only 15 mm x 8mm.
A few primary reasons why an I2C interface isattractive to a design is its small size and simplicity
of only two wires. The complexity around anoptocoupler-based isolated I2C solution requiring
discrete components weakens the value of why touse the interface to begin with. Using a digital isolator
solution, like the ISO1640 and ISO1641, allows thedesigner to bring back the compact value of the I2C
interface into an isolated solution.

How to Replace Optocouplers with Digital Isolators in Standard Interface
Circuits
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Tech NoteConclusion
TI’s digital isolators provide significant benefits
compared to traditional optocoupler circuits for
isolating digital interfaces while being costcompetitive. In the case of UART, the ISO6721
provides a smaller footprint and much lower power
consumption compared to its optocoupler equivalent.
For SPI, The ISO6741 solution supports higher clock
rates beyond 10 MHz, lower power per channel, andis significantly lower cost per channel than the totempole implementation. The large optocoupler footprint
around an isolated I2C design becomes unattractive
as systems become smaller. The ISO164x family ofI2C interface isolators presents a simple and small
solution with a native capability to achieve the higher
I2C speeds. TI’s digital isolators produce a smaller
PCB footprint due to channel consolidation andreduced number of passive components, in addition
to providing robust isolation, timing specs, and EMC
performance. They also yield circuit diagrams which
are simpler to build and debug compared to their
optocoupler equivalents.
Reference• "I2C interface has galvanic isolation, wired-OR
capability, improved noise margin," EDN, July
2007.
•Eric Coates, "Using Optocouplers," Learnabout
Electronics, July 2020.

•Texas Instruments: How to isolate RS-485 forsmallest size and highest reliability•Texas Instruments: How to Design an Isolated
CAN Port for Space-Constrained Industrial
Applications

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How to Replace Optocouplers with Digital Isolators in Standard Interface
Circuits
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