Functional SPI Isolation
Assuming a constant ground voltage helps us analyze circuits, but it is not
necessarily accurate.1 A combination of parasitic inductance and large high
frequency currents creates voltage noise on circuit grounds. For example,
large high frequency currents flowing through a ground reference can result
from thousands of transistors in digital circuitry switching simultaneously.
This is bad news for sensitive analog components, such as a analog-to-digital
converters (ADC) that use the digital components’ ground.
ADuM744x family in this configuration. The ADuM7442 in Figure 2 isolates
SCLK, DIN, and DOUT; the ADuM7440 isolates four CS X signals. The three
other channels in the ADuM7440 can be used to isolate CS X signals for
three more slave devices. Alternatively, the ADuM140x family can be used
in this configuration with clock rates of up to 45 MHz. The delayed clock
implementation allows the SPI bus to be clocked at a digital isolator’s
maximum data rate.
It is important to provide a clean ground reference for analog components.
Implementing a multipoint grounding system and filtering the analog
circuits’ supply connections are useful techniques. Digital isolation allows
components to have different ground references and still maintain a communication link. For the purposes of isolating ground noise from sensitive
components where safety isolation is not required, a high isolation rating
is unnecessary. Digital isolators with 1 kV rms withstand voltages, such as
the ADuM744x family, are more than sufficient for ground noise isolation
applications.
Figure 1 shows an ADuM7441 isolating an SPI bus between a microcontroller
and an ADC, allowing them to operate with separate ground references.
Table 1 lists the maximum SPI clock frequencies that are possible with
the ADuM7441A and ADuM7441C using 5 V and 3.3 V supplies given their
maximum propagation delays.2 Refer to Reference 2 for information on
clocking isolated SPI buses.
ADuM7440
VDD1
VDD2A
GND1
GND2
VIA
VIA
CS3
VIB
VIB
CS2
VIC
VIC
VOD
VID
VDD1
VDD1B
VDD2A
GND1
GND2
ADuM7442C
VDD1
CS1
VOA
CS
VIB
VOB
SCLK
DIN
VIC
VOC
DIN
VDD2A
GND1
GND2
VDD2
VDD2A
GND1
GND2
Model
Supply
Configuration (V)
Maximum
Propagation
Delay (ns)
Maximum SCLK
Frequency (MHz)
3.3
5.0
3.3
5.0
85
75
66
50
2.9
3.3
3.7
5.0
ADuM7441A
ADuM7441C
CS
CS
CS
VDD2
SCLK
DIN
DOUT
CS
SCLK
VIB
DIN
DOUT
VIC
VIC
DOUT
VID
VDD2B
GND1
GND2
DOUT
Table 1.
VIB
VDD1B
ADC
Figure 1.
DIN
REGISTER CLOCKED
BY DELAYED SCLK
VID
VDD1B
VDD2
VDD1A
VOD
VOD
VDD1
VIA
VDD1
GND2
SCLK
VIA
I/O
GND1
VIA
SCLK
I/O CLK
VDD2A
VDD2
VDD1A
CS4
ADDITIONAL SLAVE
SELECTS
VDD2
VDD1A
CS
DOUT
An alternative implementation of an isolated SPI bus is shown in Figure 2.
In this case, the data from the ADC (DOUT) is clocked into a separate register
by a delayed version of SCLK; this implementation also allows for three more
slave devices. The SPI bus can be clocked at rates up to 12.5 MHz with the
MICROCONTROLLER
ADuM7441C
VDD1
MICROCONTROLLER
VDD2
Figure 2.
analog.com
ADC
UP TO THREE ADDITIONAL
SPI SLAVES CAN USE THE BUS
References
1
Ott, H.W. Noise Reduction Techniques in Electronic Systems. New York, NY: John Wiley & Sons,
1988. pp. 73–115.
2
Cantrell, M. “Clocking Options for SPI.”
http://www.analog.com/static/imported-files/newsletters/digital_isolation/iCoupler_0308.html
Analog Devices, Inc.
Worldwide Headquarters
Analog Devices, Inc.
One Technology Way
P.O. Box 9106
Norwood, MA 02062-9106
U.S.A.
Tel: 781.329.4700
(800.262.5643,
U.S.A. only)
Fax: 781.461.3113
Analog Devices, Inc.
Europe Headquarters
Analog Devices, Inc.
Wilhelm-Wagenfeld-Str. 6
80807 Munich
Germany
Tel: 49.89.76903.0
Fax: 49.89.76903.157
Analog Devices, Inc.
Japan Headquarters
Analog Devices, KK
New Pier Takeshiba
South Tower Building
1-16-1 Kaigan, Minato-ku,
Tokyo, 105-6891
Japan
Tel: 813.5402.8200
Fax: 813.5402.1064
Share this on: ©2011 Analog Devices, Inc. All rights reserved.
Trademarks and registered trademarks are the
property of their respective owners.
T10184-0-9/11
Follow ADI: analog.com
twitter.com/adi_news
Analog Devices, Inc.
Southeast Asia
Headquarters
Analog Devices
22/F One Corporate Avenue
222 Hu Bin Road
Shanghai, 200021
China
Tel: 86.21.2320.8000
Fax: 86.21.2320.8222