LTC6820
applicaTions inForMaTion
Capacitive Isolation Barrier
use a transformer with a center tap and a common mode
choke as shown in Figure 19. The center tap of the trans-
former should be bypassed with a 27pF capacitor. The
center tap capacitor will help attenuate common mode
signals. Large center tap capacitors should be avoided as
they will prevent the isoSPI transmitters common mode
voltage from settling.
In some applications, where the environment is relatively
noise free and only galvanic isolation is required, capaci-
tors can be used in place of transformers as the isolation
barrier. With capacitive coupling, the twisted pair cable
is driven by a voltage and is subject to signal loss with
cable length. This low cost isolated solution can be suit-
able for short distance interconnections (1 meter or less),
such as between adjacent circuit boards or across a large
PCB. The capacitors will provide galvanic isolation, but no
common mode rejection. This option uses the drivers in
a different way, by using pull up resistors to maintain the
To improve common mode current rejection a common
mode choke should also be placed in series with the IP
and IM lines of the LTC6820. The common mode choke
willbothincreaseEMIimmunityandreduceEMIemission.
When choosing a common mode choke, the differential
modeimpedanceshouldbe20Ωorlessforsignals50MHz
and below. Generally common mode chokes similiar to
those used in Ethernet applications are recommended.
common mode near V , only the sinking drive current
DD
has any effect. Figure 18 shows an example application
circuit using a capacitive isolation barrier capable of driv-
ing 1 meter of cable.
Table 8. Recommended Common Mode Chokes
VOLTAGE
DIFFERENTIAL COMMON MODE
IMPEDANCE AT IMPEDANCE AT
MANUFACTURER PART NUMBER
CAPACITANCE
RATING
MANUFACTURER PART NUMBER
TDK ACT45B-220-2P
50MHz
50MHz
Murata
GCM188R72A104KA64
100nF
100V
20Ω
5000Ω
EMC
When using the LTC6820, for the best electromagnetic
compatibility (EMC) performance it is recommended to
Capacitive Isolation
µC
LTC6820
MSTR
LTC6820
LTC2640
MSTR
100nF
100nF
SDI
IP
IP MOSI
MISO
SDO
MOSI
MISO
SCK
CS
SDI
SCK
CS
SCK
CS
IM
IM SCK
CS
6820 F18
Figure 18. Capacitive Isolation Barrier
LTC6820
27pF
IP
IM
120Ω
6820 F19
Figure 19. Connection of Transformer and Common Mode Choke
6820f
21
Linear [ Linear ]
