Linear Technology LTC6804-1 - Page 71

LTC6804-1/LTC6804-2
71
680412fc
For more information www.linear.com/LTC6804-1
applicaTions inForMaTion
Connecting Multiple LTC6804-1s on the Same PCB
When connecting multiple LTC6804-1 devices on the same
PCB, only a single transformer is required between the
LTC6804-1 isoSPI ports. The absence of the cable also
reduces the noise levels on the communication lines and
often only a split termination is required. Figure 44 shows
an example application that has multiple LTC6804-1s
on the same PCB, communicating to the bottom MCU
through an LTC6820 isoSPI driver. If a transformer with
a center tap is used, a capacitor can be added for better
noise rejection. Additional noise filtering can be provided
with discrete common mode chokes (not shown) placed
to both sides of the single transformer.
On single board designs with low noise requirements, it
is possible for a simplified capacitor-isolated coupling as
shown in Figure 45 to replace the transformer. Dual Zener
diodes are used at each IC to clamp the common mode
voltage to stay within the receiver’s input range. The op
-
tional common mode choke (CMC) provides noise rejection
with symmetrically tapped termination. The
590Ω
resistor
creates a resistor divider with the termination resistors and
attenuates common mode noise. The 590Ω value is chosen
to provide the most noise attenuation while maintaining
sufficient differential signal. The circuit is designed such
that I
B
and V
ICMP
are the same as would be used for a
transformer based system with cables over 50m.
10nF
GNDB
GNDB
GNDB
V
REG
10nF
GNDB
V
REG
590Ω
590Ω
1nF1nF
10nF
GNDA
GNDA
GNDA
V
REG
10nF
GNDA
V
REG
590Ω
590Ω
1nF1nF
1nF1nF
680412 F45
1.5k 499Ω
•
•
1.5k 499Ω
•
•
IPA
IBIAS
ICMP
LTC6804-1
IMA
100Ω
100Ω
3.3V
3.3V
100Ω
100Ω
3.3V
3.3V
IPB
IMB
V
–
CMC
IPA
IBIAS
ICMP
LTC6804-1
IMA
100Ω
100Ω
3.3V
3.3V
100Ω
100Ω
3.3V
3.3V
IPB
IMB
V
–
CMC
Figure 45. Capacitive Isolation Coupling for LTC6804-1s on the Same PCB