LTC6820
22
6820fb
For more information www.linear.com/LTC6820
APPLICATIONS INFORMATION
Figure 20. Example Layout
Layout of the isoSPI signal lines also plays a significant
role in maximizing the immunity of a circuit. The following
layout guidelines should be followed:
1. The transformer should be placed as close to the isoSPI
cable connector as possible. The distance should be
kept less than 2cm. The LTC6820 should be placed at
least 1cm to 2cm away from the transformer to help
isolate the IC from the magnetic coupling fields.
2. On the top layer, no ground plane should be placed
under the magnetic, the isoSPI connector, or in between
the transformer and the connector.
3. The IP and IM traces should be isolated from surround
-
ing circuits. No traces should cross the IP and IM lines,
unless separated by a ground plane within the printed
circuit board.
The isoSPI drive currents are programmable and allow
for a tradeoff between power consumption and noise
immunity. The noise immunity of the LTC6820 has been
evaluated using a bulk current injection (BCI) test. The
BCI test injects current into the twisted-pair lines at set
levels over a frequency range of 1MHz to 400MHz. With
the minimum I
B
current, 0.1mA, the isoSPI serial link has
been shown to pass a 40mA BCI test with no bit errors.
A 40mA BCI test level is sufficient for most industrial ap
-
plications. Automotive applications tend to have a higher
BCI requirement so the recommended I
B
is set to 1mA, the
maximum power level. The isoSPI system has been shown
to pass a 200mA BCI test with no transmitted bit errors.
The 200mA test level is typical for automotive testing.
Software Layer
The isoSPI physical layer has high immunity to EMI and
is not particularly susceptible to bit errors induced by
noise, but for best results in a high noise environment it
is recommended to implement a software layer that uses
an error detection code like a cyclic redundancy check
or check sum. Error detection codes will allow software
detection of any bit error and will notify the system to retry
the last erroneous serial communication.
1.5cm
IP
IM
1cm
CONNECTOR
