Isolated RS485 Transceiver Breaks Ground Loops – Design Note 228
Mitchell Lee Previously, isolation was achieved by using at least three
optoisolators and a separate isolated power supply. The
LTC1535 replaces not only the optoisolators, but also the
power supply, as it includes an on-chip DC/DC converter.
Other features include selectable driver slew rate to reduce
EMI and susceptibility to reflections, full-duplex pinout
and fail-safe detection of open and shorted lines.
The LTC1535 consists of two separate dice assembled on
a proprietary, isolated lead frame. The lead frame includes
integral coupling capacitors that bridge the isolation barrier
and exhibit 2,500VRMS guaranteed standoff. Data communication takes place via the coupling capacitors, while
an on-chip, 400kHz push-pull converter sends power to the
isolated side through a small transformer. Total common
mode capacitance across the barrier amounts to less than
20pF, with the transformer accounting for about 16pF of
the total. Figure 1 shows the complete circuit for a fully
isolated RS485 port.
Internally, the two halves of the LTC1535 communicate
in a ping-pong fashion, first sending transmit data to the
isolated side and then sending receive data back to the
nonisolated side. The sampling nature of the internal communications link means that some jitter is introduced into
the data; this limits the useful baud rate to approximately …