8

LTC1690

3

1

5

6

D

120

Ω

2

1

8

7

R

DRIVER

LTC1690

5V

LTC1690

RECEIVER

120

Ω

SHIELD

2

4

7

8

R

3

4

6

5

D

RECEIVER

DRIVER

1690 F06

SHIELD

0.01

µF

5V

0.01

µF

Figure 6. Typical Application

A typical application is shown in Figure 6. Two twisted pair

wires connect two driver/receiver pairs for full duplex data

transmission. Note that the driver and receiver outputs are

always enabled. If the outputs must be disabled, use the

LTC491. There are no restrictions on where the chips are

connected, and it isn’t necessary to have the chips con-

nected to the ends of the wire. However, the wires must be

terminated at the ends with a resistor equal to their

characteristic impedance, typically 120

Ω. Because only

one driver can be connected on the bus, the cable need

only be terminated at the receiving end. The optional

shields around the twisted pair are connected to GND at

one end and help reduce unwanted noise.

The LTC1690 can be used as a line repeater as shown in

Figure 7. If the cable is longer that 4000 feet, the LTC1690

is inserted in the middle of the cable with the receiver

output connected back to the driver input.

Receiver Fail-Safe

Some encoding schemes require that the output of the

receiver maintains a known state (usually a logic 1) when

data transmission ends and all drivers on the line are

forced into three-state. The receiver of the LTC1690 has a

fail-safe feature which guarantees the output to be in a

logic 1 state when the receiver inputs are left floating or

shorted together. This is achieved without external com-

ponents by designing the trip-point of the LTC1690 to be

within – 200mV to –10mV. If the receiver output must be

a logic 0 instead of a logic 1, external components are

required.

The LTC1690 fail-safe receiver is designed to reject fast

–7V to 12V common mode steps at its inputs. The slew

rate that the receiver will reject is typically 400V/

µs, but

–7V to 12V steps in 10ns can be tolerated if the frequency

of the common mode step is moderate (<600kHz).

Driver-Receiver Crosstalk

The driver outputs generate fast rise and fall times. If the

LTC1690 receiver inputs are not terminated and floating,

switching noise from the LTC1690 driver can couple into

the receiver inputs and cause the receiver output to glitch.

This can be prevented by ensuring that the receiver inputs

are terminated with a 100

Ω or 120Ω resistor, depending

on the type of cable used. A cable capacitance that is

greater than 10pF (

≈1ft of cable) also prevents glitches if

no termination is present. The receiver inputs should not

be driven typically above 8MHz to prevent glitches.