6

LTC1520

Theory of Operation

Unlike typical line receivers whose propagation delay can

vary by as much as 500% from package to package and

show significant temperature drift, the LTC1520 employs

a novel architecture that produces a tightly controlled and

temperature compensated propagation delay. The differ-

ential timing skew is also minimized between rising and

falling output edges, and the propagation delays of any

two receivers within a package are very tightly matched.

The precision timing features of the LTC1520 reduce

overall system timing constraints by providing a narrow

6ns window during which valid data appears at the re-

ceiver output. This output timing window applies to all

receivers in separate packages over all operating tempera-

tures thereby making the LTC1520 well suited for high

speed parallel data transmission applications such as

backplanes.

In clocked data systems, the low skew minimizes duty

cycle distortion of the clock signal. The LTC1520 can

propagate signals at frequencies up to 25MHz (50Mbps)

with less than 5% duty cycle distortion. When a clock

signal is used to retime parallel data, the maximum recom-

mended data transmission rate is 25Mbps to avoid timing

errors due to clock distortion.

Rail-to-rail input common mode range enables the LTC1520

to be used in both single-ended and differential applica-

tions with transmission distances up to 100 feet. Thermal

shutdown and short-circuit protection prevent latchup

damage to the LTC1520 during fault conditions.

Single-Ended Applications

Over short distances, the LTC1520 can be configured to

receive single-ended data by tying one input to a fixed bias

voltage and connecting the other input to the driver output.

In such applications, standard high speed CMOS logic

may be used as a driver for the LTC1520. The receiver trip

points may be easily adjusted to accommodate different

driver output swings by changing the resistor divider at the

fixed input. Figure 6a shows a single-ended receiver

configuration with the driver and receiver connected via

PC traces. Note that at very high speeds, transmission line

and driver ringing effects have to be considered. Motorola’s

MECL System Design Handbook serves as an excellent

reference for transmission line and termination effects. To

mitigate transmission errors and duty cycle distortion due

to driver ringing, a small output filter or a dampening

transmit single-ended data over distances up to 10 feet,

twisted pair is recommended with the unused wire

grounded at both ends (Figure 7).

Figure 7. Medium Distance Single-Ended Transmission

Using a CMOS Driver

–

+

1/4 LTC1520

10-FT TWISTED PAIR

MC74ACT04

MC74AC04

0.01

µF

120

Ω

3.3k

5V

2.2k

1520 F07

Figure 6b. Techniques to Minimize Driver Ringing

10

Ω

PC TRACE

OR

PC TRACE

0.01

µF

MC74AC04

1520 F06b

10pF

10

Ω

Figure 6a. Single-Ended Receiver

–

+

2.2k

5V

PC TRACE

1/4 LTC1520

MC74ACT04

(TTL INPUT)

MC74AC04

(CMOS INPUT)

2.2k

1520 F06a

0.01

µF

Differential Transmission

The LTC1520 is well suited for medium distance differen-

tial transmission due to its rail-to-rail input common mode

range. Clock rates up to 25MHz can be transmitted over

100 feet of high quality twisted pair. Figure 8 shows the