8

LT1394

–

+

LT1394

1394 F02

FET PROBE

FET PROBE

* TOTAL LEAD LENGTH INCLUDING DEVICE PIN.

SOCKET AND CAPACITOR LEADS SHOULD BE

LESS THAN 0.5 IN. USE GROUND PLANE

25

Ω

25

Ω

5V

0.01

µF*

0.01

µF

10k

50

Ω

V1**

2N3866

0V

–3V

–5V

–5V

50

Ω

PULSE

IN

750

Ω

400

Ω

0.1

µF

130

Ω

0V

–100mV

Q

Q

Figure 2. Response Time Test Circuit

circuit is the lack of feedthrough from the generator to the

comparator input. This prevents overshoot on the com-

parator input, which would give a false fast reading on

comparator response time.

To adjust the circuit for exactly 5mV overdrive, V1 is

adjusted so that the LT1394 output under test settles to

1.4V (in the linear region). Then V1 is changed by – 1V to

set overdrive to 5mV.

High Speed Design Techniques

A substantial amount of design effort has made the LT1394

relatively easy to use. It is much less prone to oscillation

than some slower comparators, even with slow input

signals. However, as with any high speed comparator,

there are a number of pitfalls which may arise because of

PC board layout and design. The most common problems

involve power supply bypassing. Bypassing is necessary

to maintain low supply impedance. DC resistance and

inductance in supply wires and PC traces can quickly build

up to unacceptable levels. This allows the supply line to

move with changing internal current levels of the con-

nected devices. This will almost always result in improper

operation. In addition, adjacent devices connected through

an unbypassed supply can interact with each other through

the finite supply impedances. Bypass capacitors furnish a

simple solution to this problem by providing a local

reservoir of energy at the device, keeping supply imped-

ances low.

Bypass capacitors should be as close as possible to the

LT1394. A good high frequency capacitor such as a 0.1

µF

ceramic is recommended, in parallel with a larger capaci-

tor such as a 4.7

µF tantalum.

Poor trace routes and high source impedances are also

common sources of problems. Be sure to keep trace

lengths as short as possible, and avoid running any output

trace adjacent to an input trace to prevent unnecessary

coupling. If output traces are longer than a few inches, be

sure to terminate them with a resistor to eliminate any

reflections that may occur. Resistor values are typically

250

Ω to 400Ω. Also, be sure to keep source impedances

as low as possible, preferably 1k

Ω or less.

Crystal Oscillators

Figure 3’s circuits are crystal oscillators. In the circuit (a)

the resistors at the LT1394’s positive input set a DC bias

point. The 2k-0.068

µF path sets up phase shifted feedback

and the circuit looks like a wideband unity-gain follower at

DC. The crystal’s path provides resonant positive feed-

back and stable oscillation occurs. The circuit (b) is

similar, but supports oscillation frequencies to 30MHz.

Above 10MHz, AT-cut crystals operate in overtone mode.

Because of this, oscillation can occur at multiples of the

desired frequency. The damper network rolls off gain at

high frequency, ensuring proper operation.

Switchable Output Crystal Oscillator

Figure 4 permits crystals to be electronically switched by

logic commands. This circuit is similar to the previous

examples, except that oscillation is only possible when

one of the logic inputs is biased high.