8

LT1671

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 LT1671 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 LT1671

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 problems which may arise because

of PC board layout and design. The most common prob-

lem involves power supply bypassing. Bypassing is nec-

essary 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

connected devices. This will almost always result in

improper operation. In addition, adjacent devices con-

nected 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 impedances low.

Bypass capacitors should be as close as possible to the

LT1671. 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.

About Level Shifts

The LT1671’s logic output will interface with many cir-

cuits directly. Many applications, however, require some

form of level shifting of the output swing. With LT1671-

based circuits this is not trivial because it is desirable to

maintain very low delay in the level shifting stage. When

designing level shifters, keep in mind that the TTL output

of the LT1671 is a sink-source pair (Figure 3) with good

ability to drive capacitance (such as feedforward capaci-

tors). Figure 4 shows a noninverting voltage gain stage

with a 15V output. When the LT1671 switches, the base-

emitter voltages at the 2N2369 reverse, causing it to

switch very quickly. The 2N3866 emitter-follower gives a

low impedance output and the Schottky diode aids cur-

rent sink capability.

–

+

LT1671

1671 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