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LT1016

APPLICATIO S I FOR ATIO

The circuit shown in Figure 1 is the best

electronic means

of generating a known fast, clean step to test comparators.

It uses a very fast transistor in a common base configura-

tion. The transistor is switched “off” with a fast edge from

the generator and the collector voltage settles to exactly

0V in just a few nanoseconds. The most important feature

of this circuit is the lack of feedthrough from the generator

to the comparator input. This prevents overshoot on the

comparator input that would give a false fast reading on

comparator response time.

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

adjusted so that the LT1016 output under test settles to

1.4V (in the linear region). Then V1 is

changed –5V to set

overdrive at 5mV.

The test circuit shown measures low to high transition on

the “+” input. For opposite polarity transitions on the

output, simply reverse the inputs of the LT1016.

High Speed Design Techniques

A substantial amount of design effort has made the LT1016

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

and other vagaries than some slower comparators, even

with slow input signals. In particular, the LT1016 is stable

in its linear region, a feature no other high speed compara-

tor has. Additionally, output stage switching does not

appreciably change power supply current, further enhanc-

ing stability. These features make the application of the

50GHz gain-bandwidth LT1016 considerably easier than

other fast comparators. Unfortunately, laws of physics

dictate that the circuit

environment the LT1016 works in

must be properly prepared. The performance limits of high

speed circuitry are often determined by parasitics such as

stray capacitance, ground impedance and layout. Some of

these considerations are present in digital systems where

designers are comfortable describing bit patterns and

memory access times in terms of nanoseconds. The

LT1016 can be used in such fast digital systems and

Figure 2 shows just how fast the device is. The simple test

circuit allows us to see that the LT1016’s (Trace B)

response to the pulse generator (Trace A) is as fast as a

TTL inverter (Trace C) even when the LT1016 has only

millivolts of input signal! Linear circuits operating with

this kind of speed make many engineers justifiably wary.

Nanosecond domain linear circuits are widely associated

with oscillations, mysterious shifts in circuit characteris-

tics, unintended modes of operation and outright failure to

function.

–

PULSE

–100mV

–3V

–5V

0.1

µF

Ω

400

Ω

130

Ω

750

Ω

10k

2N3866

†

0.01

µF

0.01

µF**

LT1016

10 SCOPE PROBE

(CIN

≈ 10pF)

10 SCOPE PROBE

(CIN

≈ 10pF)

* SEE TEXT FOR CIRCUIT EXPLANATION

** TOTAL LEAD LENGTH INCLUDING DEVICE PIN.

SOCKET AND CAPACITOR LEADS SHOULD BE

LESS THAN 0.5 IN. USE GROUND PLANE

† (VOS + OVERDRIVE) • 1000

1016 F01

Figure 1. Response Time Test Circuit

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