LT1016IS8 datasheet(10/20 Pages) LINER

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LT1016

2V/DIV

100ns/DIV

1016 F09

Figure 9. Transition Instabilities Due to No Ground Plane

In Figure 7 the probes are properly selected and applied

but the LT1016’s output rings and distorts badly. In this

case, the probe ground lead is too long. For general

purpose work most probes come with ground leads about

six inches long. At low frequencies this is fine. At high

speed, the long ground lead looks inductive, causing the

ringing shown. High quality probes are always supplied

with some short ground straps to deal with this problem.

Some come with very short spring clips which fix directly

to the probe tip to facilitate a low impedance ground

connection. For fast work, the ground connection to the

probe should not exceed one inch in length.

Keep the

probe ground connection as short as possible.

Figure 8 shows the LT1016’s output (Trace B) oscillating

near 40MHz as it responds to an input (Trace A). Note that

the input signal shows artifacts of the oscillation. This

example is caused by improper grounding of the compara-

tor. In this case, the LT1016’s GND pin connection is

one inch long. The ground lead of the LT1016 must be as

short as possible and connected directly to a low imped-

ance ground point. Any substantial impedance in the

LT1016’s ground path will generate effects like this. The

reason for this is related to the necessity of bypassing the

Figure 7. Typical Results Due to Poor Probe Grounding

Figure 8. Excessive LT1016 Ground Path

Resistance Causes Oscillation

TRACE A

1V/DIV

TRACE B

2V/DIV

100ns/DIV

1016 F08

1V/DIV

20ns/DIV

1016 F07

power supplies. The inductance created by a long device

ground lead permits mixing of ground currents, causing

undesired effects in the device. The solution here is

simple.

Keep the LT1016’s ground pin connection as short

(typically 1/4 inch) as possible and run it directly to a low

impedance ground. Do not use sockets.

Figure 9 addresses the issue of the “low impedance

ground,” referred to previously. In this example, the

output is clean except for chattering around the edges.

This photograph was generated by running the LT1016

without a “ground plane.” A ground plane is formed by

using a continuous conductive plane over the surface of

the circuit board. The only breaks in this plane are for the

circuit’s necessary current paths. The ground plane serves

two functions. Because it is flat (AC currents travel along

the surface of a conductor) and covers the entire area of

the board, it provides a way to access a low inductance

ground from anywhere on the board. Also, it minimizes the

effects of stray capacitance in the circuit by referring them

to ground. This breaks up potential unintended and harm-

ful feedback paths.

Always use a ground plane with the

LT1016 when input signal levels are low or slow moving.

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