LT1016_03 Datasheet, PDF (10/20 Pages) Linear Technology – UltraFast Precision 10ns Comparator

LT1016
APPLICATIO S I FOR ATIO
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
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.
1V/DIV
TRACE A
1V/DIV
TRACE B
2V/DIV
20ns/DIV
1016 F07
Figure 7. Typical Results Due to Poor Probe Grounding
2V/DIV
100ns/DIV
1016 F08
Figure 8. Excessive LT1016 Ground Path
Resistance Causes Oscillation
10
100ns/DIV
1016 F09
Figure 9. Transition Instabilities Due to No Ground Plane