LT1016_15 Datasheet, PDF (10/20 Pages) Linear Technology – UltraFast Precision 10ns Comparator

LT1016
Applications Information
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 pur-
pose 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 con-
nection 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 com-
parator. 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 impedance
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
harmful 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
1016fc