ADCMP565
Rev. 0 | Page 9 of 16
APPLICATION INFORMATION
The ADCMP565 comparators are very high speed devices.
Consequently, high speed design techniques must be employed
to achieve the best performance. The most critical aspect of any
ADCMP565 design is the use of a low impedance ground plane.
A ground plane, as part of a multilayer board, is recommended
for proper high speed performance. Using a continuous con-
ductive plane over the surface of the circuit board can create
this, allowing breaks in the plane only for necessary signal
paths. The ground plane provides a low inductance ground,
eliminating any potential differences at different ground points
throughout the circuit board caused by ground bounce. A
proper ground plane also minimizes the effects of stray
capacitance on the circuit board.
It is also important to provide bypass capacitors for the power
supply in a high speed application. A 1 µF electrolytic bypass
capacitor should be placed within 0.5 inches of each power
supply pin to ground. These capacitors will reduce any potential
voltage ripples from the power supply. In addition, a 10 nF
ceramic capacitor should be placed as close as possible from the
power supply pins on the ADCMP565 to ground. These
capacitors act as a charge reservoir for the device during high
frequency switching.
The LATCH ENABLE input is active low (latched). If the
latching function is not used, the LATCH ENABLE input
should be grounded (ground is an ECL logic high), and the
complementary input,
LATCH ENABLE
, should be tied to
−2.0
V. This will disable the latching function.
Occasionally, one of the two comparator stages within the
ADCMP565 will not be used. The inputs of the unused com-
parator should not be allowed to float. The high internal gain
may cause the output to oscillate (possibly affecting the
comparator that is being used) unless the output is forced into a
fixed state. This is easily accomplished by ensuring that the two
inputs are at least one diode drop apart, while also appropriately
connecting the LATCH ENABLE and
LATCH ENABLE
inputs
as described above.
The best performance is achieved with the use of proper ECL
terminations. The open emitter outputs of the ADCMP565 are
designed to be terminated through 50 Ω resistors to −2.0 V, or
any other equivalent ECL termination. If a −2.0 V supply is not
available, an 82 Ω resistor to ground and a 130 Ω resistor to
−5.2 V provide a suitable equivalent. If high speed ECL signals
must be routed more than a centimeter, microstrip or stripline
techniques may be required to ensure proper transition times
and prevent output ringing.
CLOCK TIMING RECOVERY
Comparators are often used in digital systems to recover clock
timing signals. High speed square waves transmitted over a
distance, even tens of centimeters, can become distorted due to
stray capacitance and inductance. Poor layout or improper
termination can also cause reflections on the transmission line,
further distorting the signal waveform. A high speed com-
parator can be used to recover the distorted waveform while
maintaining a minimum of delay.
OPTIMIZING HIGH SPEED PERFORMANCE
As with any high speed comparator amplifier, proper design and
layout techniques should be used to ensure optimal perform-
ance from the ADCMP565. The performance limits of high
speed circuitry can easily be a result of stray capacitance,
improper ground impedance, or other layout issues.
Minimizing resistance from source to the input is an important
consideration in maximizing the high speed operation of the
ADCMP565. Source resistance in combination with equivalent
input capacitance could cause a lagged response at the input,
thus delaying the output. The input capacitance of the
ADCMP565 in combination with stray capacitance from an
input pin to ground could result in several picofarads of
equivalent capacitance. A combination of 3 kΩ source resistance
and 5 pF of input capacitance yields a time constant of 15 ns,
which is significantly slower than the sub 500 ps capability of
the ADCMP565. Source impedances should be significantly less
than 100 Ω for best performance.
Sockets should be avoided due to stray capacitance and induc-
tance. If proper high speed techniques are used, the ADCMP565
should be free from oscillation when the comparator input
signal passes through the switching threshold.
COMPARATOR PROPAGATION
DELAY DISPERSION
The ADCMP565 has been specifically designed to reduce
propagation delay dispersion over an input overdrive range of
100 mV to 1 V. Propagation delay overdrive dispersion is the
change in propagation delay that results from a change in the
degree of overdrive (how far the switching point is exceeded by
the input). The overall result is a higher degree of timing
accuracy since the ADCMP565 is far less sensitive to input
variations than most comparator designs.
Propagation delay dispersion is a specification that is important
in critical timing applications such as ATE, bench instruments,
and nuclear instrumentation. Overdrive dispersion is defined
