ADCMP551_0410 AD [Analog Devices], ADCMP551_0410 Datasheet - Page 11
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ADCMP551_0410
Manufacturer Part Number
ADCMP551_0410
Description
Single-Supply, High Speed PECL/LVPECL Comparators
Manufacturer
AD [Analog Devices]
Datasheet
1.ADCMP551_0410.pdf
(16 pages)
APPLICATION INFORMATION
The comparators in the ADCMP55x series are very high speed
devices. Consequently, high speed design techniques must be
employed to achieve the best performance. The most critical
aspect of any ADCMP55x 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 conductive 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 reduce any potential
voltage ripples from the power supply. In addition, a 10 nF
ceramic capacitor should be placed as close to the power supply
pins as possible on the ADCMP55x 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 pins
may be left open. The internal pull-ups on the latch pins set the
latch to transparent mode. If the latch is to be used, valid PECL
voltages are required on the inputs for proper operation. The
PECL voltages should be referenced to V
Occasionally, one of the two comparator stages within the
ADCMP551/ADCMP552 is not used. The inputs of the unused
comparator 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 previously.
The best performance is achieved with the use of proper PECL
terminations. The open-emitter outputs of the ADCMP55x are
designed to be terminated through 50 Ω resistors to
V
speed PECL signals must be routed more than a centimeter,
microstrip or stripline techniques may be required to ensure
proper transition times and prevent output ringing.
CCO
− 2.0 V or any other equivalent PECL termination. If high
CCI
.
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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
comparator 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
performance from the ADCMP55x. 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
ADCMP55x. Source resistance in combination with equivalent
input capacitance can cause a lagged response at the input, thus
delaying the output. The input capacitance of the ADCMP55x,
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
input capacitance yields a time constant of 15 ns, which is
significantly slower than the 500 ps capability of the
ADCMP55x. Source impedances should be significantly less
than 100 Ω for best performance.
Sockets should be avoided due to stray capacitance and
inductance. If proper high speed techniques are used, the
ADCMP55x should be free from oscillation when the
comparator input signal passes through the switching threshold.
COMPARATOR PROPAGATION DELAY
DISPERSION
The ADCMP55x has been specifically designed to reduce
propagation delay dispersion over an input overdrive range of
20 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 ADCMP55x is far less sensitive to input
variations than most comparator designs.
ADCMP551/ADCMP552/ADCMP553