ADCMP572/ADCMP573 Data Sheet
Rev. B | Page 10 of 14
V
CCO
is the signal return for the output stage and V
CCO
pins
should of course be connected to a supply plane for maximum
performance.
OPTIMIZING HIGH SPEED PERFORMANCE
As with any high speed comparator, proper design and layout
techniques are essential to obtaining the specified performance.
Stray capacitance, inductance, inductive power and ground
impedances, or other layout issues can severely limit performance
and often cause oscillation. Discontinuities along input and
output transmission lines can severely limit the specified pulse
width dispersion performance.
For applications working in a 50 Ω environment, input and
output matching has a significant impact on data dependent (or
deterministic) jitter (DJ) and on pulse width dispersion
performance. The ADCMP572/ADCMP573 comparators
provide internal 50 Ω termination resistors for both the V
P
and
V
N
inputs, and the ADCMP572 provides 50 Ω back terminated
outputs. The return side for each input termination is pinned
out separately with the V
TP
and V
TN
pins, respectively. If a 50 Ω
termination is desired at one or both of the V
P
/V
N
inputs, then
the V
TP
and V
TN
pins can be connected (or disconnected) to
(from) the desired termination potential as required. The
termination potential should be carefully bypassed using high
quality bypass capacitors as discussed earlier to prevent undesired
aberrations on the input signal due to parasitic inductance in
the circuit board layout. If a 50 Ω input termination is not
desired, either one or both of the V
TP
/V
TN
termination pins can
be left disconnected. In this case, the pins should be left floating
with no external pull-downs or bypassing capacitors.
When leaving an input termination disconnected, the internal
resistor acts as a small stub on the input transmission path and
can cause problems for very high speed inputs. Reflections
should then be expected from the comparator inputs because
they no longer provide matched impedance to the input path
leading to the device. In this case, it is important to back match
the drive source impedance to the input transmission path to
minimize multiple reflections. For applications in which the
comparator is very close to the driving signal source, the source
impedance should be minimized. High source impedance in
combination with parasitic input capacitance of the comparator
might cause an undesirable degradation in bandwidth at the
input, therefore degrading the overall response. Although the
ADCMP572/ADCMP573 comparators have been designed to
minimize input capacitance, some parasitic capacitance is
inevitable. It is therefore recommended that the drive source
impedance be no more than 50 Ω for best high speed performance.
COMPARATOR PROPAGATION
DELAY DISPERSION
The ADCMP572/ADCMP573 comparators are designed to
reduce propagation delay dispersion over a wide input overdrive
range of 5 mV to 500 mV. Propagation delay dispersion is variation
in the propagation delay that results from a change in the degree of
overdrive or slew rate (how far or how fast the input signal
exceeds the switching threshold).
Propagation delay dispersion is a specification that becomes
important in high speed, time-critical applications such as data
communication, automatic test and measurement, instrumenta-
tion, and event driven applications such as pulse spectroscopy,
nuclear instrumentation, and medical imaging. Dispersion is
defined as the variation in propagation delay as the input over-
drive conditions vary (Figure 17 and Figure 18). For the
ADCMP572/ADCMP573, dispersion is typically <15 ps
because the overdrive varies from 10 mV to 500 mV, and the
input slew rate varies from 2 V/ns to 10 V/ns. This specification
applies for both positive and negative signals since the
ADCMP572/ADCMP573 has substantially equal delays for
either positive going or negative going inputs.
Q/Q OUTPUT
INPUT VOLTAGE
500mV OVERDRIVE
10mV OVERDRIVE
DISPERSION
V
N
± V
OS
04409-0-027
Figure 17. Propagation Delay—Overdrive Dispersion
Q/Q OUTPUT
INPUT VOLTAGE
10V/ns
1V/ns
DISPERSION
V
N
± V
OS
04409-0-028
Figure 18. Propagation Delay—Slew Rate Dispersion
