adcmp581 Analog Devices, Inc., adcmp581 Datasheet - Page 12

• Manufacturer Part Number: adcmp581
• Description: Ultrafast Sige Voltage Comparator
• Manufacturer: Analog Devices, Inc.

ADCMP580/ADCMP581/ADCMP582
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 can cause oscillation. Discontinuities along input and output
transmission lines can also severely limit the specified pulse
width dispersion performance.
For applications in a 50 Ω environment, input and output
matching have a significant impact on data-dependent (or
deterministic) jitter (DJ) and pulse width dispersion
performance. The ADCMP58x family of comparators provides
internal 50 Ω termination resistors for both V
The return side for each termination is pinned out separately
with the V
is desired at one or both of the V
pins can be connected (or disconnected) to (from) the desired
termination potential as appropriate. The termination potential
should be carefully bypassed using ceramic capacitors as dis-
cussed previously to prevent undesired aberrations on the input
signal due to parasitic inductance in the termination return
path. If a 50 Ω termination is not desired, either one or both
of the V
case, the open pins should be left floating with no external pull
downs or bypassing capacitors.
For applications that require high speed operation but do not
have on-chip 50 Ω termination resistors, some reflections
should be expected, because the comparator inputs can no
longer provide matched impedance to the input trace leading
up to the device. It then becomes important to back-match the
drive source impedance to the input transmission path leading
to the input to minimize multiple reflections. For applications
in which the comparator is less than 1 cm from the driving
signal source, the source impedance should be minimized. High
source impedance in combination with parasitic input capaci-
tance of the comparator could cause undesirable degradation
in bandwidth at the input, thus degrading the overall response.
It is therefore recommended that the drive source impedance
should be no more than 50 Ω for best high speed performance.
TP
/V
TP
TN
and V
termination pins can be left disconnected. In this
TN
pins, respectively. If a 50 Ω termination
P
/V
N
inputs, the V
P
and V
TP
and V
N
inputs.
TN
Rev. A | Page 12 of 16
COMPARATOR PROPAGATION DELAY DISPERSION
The ADCMP58x family of comparators has been specifically
designed to reduce propagation delay dispersion over a wide
input overdrive range of 5 mV to 500 mV. Propagation delay
dispersion is a change in propagation delays that results
from a change in the degree of overdrive or slew rate (how far
or fast the input signal exceeds the switching threshold). The
overall result is a higher degree of timing accuracy.
Propagation delay dispersion is a specification that becomes
important in critical timing applications, such as data commu-
nications, automatic test and measurement, instrumentation,
and event-driven applications, such as pulse spectroscopy,
nuclear instrumentation, and medical imaging. Dispersion
is defined as the variation in the overall propagation delay as
the input overdrive conditions are changed (see Figure 26 and
Figure 27). For the ADCMP58x family of comparators, disper-
sion is typically <25 ps, because the overdrive varies from 5 mV
to 500 mV, and the input slew rate varies from 1 V/ns to 10 V/ns.
This specification applies for both positive and negative signals
because the ADCMP58x family of comparators has almost
equal delays for positive- and negative-going inputs.
INPUT VOLTAGE
Q/Q OUTPUT
INPUT VOLTAGE
Q/Q OUTPUT
Figure 26. Propagation Delay—Overdrive Dispersion
Figure 27. Propagation Delay—Slew Rate Dispersion
10V/ns
1V/ns
500mV OVERDRIVE
5mV OVERDRIVE
DISPERSION
DISPERSION
V
V
N
N
± V
± V
OS
OS