NEW COMPARATOR PRODUCT LINE
TARGETS MEDICAL, INDUSTRIAL & ATE
Feature Article from Hittite Microwave Corporation
New Comparators Deliver 20 Gbps Speed at 50% Lower Power Consumption
vs. Traditional Comparator Solutions
A
comparator is a fundamental
analog building block which can
be used to determine whether an
input voltage is higher or lower
than a reference voltage, and set the output
to one of two levels. Comparators can be used in
a wide range of applications, such as high speed trigger
circuits, clock and data recovery, and 1-Bit analog-to-digital
conversion. Comparators are also key components in many
industrial wideband data acquisition, ATE, and real time
medical imaging systems. In these systems, the common
design trend is to simplify the complexity of all of the stages
from signal acquisition to analog-to-digital conversion
(ADC), resulting in digitization of signals at higher sample
rates. Consequently, many traditional comparator circuits
are now operating at higher clock and data rates, and this
has resulted in more stringent specifications for random and
deterministic jitter, lower propagation delay, reduced data
skew values and even lower power consumption.
To meet these requirements Hittite Microwave now offers
a full line of high speed, low power comparators for use in
the above mentioned applications. Hittite’s comparators
currently include six products, which can be sub-divided
into two categories: the level-latched family which
includes part numbers HMC674LC3C, HMC675LC3C and
HMC676LC3C; and the clocked or edge-latched family
which includes part numbers HMC874LC3C, HMC875LC3C,
and HMC876LC3C. Both families share the same proven
input topology that offers the advantage of high input
bandwidth (>10 GHz), low propagation delay dispersion,
COMPUTER
INTERFACE
ERROR
AMP
CLOCKED
COMP
HMC876LC3C
REF 4
TIMING
ACTIVE
LOAD
LATCHED
COMP
HMC874LC3C
REF 3
FORMATTER /
DESKEWER
DRIVER
CLOCKED
COMP
DUT
LATCHED
COMP
HMC875LC3C
REF 2
REF 1
HMC675LC3C
Figure 1: Typical Use of Hittite Comparators in an ATE Application
Hittite Microwave Corporation
February 2010
NEW COMPARATOR PRODUCT LINE IDEAL FOR
INP
FRONT
END
Q
INP
Q
INN
FRONT
END
LATCH
INN
LE
Q
MASTER
SLAVE
Q
LE
CLK
CLK
HMC874/875/876LC3C
CLOCKED-LATCHED
COMPARATORS
HMC674/675/676LC3C
LEVEL-LATCHED
COMPARATORS
Figure 2: Comparator Circuit Topologies: Level-Latched vs. Clocked (or Edge-Latched)
low jitter, and high input-common-mode-range. At the same
time, both families maintain very low power and low thermal
hysteresis. Figure 2 highlights the topology differences
between the two families. Devices from both families are
offered with reduced output swing ECL, PECL, and CML logic
driver versions. Table 1 compares the key specifications of
both families.
Level-Latched Comparator Family
HMC674LC3C, HMC675LC3C and HMC676LC3C are
monolithic, fast comparators which feature latch inputs and
reduced swing PECL, CML and ECL output drivers res-
pectively. These comparators support 10 Gbps operation
and exceed the performance of their closest competitors
by providing approximately double the speed with almost
50% reduction in power dissipation. The HMC67xLC3C
comparator family also exhibits 85 ps propagation delay and
60 ps minimum pulse width with only 0.2 ps rms jitter (RJ).
The most important characteristic of a comparator is to
preserve the phase and timing information at the instant that
the input voltage crosses the reference. Any aberration of
this instant is due to dispersion and jitter which results in
distortion of the phase/timing information. The ability to latch
this information is important in noisy environments or when
the user is interested in a timing window for the threshold
crossing.
Table 1: Key Specifications Comparison of the Level-Latched Family
vs. the Clocked Comparator Family (typical values shown)
Parameter
Level-Latched
Family
Clocked
Family
Units
Equivalent Input BW
10
10
GHz
Input Clock Rate
n/a
20
Gbps
Tpd (Propagation Delay)
85
120
ps
Tpd Dispersion for 50 mV < VOD < 1 V
10
10
ps
Tpd Dispersion for -1.75 V < Vcm < +1.75 V
8
8
ps
Minimum Input Pulse Width
(VOD = ±100 mV)
60
60
ps
Deterministic Jitter (p-p)
2
<3
ps
Random Jitter (rms)
Input CM
Power Dissipation (Pd)
LE / LE Pins Termination
0.2
0.2
ps
-1.75 to +1.75
-1.75 to +1.75
V
100 - 140
130 - 150
mW
8000
50
Ohms
0 to 25
mV
(High Impedance to
all track only)
Hysteresis Control
0 to 25
The level-latched comparator family
also offers several applicationspecific options to the user. First,
the latch pins (LE/LE) can be left
floating (see Figure 2), such that
the comparator is in the unlatched
or transparent mode. This feature is
useful for temporal alignment, which
is a critical requirement in MRI or
medical ultrasound applications.
The high gain of the comparator
allows the user to detect whether
the input level has exceeded the
reference level without requiring
additional amplification prior to the
output stage. No additional LE/
LE voltage is required for these
applications.
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MEDICAL, INDUSTRIAL & ATE APPLICATIONS
If the LE/LE latch pins are driven, then the comparator can
attain its latched phase. In this mode, the user can store (or
latch) the actual crossover instant. The comparator remains
in the latched mode when the LE voltage level exceeds
the LE voltage level. To reverse to the transparent mode
the voltage levels are reversed, (i.e. the LE voltage level is
less than the LE voltage level).
20
One of the most important characteristics of any comparator
is propagation delay - the time required for the output to
switch from a low state to a high state, when the input voltage
crosses the reference level. A more critical issue however, is
the change in propagation delay as a function of overdrive
voltage (VOD). This change in propagation delay is called
dispersion, as shown in Figure 4. Hittite’s comparators exhibit minimal dispersion, which is highly beneficial in
applications where phase preservation is critical.
15
12.5
12
10
ΔtPD (ps)
HYSTERESIS (mV)
16
8
4
Rising Edge
Falling Edge
7.5
5
2.5
0
0
10
100
1000
10000
RESISTANCE (Ohm)
Figure 3: Comparator Hysteresis vs. Rhys Control
Resistor for the HMC674LC3C
The level-latched comparators have been optimized to
significantly reduce thermal hysteresis. In some applications
however, the differential input to the comparator may be
superimposed on a slow varying signal, which could cause the
comparator to “chatter.” In those applications, it is beneficial
for the user to create a known amount of hysteresis (nonthermal). The level-latched comparator family accomplishes
this by connecting the HYS pin through a resistor to the
negative terminal (Vee). The amount of hysteresis may be
programmed from 0 to 25 mV by selecting the appropriate
resistor value, as shown in Figure 3.
-2.5
-5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
COMMON MODE VOLTAGE (mV)
Figure 5: Propagation Delay vs. Common Mode Voltage
for the HMC674LC3C
Equally important for a comparator is its ability to accept a
large common mode voltage at its input without affecting the
propagation delays. Hittite’s comparators can accept input
common mode voltages between +1.75 to -1.75V, which
makes them ideal for a wide range of applications (Figure 5).
Finally, jitter is an important parameter of the comparator
as it characterizes the variation in response time over a
large number of sampled input voltages. This is especially
11
DISPERSION (ps)
9
Rising Edge
Falling Edge
7
5
3
1
-1
0
10
20
30
40
50
60
70
80
90
100
OVERDRIVE VOLTAGE (mV)
Figure 4: Dispersion vs. Overdrive Voltage
for the HMC674LC3C
Figure 6: HMC674LC3C Eye Diagram
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HITTITE’S NEW COMPARATOR PRODUCT LINE
important in ATE channels where any source of noise can
contaminate the desired information. Figure 6 shows a
typical eye diagram of Hittite level-latched comparators.
The level-latched comparators are housed in compact,
ceramic, RoHS-compliant, 3x3 mm SMT packages and are
specified for operation from -40°C to +85°C.
The Clocked Comparator Family
The clocked comparator family
is unique in the market today,
offering 20 Gbps speed at 150
mW power consumption, which
represents 50% less power
than traditional comparator
solutions. The HMC874LC3C,
HMC875LC3C and the HMC876LC3C are monolithic, ultra
fast clocked comparators that are ideal for ATE and medical
imaging applications where high speed, high performance
and low power are critical requirements.
The clocked comparator family is an extension of the levellatched family, and it relies on the same input front end circuit.
The additional latch inserted between the first latch and the
output stage (Figure 2) changes the latching instance from a
voltage level to an edge of the clock signal.
Consider the application of a latch signal applied to the
level-latched comparator. If the signal has a 50% duty cycle,
then the timing allowed for the comparison of the input and
the threshold voltage is half of this period (during the other
half of the period, the device is in its latched phase). In
contrast to the level-latched comparator, the edge-latched
comparator accomplishes the latching function on the rising
edge of the clock. This is advantageous since an entire clock
period can now be dedicated to the next comparison phase.
Unlike the level-latched comparators, the CLK/CLK ports
of the clocked comparators are terminated with 50 Ohm
resistors, so these pins cannot be left floating to create a
transparent (unlatched) mode.
However, all other features of the level-latched family are
either maintained or improved. The clocked comparator
family features high input bandwidth of 10 GHz, low
propagation delay dispersion of 10 ps, low random jitter
of 0.2 ps, and high input-common-mode-range of ±1.75V.
Clock-to-data output delay is specified at only 120 ps.
Although the clocked comparator family is normally specified to operate with clocks of 20 GHz, up to 25 GHz can
be achieved if logic output swings lower than 300 mV are
acceptable.
The clocked comparators are housed in compact, ceramic,
RoHS-compliant, 3x3 mm SMT packages and are specified
for operation from -40 °C to +85 °C.
Summary
Hittite Microwave continues to lead the way with state-ofthe-art innovations in high speed, low power comparator
components.
The level-latched and clocked comparator families were
developed to be used in applications where speed/power
dissipation is defined as an important figure of merit. The
larger the ratio, the better these comparators are suited
to the task. For example, in an ATE system, thousands of
comparators may be required, so power comes at a large
premium. Similar requirements arise in the medical field
(MRI/Ultrasound) even though the bandwidth requirements
are reduced. Clearly, low dispersion vs. input slew and
overdrive conditions as well as low jitter is important in
medical equipment applications as it translates to low
noise since the frequency used in penetrating different
tissue depths is varied. The low dispersion vs. overdrive
specification allows the multi-channel sensors to be aligned
temporally and therefore require fewer calibration cycles.
Designers looking for versions of these products with
alternate pinouts, alternate DC power supply voltages or
higher operating speeds should contact Hittite Microwave
directly. Multiple comparator and/or logic functions may
also be combined to provide products with higher levels
of integration, and these may be made available in SMT
packages or module format to meet the needs of custom
applications. Data sheets and supporting information for
Hittite’s comparators are available at www.hittite.com.
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