EL5285C - Preliminary
Dual and Window 4ns High-Speed Comparators
Applications Information
imum slew rate requirements. In some applications, it
may be helpful to apply some positive feedback (hyster-
esis) between the output and the positive input. The
hysteresis effectively causes one comparator's input
voltage to move quickly past the other, thus taking the
input out of the region where oscillation occurs. For the
EL5285C, the propagation delay increases when the
input slew rate increases for low overdrive voltages.
With high overdrive voltages, the propagation delay
does not change much with the input slew rate.
Power Supplies and Circuit Layout
The EL5285C comparator operates with single and dual
supply with 5V to 12V between VS+ and VS-. The out-
put side of the comparator is supplied by a single supply
from 2.7V to 5V. The rail to rail output swing enables
direct connection of the comparator to both CMOS and
TTL logic circuits. As with many high speed devices,
the supplies must be well bypassed. Elantec recom-
mends a 4.7µF tantalum in parallel with a 0.1µF
ceramic. These should be placed as close as possible to
the supply pins. Keep all leads short to reduce stray
capacitance and lead inductance. This will also mini-
mize unwanted parasitic feedback around the
comparator. The device should be soldered directly to
the PC board instead of using a socket. Use a PC board
with a good, unbroken low inductance ground plane.
Good ground plane construction techniques enhance sta-
bility of the comparators.
Latch Pin Dynamics
The EL5285C contains a “transparent” latch for each
channel. The latch pin is designed to be driven with
either a TTL or CMOS output. When the latch is con-
nected to a logic high level or left floating, the
comparator is transparent and immediately responds to
the changes at the input terminals. When the latch is
switched to a logic low level, the comparator output
remains latched to its value just before the latch’s high-
to-low transition. To guarantee data retention, the input
signal must remain the same state at least 1ns (hold time)
after the latch goes low and at least 2ns (setup time)
before the latch goes low. When the latch goes high, the
new data will appear at the output in approximately 6ns
(latch propagation delay).
Input Voltage Considerations
The EL5285C’s input range is specified from 0.1V
below VS- to 2.25V below VS+. The criterion for the
input limit is that the output still responds correctly to a
small differential input signal. The differential input
stage is a pair of PNP transistors, therefore, the input
bias current flows out of the device. When either input
signal falls below the negative input voltage limit, the
parasitic PN junction formed by the substrate and the
base of the PNP will turn on, resulting in a significant
increase of input bias current. If one of the inputs goes
above the positive input voltage limit, the output will
still maintain the correct logic level as long as the other
input stays within the input range. However, the propa-
gation delay will increase. When both inputs are outside
the input voltage range, the output becomes unpredict-
able. Large differential voltages greater than the supply
voltage should be avoided to prevent damages to the
input stage.
Hysteresis
Hysteresis can be added externally. The following two
methods can be used to add hysteresis.
Inverting comparator with hysteresis:
V
REF
R
3
R
R
2
+
-
1
V
IN
Input Slew Rate
R3 adds a portion of the output to the threshold set by R1
and R2. The calculation of the resistor values are as
follows:
Most high speed comparators oscillate when the voltage
of one of the inputs is close to or equal to the voltage on
the other input due to noise or undesirable feedback. For
clean output waveform, the input must meet certain min-
8
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