8

EL5281C

Dual 8ns High-Speed Comparator

Applications Information

Power Supplies and Circuit Layout

The EL5281C comparator operates with single and dual

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 comparator.

Input Voltage Considerations

The EL5281C input range is specified from 0.1V below

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 cor-

rect logic level as long as the other input stays within the

input range. However, the propagation delay will

increase. When both inputs are outside the input voltage

range, the output becomes unpredictable. Large differ-

ential voltages greater than the supply voltage should be

avoided to prevent damages to the input stage. Inputs of

unused channels should not be left floating. They should

be driven to a known state. For example, one input can

be tied to ground and the other input can be connected to

some voltage reference (like ±100mV) to avoid oscilla-

tion in the output due to unwanted output to input

feedback.

Input Slew Rate

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-

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

EL5281C, 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.

Latch Pin Dynamics

The EL5281C 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).

Hysteresis

Hysteresis can be added externally. The following two

methods can be used to add hysteresis.

Inverting comparator with hysteresis:

+

-