7

Simplified Schematic

Applications Information

The EL2252 is very easy to use and is relatively oscillation-

free, but a few items must be attended. The first is that both

supplies should be bypassed closely. 1µF tantalums are very

good and no additional smaller capacitors are necessary.

The EL2252 requires V- to be at least 5V to preserve AC

performance. V+ must be at least 6V for a TTL output swing,

8V for CMOS outputs.

The input voltage range will be referred to the more positive

of the two inputs. That is, bringing an input as negative as V-

will not cause problems; it's the other input's level that must

be considered. The typical input range is +13/-12V when the

supplies are ±15V. This range diminishes over temperature

and varies with processing; it is wise to set power supplies

such that V+ is 5V more positive than the most positive input

signal and V- more negative than 6V below the most

negative input. ±12V supplies will easily encompass all

CMOS and ECL logic inputs. If the input exceeds the

device's common-mode input capability, the EL2252

propagation delay and input bias current will increase. Fault

currents will occur with inputs a diode below V- or above V+.

within the absolute maximum ratings.

One of the few ways in which oscillations can be induced is

by connecting a high-Q reactive source impedance to the

EL2252 inputs. Such sources are long wires and

unterminated coaxial lines. The source impedance should

be de-Q'ed. One method is to connect a series resistor to the

EL2252 input of around 100

Ω value. More resistance will

calm the system more effectively, but at the expense of

comparator response time. Another method is to install a

“snubber” network from comparator input to ground. A

snubber is a resistor in series with a small capacitor, around

100

Ω and 33pF. Each physical and electrical environment

will require different treatments, although many need none.

The major use of the HYS pin is to suppress noise

superimposed on the input signal. By shorting the HYS pin

comparator input. Leaving the pin open, or more

appropriately, grounding the HYS pin removes all hysteresis.

Connecting a resistor between HYS and V- allows an

adjustment of the peak-to-peak hysteresis level.

Unfortunately, an external resistor cannot track the internal

devices properly, so temperature and unit-to-unit variations

of hysteresis are increased. The relationship between the

resistor and resulting hysteresis level is not linear, but a 1.5k

resistor will approximately halve the nominal value.

The time delay of the EL2252 will increase by about 0.7ns

when using full hysteresis.

The EL2252 is specifically designed to be tolerant of large

inputs. It will exhibit very much increased delay times for

input overdrives below 100mV. If very small overdrives must

be sensed, the EL2018 or EL2019 comparators would be

good choices, although they lose accuracies with signal

input Slew Rates above 400V/µs. The EL2252 keeps its

timing accuracy with input Slew Rates between 100V/µs and

4000V/µs of input Slew Rate.

The output stage drives tens of pF load capacitances without

increased overshoot, but propagation delay increases about

ONE COMPARATOR

EL2252