EL2157CS datasheet(15/19 Pages) INTERSIL

15 / 19 page

Figure 1 below is a unity gain connected EL2157 being

driven by a 3VP-P sinewave, with 2.25V applied to the

CLAMP pin. The resulting output waveform, with its output

being clamped to 2.25V, is shown in Figure 2.

Figure 3 shows the output of the same circuit being driven by

a 0.5V to 2.75V square wave, as the clamp voltage is varied

from 1.0V to 2.5V, as well as the unclamped output signal.

The rising edge of the signal is clamped to the voltage

applied to the CLAMP pin almost instantaneously. The

output recovers from the clamped mode within 5ns - 7ns,

depending on the clamp voltage. Even when the CLAMP pin

is taken 0.2V below the minimum 1.2V specified, the output

is still clamped and recovers in about 11ns.

The clamp accuracy is affected by 1) the CLAMP pin

voltage, 2) the input voltage, and 3) the load resistor.

Depending upon the application, the accuracy may be as

little as a few tens of millivolts to a few hundred millivolts. Be

sure to allow for these inaccuracies when choosing the

clamp voltage. Curves of Clamp Accuracy vs VCLAMP, and

VIN for 3 values of RL are included in the Typical

Performance Curves Section.

Unlike amplifiers that clamp at the input and are therefore

limited to non-inverting applications only, the EL2157 output

clamp architecture works for both inverting and non-inverting

gain applications. There is also no maximum voltage

difference limitation between VIN and VCLAMP which is

common on input clamped architectures.

The voltage clamp operates for any voltage between +1.2V

above the GND pin, and the minimum output voltage swing,

VOP. Forcing the CLAMP pin much below +1.2V can

saturate transistors and should therefore be avoided.

Forcing the CLAMP pin above VOP simply de-activates the

CLAMP feature. In other words, one cannot expect to clamp

any voltage higher than what the EL2157 can drive to in the

first place. If the clamp feature is not desired, either let the

CLAMP pin float or connect it to the VS+ pin.

EL2157 Comparator Application

The EL2157 can be used as a very fast, single supply

comparator by utilizing the clamp feature. Most op amps

used as comparators allow only slow speed operation

because of output saturation issues. However, by applying a

DC voltage to the CLAMP pin of the EL2157, the maximum

output voltage can be clamped, thus preventing saturation.

Figure 4 below is the EL2157 implemented as a comparator.

2.5V DC is applied to the CLAMP pin, as well as the IN- pin.

A differential signal is then applied between the inputs.

Figure 5 shows the output square wave that results when a

±1V, 10MHz triangular wave is applied, while Figure 6 is a

graph of propagation delay vs. overdrive as a square wave is

presented at the input.

FIGURE 1.

FIGURE 2.

FIGURE 3.

FIGURE 4.

FIGURE 5.

EL2150, EL2157

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