EL2257CS datasheet(13/17 Pages) INTERSIL

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voltage higher than +1.5V at the inputs will be clamped and

+3V will be seen at the output. Each amplifier of the EL2257

have their own CLAMP pin, so individual clamp levels may

be set.

Figure 1 below is the EL2257 with each amplifier unity gain

connected. Amplifier A is being driven by a 3VP-P sinewave

and has 2.25V applied to CLAMPA, while amplifier B is

driven by a 3VP-P triangle wave and 1.5V is applied to

CLAMPB. The resulting output waveforms, with their outputs

being clamped is shown in Figure 2.

Figure 3 shows the output of amplifier A 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 5–7ns,

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 up 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 EL2257 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 EL2257 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.

EL2257 Comparator Application

The EL2257 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 EL2257, the maximum

output voltage can be clamped, thus preventing saturation.

Figure 4 is amplifier A of an EL2257 implemented as a

comparator. 2.25V 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.

EL2257

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