EL2157CS datasheet(14/20 Pages) ELANTEC

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EL2150CEL2157C

125 MHz Single Supply Clamping Op Amps

Applications Information

Contd

Output Voltage Clamp

The EL2157C amplifier has an output voltage

clamp This clamping action is fast being acti-

vated almost instantaneously and being deacti-

vated in k 7 ns and prevents the output voltage

from going above the preset clamp voltage This

can be very helpful when the EL2157C is used to

drive an AD converter as some converters can

require long times to recover if overdriven The

output voltage remains at the clamp voltage level

as long as the product of the input voltage and

the gain setting exceeds the clamp voltage If the

EL2157C is connected in a gain of 2 for example

and a3V DC is applied to the CLAMP pin any

voltage higher than a15V at the inputs will be

clamped and a3V will be seen at the output

Figure 1 below is a unity gain connected

EL2157C being driven by a 3Vp-p sinewave with

225V applied to the CLAMP pin The resulting

output waveform with its output being clamped

to 225V is shown in Figure 2

2150 – 59

Figure 1

2150 – 60

Figure 2

Figure 3 shows the output of the same circuit

being driven by a 05V to 275V square wave as

the clamp voltage is varied from 10V to 25V as

well as the unclamped output signal The rising

edge of the signal is clamped to the voltage ap-

plied to the CLAMP pin almost instantaneously

The output recovers from the clamped mode

within5-7ns depending on the clamp voltage

Even when the CLAMP pin is taken 02V below

the minimum 12V specified the output is still

clamped and recovers in about 11 ns

2150 – 61

Figure 3

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 ac-

curacy 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 volt-

age Curves of Clamp Accuracy vs VCLAMP and

VIN for 3 values of RL are included in the Typi-

cal Performance Curves Section

Unlike amplifiers that clamp at the input and are

therefore limited to non-inverting applications

only

the EL2157C 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 architec-

tures

The voltage clamp operates for any voltage be-

tween a12V above the GND pin and the mini-

mum output voltage swing VOP Forcing the

CLAMP pin much below a12V can saturate

transistors and should therefore be avoided

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