el2257 Intersil Corporation, el2257 Datasheet - Page 13

• Manufacturer Part Number: el2257
• Description: 125mhz Single Supply, Clamping Op Amp
• Manufacturer: Intersil Corporation

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 3V
and has 2.25V applied to CLAMPA, while amplifier B is
driven by a 3V
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.
P-P
triangle wave and 1.5V is applied to
FIGURE 1.
FIGURE 2.
13
P-P
sinewave
EL2257
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. V
3 values of R
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 V
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,
V
transistors and should therefore be avoided. Forcing the
CLAMP pin above V
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 V
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 I
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.
OP
. Forcing the CLAMP pin much below +1.2V can saturate
N
- pin. A differential signal is then applied between
L
are included in the Typical Performance
OP
simply de-activates the CLAMP
FIGURE 3.
IN
and V
S
+ pin.
CLAMP
CLAMP
which is
and V
IN
for