lf451 National Semiconductor Corporation, lf451 Datasheet - Page 6

• Manufacturer Part Number: lf451
• Description: Wide-bandwidth Jfet-input Operational Amplifier
• Manufacturer: National Semiconductor Corporation

Application Hints
Exceeding the positive common-mode limit on a single input
will not change the phase of the output however if both
inputs exceed the limit the output of the amplifier will be
forced to a high state
The amplifier will operate with a common-mode input volt-
age equal to the positive supply however the gain band-
width and slew rate may be decreased in this condition
When the negative common-mode voltage swings to within
3V of the negative supply an increase in input offset voltage
may occur
The LF451 is biased by a zener reference which allows nor-
mal circuit operation on
ages less than these may result in lower gain bandwidth and
slew rate
The LF451 will drive a 2 kX load resistance to
the full temperature range of 0 C to
is forced to drive heavier load currents however an in-
crease in input offset voltage may occur on the negative
voltage swing and finally reach an active current limit on
both positive and negative swings
Precautions should be taken to ensure that the power sup-
ply for the integrated circuit never becomes reversed in po-
larity or that the unit is not inadvertently installed backwards
in a socket as an unlimited current surge through the result-
ing forward diode within the IC could cause fusing of the
internal conductors and result in a destroyed unit
As with most amplifiers care should be taken with lead
dress component placement and supply decoupling in or-
der to ensure stability For example resistors from the out-
put to an input should be placed with the body close to the
Detailed Schematic
g
4V power supplies Supply volt-
(Continued)
a
70 C If the amplifier
g
10V over
6
input to minimize ‘‘pick-up’’ and maximize the frequency of
the feedback pole by minimizing the capacitance from the
input to ground
A feedback pole is created when the feedback around any
amplifier is resistive The parallel resistance and capaci-
tance from the input of the device (usually the inverting in-
put) to AC ground set the frequency of the pole In many
instances the frequency of this pole is much greater than
the expected 3 dB frequency of the closed loop gain and
consequently there is negligible effect on stability margin
However if the feedback pole is less than approximately 6
times the expected 3 dB frequency a lead capacitor should
be placed from the output to the input of the op amp The
value of the added capacitor should be such that the RC
time constant of this capacitor and the resistance it parallels
is greater than or equal to the original feedback pole time
constant
The benefit of the S O package results from its very small
size It follows however that the die inside the S O pack-
age is less protected from external physical forces than a
die in a standard DIP would be because there is so much
less plastic in the S O Therefore not following certain pre-
cautions when board mounting the LF451CM can put me-
chanical stress on the die lead frame and or bond wires
This can cause shifts in the LF451CM’s parameters even
causing them to exceed limits specified in the Electrical
Characteristics For recommended practices in LF451CM
surface mounting refer to Application Note AN-450 ‘‘Sur-
face Mounting Methods and Their Effect on Product Reli-
ability’’ and to Section 6 ‘‘Surface Mount’’ found in any
Rev 1 Linear Databook volume
TL H 9660 – 11