Application Hints (Continued)
common-mode range again puts the input stage and thus
the amplifier in a normal operating mode.
wards in a socket as an unlimited current surge through the
resulting forward diode within the IC could cause fusing of
the internal conductors and result in a destroyed unit.
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.
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
input to minimize ‘‘pick-up’’ and maximize the frequency of
the feedback pole by minimizing the capacitance from the
input to ground.
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.
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 LF351 is biased by a zener reference which allows nor-
g
mal circuit operation on 4V power supplies. Supply volt-
ages less than these may result in lower gain bandwidth and
slew rate.
g
The LF351 will drive a 2 kX load resistance to 10V over
a
the full temperature range of 0 C to 70 C. If the amplifier
§
§
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 supply
for the integrated circuit never becomes reversed in polarity
or that the unit is not inadvertently installed back-
Detailed Schematic
TL/H/5648–9
7
NSC [ National Semiconductor ]
