Application Hints**
Protecting Against Gross
Fault Conditions
*Protects input
²Protects output TL/H/7752– 26
³Protects outputÐnot needed when R4 is used.
Compensating for Stray Input Capacitances
or Large Feedback Resistor
C2 eR1 CS
R2
TL/H/7752– 27
Isolating Large Capacitive Loads
TL/H/7752– 28
Although the LM101A is designed for trouble free operation,
experience has indicated that it is wise to observe certain
precautions given below to protect the devices from abnor-
mal operating conditions. It might be pointed out that the
advice given here is applicable to practically any IC op amp,
although the exact reason why may differ with different de-
vices.
When driving either input from a low-impedance source, a
limiting resistor should be placed in series with the input
lead to limit the peak instantaneous output current of the
source to something less than 100 mA. This is especially
important when the inputs go outside a piece of equipment
where they could accidentally be connected to high voltage
sources. Large capacitors on the input (greater than 0.1 mF)
should be treated as a low source impedance and isolated
with a resistor. Low impedance sources do not cause a
problem unless their output voltage exceeds the supply volt-
age. However, the supplies go to zero when they are turned
off, so the isolation is usually needed.
The output circuitry is protected against damage from shorts
to ground. However, when the amplifier output is connected
to a test point, it should be isolated by a limiting resistor, as
test points frequently get shorted to bad places. Further,
when the amplifer drives a load external to the equipment, it
is also advisable to use some sort of limiting resistance to
preclude mishaps.
Precautions should be taken to insure that the power sup-
plies for the integrated circuit never become reversedÐ
even under transient conditions. With reverse voltages
greater than 1V, the IC will conduct excessive current, fus-
ing internal aluminum interconnects. If there is a possibility
of this happening, clamp diodes with a high peak current
rating should be installed on the supply lines. Reversal of
the voltage between Vaand Vbwill always cause a prob-
lem, although reversals with respect to ground may also
give difficulties in many circuits.
The minimum values given for the frequency compensation
capacitor are stable only for source resistances less than
10 kX, stray capacitances on the summing junction less
than 5 pF and capacitive loads smaller than 100 pF. If any
of these conditions are not met, it becomes necessary to
overcompensate the amplifier with a larger compensation
capacitor. Alternately, lead capacitors can be used in the
feedback network to negate the effect of stray capacitance
and large feedback resistors or an RC network can be add-
ed to isolate capacitive loads.
Although the LM101A is relatively unaffected by supply by-
passing, this cannot be ignored altogether. Generally it is
necessary to bypass the supplies to ground at least once on
every circuit card, and more bypass points may be required
if more than five amplifiers are used. When feed-forward
compensation is employed, however, it is advisable to by-
pass the supply leads of each amplifier with low inductance
capacitors because of the higher frequencies involved.
**Pin connections shown are for 8-pin packages.
7