Application Hints
Input Voltages
Electrostatic shielding of high impedance circuitry is advisa-
ble.
The LH0101 operational amplifier contains JFET input de-
vices which exhibit high reverse breakdown voltages from
gate to source or drain. This eliminates the need for input
clamp diodes, so that high differential input voltages may be
applied without a large increase in input current. However,
neither input voltage should be allowed to exceed the nega-
tive supply as the resultant high current flow may destroy
the unit.
Error voltages can also be generated in the external circuit-
ry. Thermocouples formed between dissimilar metals can
cause hundreds of microvolts of error in the presence of
temperature gradients.
Since the LH0101 can deliver large output currents, careful
attention should be paid to power supply, power supply by-
passing and load currents. Incorrect grounding of signal in-
puts and load can cause significant errors.
Exceeding the negative common-mode limit on either input
will cause a reversal of the phase to the output and force
the amplifier output to the corresponding high or low state.
Exceeding the negative common-mode limit on both inputs
will force the amplifier output to a high state. In neither case
does a latch occur since raising the input back within the
common-mode range again puts the input stage and thus
the amplifier in a normal operating mode.
Every attempt should be made to achieve a single point
ground system as shown in the figure below.
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.
These amplifiers will operate with the common-mode input
voltage equal to the positive supply. In fact, the common-
mode voltage may exceed the positive supply by approxi-
mately 100 mV, independent of supply voltage and over the
full operating temperature range. The positive supply may
therefore be used as a reference on an input as, for exam-
ple, in a supply current monitor and/or limiter.
With the LH0101 there is a temptation to remove the bias
current compensation resistor normally used on the non-in-
verting input of a summing amplifier. Direct connection of
the inputs to ground or a low-impedance voltage source is
not recommended with supply voltages greater than 3V.
The potential problem involves loss of one supply which can
cause excessive current in the second supply. Destruction
of the IC could result if the current to the inputs of the de-
vice is not limited to less than 100 mA or if there is much
more than 1 mF bypass on the supply buss.
TL/K/5558–7
FIGURE 1. Single-Point Grounding
Bypass capacitor C should be used if the lead lengths of
BX
bypass capacitors C are long. If a single point ground sys-
B
tem is not possible, keep signal, load, and power supply
from intermingling as much as possible. For further informa-
tion on proper grounding techniques refer to ‘‘Grounding
and Shielding Techniques in Instrumentation’’ by Morrison,
and ‘‘Noise Reduction Techniques in Electronic Systems’’
by Ott (both published by John Wiley and Sons).
Although difficulties can be largely avoided by installing
clamp diodes across the supply lines on every PC board, a
conservative design would include enough resistance in the
input lead to limit current to 10 mA if the input lead is pulled
to either supply by internal currents. This precaution is by no
means limited to the LH0101.
Leads or PC board traces to the supply pins, short-circuit
current limit pins, and the output pin must be substantial
enough to handle the high currents that the LH0101 is capa-
ble of producing.
Layout Considerations
Short Circuit Current Limiting
When working with circuitry capable of resolving pico-am-
pere level signals, leakage currents in circuitry external to
the op amp can significantly degrade performance. High
quality insulation is a must (Kel-F and Teflon rate high).
Proper cleaning of all insulating surfaces to remove fluxes
and other residues is also required. This includes the IC
package as well as sockets and printed circuit boards.
a
should be shorted to V and SC should be shorted to
Should current limiting of the output not be necessary, SC
a
b
. Remember that the short circuit current limit is depen-
b
V
dent upon the total resistance seen between the supply and
current limit pins. This total resistance includes the desired
resistor plus leads, PC Board traces, and solder joints.* As-
suming a zero TCR current limit resistor, typical temperature
coefficient of the short circuit current will be approximately
When operating in high humidity environments or near 0 C,
§
some form of surface coating may be necessary to provide
a moisture barrier.
.3%/ C.
§
0.6
*Short circuit current will be limited to approximately
.
The effects of board leakage can be minimized by encircling
the input circuitry with a conductive guard ring operated at a
potential close to that of the inputs.
RSC
6
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