Application Hints

Input Voltages

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

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

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 andor 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

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

Layout Considerations

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

When operating in high humidity environments or near 0 C

some form of surface coating may be necessary to provide

a moisture barrier

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

Electrostatic shielding of high impedance circuitry is advisa-

ble

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

Every attempt should be made to achieve a single point

ground system as shown in the figure below

TLK5558 – 7

FIGURE 1 Single-Point Grounding

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)

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

Short Circuit Current Limiting

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

3% C

Short circuit current will be limited to approximately

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