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AN-242

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Applying a New Precision Op Amp

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National Semiconductor

Application Note 242

April 1980

Applying a New

Precision Op Amp

Robert J. Widlar

Apartado Postal 541

Puerto Vallarta, Jalisco

Mexico

Bob Pease and Mineo Yamatake

National Semiconductor Corporation

Santa Clara, California

U.S.A.

Transistors with typical current gains of 5000 have been

used in the manufacture of the LM11. The input stage em-

ploys a Darlington connection that has been modified so

that offset voltage and drift are not degraded. The typical

input currents, plotted in Figure 1, demonstrate the value of

the approach.

Abstract: A new bipolar op amp design has advanced the

state of the art by reducing offset voltage and bias current

errors. Its characteristics are described here, indicating an

ultimate input resolution of 10 mV and 1 pA under laboratory

conditions. Practical circuits for making voltmeters, amme-

ters, differential instrumentation amplifiers and a variety of

other designs that can benefit from the improved perform-

ance are covered in detail. Methods of coupling the new

device to existing fast amplifiers to take advantage of the

best characteristics of both, even in follower applications,

are explored.

introduction

A low cost, mass-produced op amp with electrometer-type

input currents combined with low offset voltage and drift is

now available. Designated the LM11, this IC can minimize

production problems by providing accuracy without adjust-

ments, even in high-impedance circuitry. On the other hand,

if pushed to its full potential, what has been impossible in

the past becomes entirely practical.

Significantly, the LM11 is not restricted to commercial and

industrial use. Devices can be completely specified over a

TL/H/7479–1

Figure 1. Below 100 C, bias current varies almost linear-

125 C range. Preliminary data indicates that

55 C to

ly with temperature. This means that simple

circuitry can be used for compensation. Offset

current is unusually low.

reliability is the same as standard ICs qualified for military

and space applications.

The essential details of the design along with an introduc-

tion to the peculiarities of high-impedance circuits have

been presented elsewhere.* This will be expanded here.

Practical circuitry that reduces effective bias current for

those applications where performance cannot be made de-

pendent on offset current are described. In addition, circuits

combining the DC characteristics of the new part with the

AC performance of existing fast amplifiers will be shown.

This will be capped with a number of practical designs to

provide some perspective into what might be done.

The offset current of this op amp is so low that it cannot be

measured on existing production test equipment. Therefore,

it probably cannot be specified tighter than 10 pA. For crit-

ical applications, the user should have little difficulty in se-

lecting to a tighter limit.

The bias current of the LM11 equals that of monolithic FET

amplifier at 25 C. Unlike FETs, it does not double every

10 C. In fact, the drift over a 55 C to 125 C temperature

range is about the same as that of a FET op amp during

normal warm up.

dc errors

Other characteristics are summarized in Table I. It can be

seen that the common-mode rejection, supply-voltage re-

jection and voltage gain are high enough to take full advan-

tage of the low offset voltage. The unspectacular 0.3V/ms

slew rate is balanced by the 300 mA current drain.

Barring the use of chopper or reset stabilization, the best

offset voltage, drift and long-term stability are obtained us-

ing bipolar transistors for the op amp input stage. This has

been done with the LM11. On-wafer trimming further im-

proves performance. Typically,

1 mV/ C drift results.

a 100 mV offset with

*R. J. Widlar, ‘‘Working with High Impedance Op Amps’’, National Semiconductor AN-241.

1995 National Semiconductor Corporation

TL/H/7479

RRD-B30M115/Printed in U. S. A.

12 3 4 5 6 7...16

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