AN-242 Datasheet, PDF (1/16 Pages) National Semiconductor (TI) – Applying a New Precision Op Amp

Applying a New
Precision Op Amp
National Semiconductor
Application Note 242
April 1980
Robert J Widlar
Apartado Postal 541
Puerto Vallarta Jalisco
Mexico
Bob Pease and Mineo Yamatake
National Semiconductor Corporation
Santa Clara California
USA
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
b55 C to a125 C range Preliminary data indicates that
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
dc errors
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 a 100 mV offset with
1 mV C drift results
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
TL H 7479 – 1
Figure 1 Below 100 C bias current varies almost linear-
ly with temperature This means that simple
circuitry can be used for compensation Offset
current is unusually low
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 b55 C to a125 C temperature
range is about the same as that of a FET op amp during
normal warm up
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
R J Widlar ‘‘Working with High Impedance Op Amps’’ National Semiconductor AN-241
C1995 National Semiconductor Corporation TL H 7479
RRD-B30M115 Printed in U S A