TLH7769

February 1995

LM221LM321 Precision Preamplifiers

General Description

The LM221 series are precision preamplifiers designed to

operate with general purpose operational amplifiers to dras-

tically decrease dc errors Drift bias current common mode

and supply rejection are more than a factor of 50 better than

standard op amps alone Further the added dc gain of the

LM221 decreases the closed loop gain error

unregulated supplies The operating current is programma-

ble from 5 mAto200 mA so bias current offset current gain

and noise can be optimized for the particular application

while still realizing very low drift Super-gain transistors are

used for the input stage so input error currents are lower

than conventional amplifiers at the same operating current

Further the initial offset voltage is easily nulled to zero

The extremely low drift of the LM221 will improve accuracy

on almost any precision dc circuit For example instrumen-

tation amplifier strain gauge amplifiers and thermocouple

amplifiers now using chopper amplifiers can be made with

the LM221 The full differential input and high common-

mode rejection are another advantage over choppers For

applications where low bias current is more important than

drift the operating current can be reduced to low values

High operating currents can be used for low voltage noise

with low source resistance The programmable operating

current of the LM221 allows tailoring the input characteris-

tics to match those of specialized op amps

Features

Y

Guaranteed drift of LM321A02 mV C

Y

Guaranteed drift of LM221 series1 mV C

Y

Offset voltage less than 04 mV

Y

Bias current less than 10 nA at 10 mA operating current

Y

CMRR 126 dB minimum

Y

120 dB supply rejection

Y

Easily nulled offset voltage

Typical Applications

Thermocouple Amplifier with Cold Junction Compensation

TLH7769 1

Set for 298V at output with LM113

shorted Output should equal ambi-

ent temperature at 10 mV K

Adjust for output reading in C

C1995 National Semiconductor Corporation

RRD B30M115Printed in U S A