General Description

The LM221 series are precision preamplifiers designed to

operate with general purpose operational amplifiers to drastically 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.

The LM221 series operates with supply voltages from g3V

to g20V and has sufficient supply rejection to operate from

unregulated supplies. The operating current is programmable from 5 mA to 200 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, instrumentation amplifier, strain gauge amplifiers and thermocouple

amplifiers now using chopper amplifiers can be made with

the LM221. The full differential input and high commonmode 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 characteristics to match those of specialized op amps.

The LM221 is specified over a b25§C to a85§C range and

the LM321 over a 0§C to a70§C temperature range.

Features

- Guaranteed drift of LM321AÐ0.2 mV/§C

- Guaranteed drift of LM221 seriesÐ1 mV/§C

- Offset voltage less than 0.4 mV

- Bias current less than 10 nA at 10 mA operating current

- CMRR 126 dB minimum

- 120 dB supply rejection

- Easily nulled offset voltage