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