National Semiconductor LM221/LM321 Precision Preamplifiers Datasheet

The LM221 series is a precision preamplifier 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. Furthermore, the added dc gain of the LM221 decreases the closed loop gain error. The LM221 series operates with supply voltages from 3V to 20V 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. Furthermore, 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 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.

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