LM221 LM321 Precision Preamplifiers
February 1995
LM221 LM321 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
The LM221 series operates with supply voltages from
g
3V
to
g
20V and has sufficient supply rejection to operate from
unregulated supplies The operating current is programma-
ble 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 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
The LM221 is specified over a
b
25 C to
a
85 C range and
the LM321 over a 0 C to
a
70 C temperature range
Features
Y
Y
Y
Y
Y
Y
Y
Guaranteed drift of LM321A 0 2
mV
C
Guaranteed drift of LM221 series 1
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
Typical Applications
Thermocouple Amplifier with Cold Junction Compensation
Set for 2 98V at output with LM113
shorted Output should equal ambi-
ent temperature at 10 mV K
Adjust for output reading in C
TL H 7769 鈥?1
C
1995 National Semiconductor Corporation
TL H 7769
RRD-B30M115 Printed in U S A
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