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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 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 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 ambient temperature at 10 mV K Adjust for output reading in C
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C1995 National Semiconductor Corporation
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RRD-B30M115 Printed in U S A
Absolute Maximum Ratings
Supply Voltage Power Dissipation (Note 1) Differential Input Voltage (Notes 2 and 3) Input Voltage (Note 3)
g20V
500 mW
g15V g15V
Operating Temperature Range LM321A Storage Temperature Range Lead Temperature (Soldering 10 sec ) ESD rating to be determined
0 C to a 70 C
b 65 C to a 150 C
300 C
Electrical Characteristics (Note 4) LM321A
LM321A Parameter Input Offset Voltage Input Offset Current Conditions Min TA e 25 C 6 4k s RSET s 70k TA e 25 C RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k 6 4k s RSET s 70k RSET e 70k RSET e 6 4k RSET e 70k RSET e 6 4k RSET e 70k RS s 200X 6 4k s RSET s 70k Offset Voltage Nulled 0 07 3 1 VS e g15V (Note 5) RSET e 70k RSET e 6 4k RSET e 70k RSET e 6 4k RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k RL l 3 MX RSET e 70k RSOURCE e 0 35 02 mV C mV yr mA V V 140 130 126 120 20 8 dB dB dB dB VV nV 0Hz 2 02 Typ 02 03 Max 04 05 5 15 150 mV nA nA nA nA MX MX 22 0 65 25 250 1 10 mA mV nA nA nA nA pA C Units
Input Bias Current
5 50 8 08 05 15 150 05 5 3
Input Resistance
Supply Current Input Offset Voltage Input Bias Current Input Offset Current Input Offset Current Drift Average Temperature Coefficient of Input Offset Voltage Long Term Stability Supply Current Input Voltage Range
g13
a 7 b 13
Common-Mode Rejection Ratio Supply Voltage Rejection Ratio Voltage Gain Noise
126 120 118 114 12
Note 1 The maximum junction temperature of the LM321A is 85 C For operating at elevated temperature devices in the H08 package must be derated based on a thermal resistance of 150 C W junction to ambient or 18 C W junction to case Note 2 The inputs are shunted with back-to-back diodes in series with a 500X resistor for overvoltage protection Therefore excessive current will flow if a differential input voltage in excess of 1V is applied between the inputs Note 3 For supply voltages less than g15V the absolute maximum input voltage is equal to the supply voltage Note 4 These specifications apply for g5 s VS s g20V and b 55 C s TA s a 125 C unless otherwise specified With the LM221A however all temperature specifications are limited to b 25 C s TA s a 85 C and for the LM321A the specifications apply over a 0 C to a 70 C temperature range Note 5 External precision resistor 0 1% can be placed from pins 1 and 8 to 7 increase positive common-mode range Note 6 See RETS121X for LM121H 883 military specs and RET121AX for LM121AH 883 military specs
2
Absolute Maximum Ratings
If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications Supply Voltage Power Dissipation (Note 1) Differential Input Voltage (Notes 2 and 3) Input Voltage (Note 3)
g20V
Operating Temperature Range LM221 LM121A (-883) LM121 (-883) LM321 LM321A Storage Temperature Range Lead Temperature (Soldering 10 sec ) ESD rating to be determined
b 25 C to a 85 C 0 C to a 70 C b 65 C to a 150 C
500 mW
g15V g15V
260 C
Electrical Characteristics (Note 4) LM221
Parameter Input Offset Voltage Input Offset Current Conditions
LM321 LM221 Min Typ Max 07 1 10 10 100 4 04 15 10 30 300 3 30 3 1 5 25 5 35 3 1 2 02 22 25 28 280 4 40 Min LM321 Typ Max 15 2 20 18 180 mV nA nA nA nA MX MX mA mV nA nA nA nA pA C mV C mV yr mA V V dB dB dB dB VV 8 nV 0Hz Units
TA e 25 C 6 4k s RSET s 70k TA e 25 C RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k 6 4k s RSET s 70k RSET e 70k RSET e 6 4k RSET e 70k RSET e 6 4k RSET e 70k RS s 200X 6 4k s RSET s 70k Offset Voltage Nulled
Input Bias Current
Input Resistance
Supply Current Input Offset Voltage Input Bias Current Input Offset Current Input Offset Current Drift Average Temperature Coefficient of Input Offset Voltage Long Term Stability Supply Current Input Voltage Range
VS e g15V (Note 5) RSET e 70k RSET e 6 4k RSET e 70k RSET e 6 4k RSET e 70k RSET e 6 4k TA e 25 C RSET e 70k RL l 3 MX RSET e 70k RSOURCE e 0
g13
g13
a 7 b 13
a 7 b 13
Common-Mode Rejection Ratio Supply Voltage Rejection Ratio Voltage Gain Noise
120 114 120 114 16 8
114 114 114 114 12
Note 1 The maximum junction temperature of the LM221 is 100 C The maximum junction temperature of the LM321 is 85 C For operating at elevated temperature devices in the H08 package must be derated based on a thermal resistance of 150 C W junction to ambient or 18 C W junction to case Note 2 The inputs are shunted with back-to-back diodes in series with a 500X resistor for overvoltage protection Therefore excessive current will flow if a differential input voltage in excess of 1V is applied between the inputs Note 3 For supply voltages less than g15V the absolute maximum input voltage is equal to the supply voltage Note 4 These specifications apply for g5 s VS s g20V and b 55 C s TA s a 125 C unless otherwise specified With the LM221 however all temperature specifications are limited to b 25 C s TA s a 85 C and for the LM321 the specifications apply over a 0 C to a 70 C temperature range Note 5 External precision resistor 0 1% can be placed from pins 1 and 8 to 7 increase positive common-mode range
3
Typical Performance Characteristics
Input Bias Current Distribution of Offset Voltage Drift (Nulled) Distribution of Offset Voltage Drift (Nulled)
Positive Power Supply Rejection
Negative Power Supply Rejection
Input Noise Voltage
Input Noise Current
Voltage Drift
Differential Voltage Gain
Set Resistor and Set Current
Set Current
Common-Mode Limits
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4
Typical Performance Characteristics
Common-Mode Limits
(Continued) Differential Voltage Gain
Output Common-Mode Voltage
Supply Current
Offset Voltage Adjustment
Common-Mode Rejection Ratio
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Connection Diagram
Metal Can Package
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Top View
Note Pin 4 connected to case
Order Number LM121AH 883 LM121H 883 LM221H LM321H or LM321AH See NS Package Number H08C
Note Outputs are inverting from the input of the same number
5
Schematic Diagram
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6
Frequency Compensation
UNIVERSAL COMPENSATION The additional gain of the LM321 preamplifier when used with an operational amplifier usually necessitates additional frequency compensation When the closed loop gain of the op amp with the LM321 is less than the gain of the LM321 alone more compensation is needed The worst case situation is when there is 100% feedback such as a voltage follower or integrator and the gain of the LM321 is high When high closed loop gains are used for example AV e 1000 and only an addition gain of 200 is inserted by the LM321 the frequency compensation of the op amp will usually suffice The frequency compensation shown here is designed to operate with any unity-gain stable op amp Figure 1 shows the basic configuration of frequency stabilizing network In operation the output of the LM321 is rendered single ended by a 0 01 mF bypass capacitor to ground Overall frequency compensation then is achieved by an integrating capacitor around the op amp 12 2qRSETC 4 for 0 5 MHz bandwidth C e 106RSET For use with higher frequency op amps such as the LM118 the bandwidth may be increased to about 2 MHz If the closed loop gain is greater than unity ``C'' may be decreased to Bandwidth at unity-gain j Ce 4 106 ACL RSET Table I shows typical values for the two compensating capacitors for various gains and operating currents TABLE I Closed Loop Gain AV e 1 AV e 5 AV e 10 AV e 50 AV e 100 AV e 500 AV e 1000 Current Set Resistor 120 kX 68 15 10 1 - - - 60 kX 130 27 15 3 1 - - 30 kX 270 56 27 5 3 1 - 12 kX 680 130 68 15 5 1 - 6 kX 1300 270 130 27 10 3 -
This table applies for the LM108 LM101A LM741 LM118 Capacitance is in pF DESIGN EQUATIONS FOR THE LM321 SERIES 1 2 c 106 RSET Null Pot Value should be 10% of RSET 2 c 0 65V Operating Current RSET Gain AV Positive Common-Mode Limit Va b 0 6 b 0 65V c 50k RSET
(
ALTERNATE COMPENSATION The two compensation capacitors can be made equal for improved power supply rejection In this case the formula for the compensation capscitor is Ce 8 106 ACL RSET
Typical Applications
Offset adjust See table for frequency compensation
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FIGURE 1 Low Drift Op Amp Using the LM321A as a Preamp
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Typical Applications (Continued)
Gain of 1000 Instrumentation Amplifier
Offset adjust Gain trim Better than 1% linearity for input signals up to g10 mV gain stability typical a 2% from b 55 to a 125 C Match of R5 and R6 effect power supply rejection
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High Speed Inverting Amplifier with Low Drift
Bandwidth e 10 MHz Slew Rate e 40 V ms
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Medium Speed General Purpose Amplifier
Bandwidth e 3 5 MHz Slew Rate e 1 1 V ms
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Typical Applications (Continued)
Increased Common-Mode Range at High Operating Currents
Match to 0 1% Depends on close loop gain
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9
LM221 LM321 Precision Preamplifiers
Physical Dimensions inches (millimeters)
Metal Can Package (H) Order Number LM221H LM321H or LM321AH NS Package Number H08C
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National Semiconductor Corporation 1111 West Bardin Road Arlington TX 76017 Tel 1(800) 272-9959 Fax 1(800) 737-7018
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