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 g 3V to g 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. 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 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 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. ² Adjust for output reading in § C. C1995 National Semiconductor Corporation TL/H/7769 TL/H/7769 – 1 RRD-B30M115/Printed in U. S. A. LM221/LM321 Precision Preamplifiers February 1995 Absolute Maximum Ratings Supply Voltage g 20V Power Dissipation (Note 1) Operating Temperature Range LM321A 500 mW Differential Input Voltage (Notes 2 and 3) Input Voltage (Note 3) Storage Temperature Range Lead Temperature (Soldering, 10 sec.) ESD rating to be determined. g 15V g 15V 0§ C to a 70§ C b 65§ C to a 150§ C 300§ C Electrical Characteristics (Note 4) LM321A LM321A Parameter Conditions Units Min Typ Max Input Offset Voltage TA e 25§ C, 6.4k s RSET s 70k 0.2 0.4 mV Input Offset Current TA e 25§ C, RSET e 70k RSET e 6.4k 0.3 0.5 5 nA nA 5 50 15 150 nA nA Input Bias Current Input Resistance TA e 25§ C, RSET e 70k RSET e 6.4k TA e 25§ C, RSET e 70k RSET e 6.4k 2 0.2 8 MX MX Supply Current TA e 25§ C, RSET e 70k 0.8 2.2 mA Input Offset Voltage 6.4k s RSET s 70k 0.5 0.65 mV Input Bias Current RSET e 70k RSET e 6.4k 15 150 25 250 nA nA Input Offset Current RSET e 70k RSET e 6.4k 0.5 5 1 10 nA nA Input Offset Current Drift RSET e 70k 3 Average Temperature RS s 200X, 6.4k s RSET s 70k Coefficient of Input Offset Voltage Offset Voltage Nulled 0.07 Long Term Stability 3 Supply Current 1 Input Voltage Range VS e g 15V, (Note 5) RSET e 70k RSET e 6.4k g 13 pA/§ C 0.2 mV/§ C mV/yr 3.5 mA V V a 7, b 13 Common-Mode Rejection Ratio RSET e 70k RSET e 6.4k 126 120 140 130 dB dB Supply Voltage Rejection Ratio RSET e 70k RSET e 6.4k 118 114 126 120 dB dB Voltage Gain TA e 25§ C, RSET e 70k, RL l 3 MX 12 20 V/V Noise RSET e 70k, RSOURCE e 0 8 nV/0Hz 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 g 15V, the absolute maximum input voltage is equal to the supply voltage. Note 4: These specifications apply for g 5 s VS s g 20V 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 Ð0.1%Ð 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. Operating Temperature Range LM221, LM121A (-883), LM121 (-883) LM321, LM321A Supply Voltage Storage Temperature Range Lead Temperature (Soldering, 10 sec.) ESD rating to be determined. g 20V Power Dissipation (Note 1) Differential Input Voltage (Notes 2 and 3) Input Voltage (Note 3) 500 mW g 15V g 15V b 25§ C to a 85§ C 0§ C to a 70§ C b 65§ C to a 150§ C 260§ C Electrical Characteristics (Note 4) LM221, LM321 Parameter LM221 Conditions Min Typ LM321 Max Min Typ Units Max Input Offset Voltage TA e 25§ C, 6.4k s RSET s 70k 0.7 1.5 mV Input Offset Current TA e 25§ C, RSET e 70k RSET e 6.4k 1 10 2 20 nA nA TA e 25§ C, RSET e 70k RSET e 6.4k 10 100 18 180 nA nA Input Bias Current Input Resistance TA e 25§ C, RSET e 70k RSET e 6.4k 4 0.4 2 0.2 MX MX Supply Current TA e 25§ C, RSET e 70k 1.5 2.2 mA Input Offset Voltage 6.4k s RSET s 70k 1.0 2.5 mV Input Bias Current RSET e 70k RSET e 6.4k 30 300 28 280 nA nA Input Offset Current RSET e 70k RSET e 6.4k 3 30 4 40 nA nA Input Offset Current Drift RSET e 70k Average Temperature Coefficient of Input Offset Voltage RS s 200X, 6.4k s RSET s 70k Offset Voltage Nulled 3 1 Long Term Stability 5 2.5 VS e g 15V, (Note 5) RSET e 70k RSET e 6.4k pA/§ C 1 5 Supply Current Input Voltage Range 3 mV/§ C mV/yr 3.5 mA g 13 g 13 a 7, b 13 a 7, b 13 V V Common-Mode Rejection Ratio RSET e 70k RSET e 6.4k 120 114 114 114 dB dB Supply Voltage Rejection Ratio RSET e 70k RSET e 6.4k 120 114 114 114 dB dB Voltage Gain TA e 25§ C, RSET e 70k, RL l 3 MX 16 Noise RSET e 70k, RSOURCE e 0 12 8 V/V 8 nV/0Hz 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 g 15V, the absolute maximum input voltage is equal to the supply voltage. Note 4: These specifications apply for g 5 s VS s g 20V 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 Ð0.1%Ð 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 TL/H/7769 – 9 4 Typical Performance Characteristics (Continued) Common-Mode Limits Output Common-Mode Voltage Differential Voltage Gain Supply Current Offset Voltage Adjustment Common-Mode Rejection Ratio TL/H/7769 – 10 Connection Diagram Metal Can Package TL/H/7769 – 7 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 TL/H/7769 – 8 Schematic Diagram 6 Frequency Compensation Table I shows typical values for the two compensating capacitors for various gains and operating currents. 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% feedbackÐsuch as a voltage follower or integratorÐand the gain of the LM321 is high. When high closed loop gains are usedÐfor example AV e 1000Ðand 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. TABLE I 120 kX 60 kX 30 kX 12 kX 6 kX AV e 1 AV e 5 AV e 10 AV e 50 AV e 100 AV e 500 AV e 1000 68 15 10 1 – – – 130 27 15 3 1 – – 270 56 27 5 3 1 – 680 130 68 15 5 1 – 1300 270 130 27 10 3 – Current Set Resistor This table applies for the LM108, LM101A, LM741, LM118. Capacitance is in pF. DESIGN EQUATIONS FOR THE LM321 SERIES 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 Closed Loop Gain 1.2 c 106 RSET Null Pot Value should be 10% of RSET 2 c 0.65V Operating Current & RSET Gain AV & Ð Positive Common-Mode Limit & V a b 0.6 b 4 0.65V c 50k RSET 106 ACL 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. ² See table for frequency compensation. TL/H/7769 – 2 FIGURE 1. Low Drift Op Amp Using the LM321A as a Preamp 7 ( Typical Applications (Continued) Gain of 1000 Instrumentation Amplifier ³ *Offset adjust ² Gain trim ³ Better than 1% linearity for input sig- nals up to g 10 mV gain stability typical a 2% from b 55 to a 125§ C. Match of R5 and R6 effect power supply rejection TL/H/7769 – 3 High Speed* Inverting Amplifier with Low Drift *Bandwidth e 10 MHz Slew Rate e 40 V/ms TL/H/7769 – 4 Medium Speed* General Purpose Amplifier *Bandwidth e 3.5 MHz Slew Rate e 1.1 V/ms TL/H/7769 – 5 8 Typical Applications (Continued) Increased Common-Mode Range at High Operating Currents *Match to 0.1% ² Depends on close loop gain TL/H/7769 – 6 9 LM221/LM321 Precision Preamplifiers Physical Dimensions inches (millimeters) Metal Can Package (H) Order Number LM221H, LM321H or LM321AH NS Package Number H08C LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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