LM108A - National Semiconductor

TL/H/7759

LM108A/LM208A/LM308A Operational Amplifiers

May 1989

LM108A/LM208A/LM308A Operational Amplifiers

General Description

The LM108/LM108A series are precision operational ampli-

fiers having specifications about a factor of ten better than

FET amplifiers over their operating temperature range. In

addition to low input currents, these devices have extremely

low offset voltage, making it possible to eliminate offset ad-

justments, in most cases, and obtain performance ap-

proaching chopper stabilized amplifiers.

The devices operate with supply voltages from g2V to

g18V and have sufficient supply rejection to use unregulat-

ed supplies. Although the circuit is interchangeable with and

uses the same compensation as the LM101A, an alternate

compensation scheme can be used to make it particularly

insensitive to power supply noise and to make supply by-

pass capacitors unnecessary.

The low current error of the LM108A series makes possible

many designs that are not practical with conventional ampli-

fiers. In fact, it operates from 10 MXsource resistances,

introducing less error than devices like the 709 with 10 kX

sources. Integrators with drifts less than 500 mV/sec and

analog time delays in excess of one hour can be made us-

ing capacitors no larger than 1 mF.

The LM208A is identical to the LM108A, except that the

LM208A has its performance guaranteed over a b25§Cto

85§C temperature range, instead of b55§Ctoa

125§C.

The LM308A devices have slightly-relaxed specifications

and performances over a 0§Ctoa

70§C temperature range.

Features

YOffset voltage guaranteed less than 0.5 mV

YMaximum input bias current of 3.0 nA over temperature

YOffset current less than 400 pA over temperature

YSupply current of only 300 mA, even in saturation

YGuaranteed 5 mV/§C drift

Compensation Circuits

Standard Compensation Circuit

CftR1 CO

R1aR2

COe30 pF

TL/H/7759–1

**Bandwidth and slew rate are proportional to 1/Cf.**Bandwidth and slew rate are proportional to 1/Cs.

Alternate*Frequency Compensation

*Improves rejection of power supply

noise by a factor of ten.

TL/H/7759–2

Feedforward Compensation

TL/H/7759–3

C1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.

LM108A/LM208A Absolute Maximum Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

(Note 5)

Supply Voltage g20V

Power Dissipation (Note 1) 500 mW

Differential Input Current (Note 2) g10 mA

Input Voltage (Note 3) g15V

Output Short-Circuit Duration Continuous

Operating Free Air Temperature Range

LM108A b55§Ctoa

125§C

LM208A b25§Ctoa

85§C

Storage Temperature Range b65§Ctoa

150§C

Lead Temperature (Soldering, 10 sec.) (DIP) 260§C

Soldering Information

Dual-In-Line Package

Soldering (10 sec.) 260§C

Small Outline Package

Vapor Phase (60 sec.) 215§C

Infrared (15 sec.) 220§C

See An-450 ‘‘Surface Mounting Methods and Their Effect

on Product Reliability’’ for other methods of soldering sur-

face mount devices.

ESD Tolerance (Note 6) 2000V

Electrical Characteristics (Note 4)

Parameter Conditions Min Typ Max Units

Input Offset Voltage TAe25§C 0.3 0.5 mV

Input Offset Current TAe25§C 0.05 0.2 nA

Input Bias Current TAe25§C 0.8 2.0 nA

Input Resistance TAe25§C3070MX

Supply Current TAe25§C 0.3 0.6 mA

Large Signal Voltage Gain TAe25§C, VSeg15V, 80 300 V/mV

VOUT eg10V, RLt10 kX

Input Offset Voltage 1.0 mV

Average Temperature Coefficient 1.0 5.0 mV/§C

of Input Offset Voltage

Input Offset Current 0.4 nA

Average Temperature Coefficient 0.5 2.5 pA/§C

of Input Offset Current

Input Bias Current 3.0 nA

Supply Current TAe125§C 0.15 0.4 mA

Large Signal Voltage Gain VSeg15V, VOUT eg10V, 40 V/mV

RLt10 kX

Output Voltage Swing VSeg15V, RLe10 kXg13 g14 V

Input Voltage Range VSeg15V g13.5 V

Common Mode Rejection Ratio 96 110 dB

Supply Voltage Rejection Ratio 96 110 dB

Note 1: The maximum junction temperature of the LM108A is 150§C, while that of the LM208A is 100§C. For operating at elevated temperatures, devices in the H08

package must be derated based on a thermal resistance of 160§C/W, junction to ambient, or 20§C/W, junction to case. The thermal resistance of the dual-in-line

package is 100§C/W, junction to ambient.

Note 2: The inputs are shunted with back-to-back diodes for overvoltage protection. Therefore, excessive current will flow if a differential input voltage in excess of

1V is applied between the inputs unless some limiting resistance is used.

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 g5V sVSsg20V and b55§CsTAs125§C, unless otherwise specified. With the LM208A, however, all temperature

specifications are limited to b25§CsTAs85§C.

Note 5: Refer to RETS108AX for LM108AH and LM108AJ-8 military specifications.

Note 6: Human body model, 1.5 kXin series with 100 pF.

LM308A Absolute Maximum Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Supply Voltage g18V

Power Dissipation (Note 1) 500 mW

Differential Input Current (Note 2) g10 mA

Input Voltage (Note 3) g15V

Output Short-Circuit Duration Continuous

Operating Temperature Range 0§Ctoa

70§C

Storage Temperature Range b65§Ctoa

150§C

H-Package Lead Temperature

(Soldering, 10 sec.) 300§C

Lead Temperature (Soldering, 10 sec.) (DIP) 260§C

Soldering Information

Dual-In-Line Package

Soldering (10 sec.) 260§C

Small Outline Package

Vapor phase (60 sec.) 215§C

Infrared (15 sec.) 220§C

See An-450 ‘‘Surface Mounting Methods and Their Effect

on Product Reliability’’ for other methods of soldering sur-

face mount devices.

ESD rating to be determined.

Electrical Characteristics (Note 4)

Parameter Conditions Min Typ Max Units

Input Offset Voltage TAe25§C 0.3 0.5 mV

Input Offset Current TAe25§C 0.2 1 nA

Input Bias Current TAe25§C 1.5 7 nA

Input Resistance TAe25§C1040MX

Supply Current TAe25§C, VSeg15V 0.3 0.8 mA

Large Signal Voltage Gain TAe25§C, VSeg15V, 80 300 V/mV

VOUT eg10V, RLt10 kX

Input Offset Voltage VSeg15V, RSe100X0.73 mV

Average Temperature Coefficient VSeg15V, RSe100X2.0 5.0 mV/§C

of Input Offset Voltage

Input Offset Current 1.5 nA

Average Temperature Coefficient 2.0 10 pA/§C

of Input Offset Current

Input Bias Current 10 nA

Large Signal Voltage Gain VSeg15V, VOUT eg10V, 60 V/mV

RLt10 kX

Output Voltage Swing VSeg15V, RLe10 kXg13 g14 V

Input Voltage Range VSeg15V g14 V

Common Mode Rejection Ratio 96 110 dB

Supply Voltage Rejection Ratio 96 110 dB

Note 1: The maximum junction temperature of the LM308A is 85§C. For operating at elevated temperatures, devices in the H08 package must be derated based on

a thermal resistance of 160§C/W, junction to ambient, or 20§C/W, junction to case. The thermal resistance of the dual-in-line package is 100§C/W, junction to

ambient.

Note 2: The inputs are shunted with back-to-back diodes for overvoltage protection. Therefore, excessive current will flow if a differential input voltage in excess of

1V is applied between the inputs unless some limiting resistance is used.

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 g5V sVSsg15V and 0§CsTAsa70§C, unless otherwise specified.

Typical Applications

Sample and Hold

²Teflon, polyethylene or polycarbonate dielectric capacitor.

Worst case drift less than 2.5 mV/sec. TL/H/7759–4

High Speed Amplifier with Low Drift and Low Input Current

TL/H/7759–5

Application Hints

A very low drift amplifier poses some uncommon application

and testing problems. Many sources of error can cause the

apparent circuit drift to be much higher than would be pre-

dicted.

Thermocouple effects caused by temperature gradient

across dissimilar metals are perhaps the worst offenders.

Only a few degrees gradient can cause hundreds of micro-

volts of error. The two places this shows up, generally, are

the package-to-printed circuit board interface and tempera-

ture gradients across resistors. Keeping package leads

short and the two input leads close together helps greatly.

Resistor choice as well as physical placement is important

for minimizing thermocouple effects. Carbon, oxide film and

some metal film resistors can cause large thermocouple er-

rors. Wirewound resistors of evanohm or manganin are best

since they only generate about 2 mV/§C referenced to cop-

per. Of course, keeping the resistor ends at the same tem-

perature is important. Generally, shielding a low drift stage

electrically and thermally will yield good results.

Resistors can cause other errors besides gradient generat-

ed voltages. If the gain setting resistors do not track with

temperature a gain error will result. For example, a gain of

1000 amplifier with a constant 10 mV input will have a 10V

output. If the resistors mistrack by 0.5% over the operating

temperature range, the error at the output is 50 mV. Re-

ferred to input, this is a 50 mV error. All of the gain fixing

resistor should be the same material.

Testing low drift amplifiers is also difficult. Standard drift

testing technique such as heating the device in an oven and

having the leads available through a connector, thermo-

probe, or the soldering iron methodÐdo not work. Thermal

gradients cause much greater errors than the amplifier drift.

Coupling microvolt signal through connectors is especially

bad since the temperature difference across the connector

can be 50§C or more. The device under test along with the

gain setting resistor should be isothermal.

Schematic Diagram

TL/H/7759–6

Connection Diagrams

Metal Can Package

TL/H/7759–7

Pin 4 is connected to the case.

**Unused pin (no internal connection) to allow for input anti-leakage guard

ring on printed circuit board layout.

Order Number LM108AH, LM208AH or LM208AH

See NS Package Number H08C

Dual-In-Line Package

TL/H/7759–8

Top View

Order Number LM108AJ-8, LM208AJ-8, LM308AJ-8,

LM308AM or LM308AN

See NS Package Number J08A, M08A or N08E

Physical Dimensions inches (millimeters)

Metal Can Package (H)

Order Number LM108AH, LM208AH or LM308AH

NS Package Number H08C

Physical Dimensions inches (millimeters) (Continued)

Ceramic Dual-In-Line Package (J)

Order Number LM108AJ-8, LM208AJ-8 or LM308AJ-8

NS Package Number J08A

S.O. Package (M)

Order Number LM308AM

NS Package Number M08A

LM108A/LM208A/LM308A Operational Amplifiers

Physical Dimensions inches (millimeters) (Continued)

Molded Dual-In-Line Package (N)

Order Number LM308AN

NS Package Number N08E

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failure to perform, when properly used in accordance support device or system, or to affect its safety or

with instructions for use provided in the labeling, can effectiveness.

be reasonably expected to result in a significant injury

to the user.

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