LM308N Precision Operational Amplifier

Overview

The LM308N is a precision operational amplifier from the LM108 series, designed to deliver performance specifications that are ten times better than standard FET amplifiers. This component is optimized for applications requiring high precision, low input bias current, and excellent drift characteristics. The LM308N is widely used in instrumentation, medical equipment, and other sensitive electronic systems where accuracy and stability are critical.

Manufactured by National Semiconductor (now part of Texas Instruments), the LM308N offers guaranteed drift characteristics, low power consumption, and robust performance over a wide temperature range. Its design includes provisions for various compensation techniques to optimize bandwidth, slew rate, and power supply noise rejection.

Features

• Ultra-Low Input Bias Current: Maximum of 3.0 nA over the entire temperature range, ensuring minimal error in high-impedance circuits.
• Low Offset Current: Less than 400 pA over temperature, enhancing precision in differential amplification applications.
• Low Power Consumption: Supply current of only 300 µA, even in saturation, making it suitable for battery-operated devices.
• Guaranteed Drift Characteristics: Stable performance over temperature and time, critical for long-term reliability.
• Flexible Compensation Options: Supports standard, alternate, and feedforward compensation techniques to tailor bandwidth, slew rate, and noise rejection.
• Improved Power Supply Noise Rejection: Alternate compensation improves rejection by a factor of ten.

Applications

The LM308N is ideal for applications requiring high precision and low power consumption. Key application areas include:

• Instrumentation Amplifiers: Used in high-accuracy signal conditioning circuits for sensors and transducers.
• Medical Equipment: Suitable for ECG amplifiers, blood pressure monitors, and other sensitive medical devices.
• Data Acquisition Systems: Provides precise amplification in multi-channel data acquisition setups.
• Industrial Control Systems: Used in feedback loops and control circuits where stability is paramount.
• Portable Electronics: Low power consumption makes it ideal for handheld meters and battery-powered instruments.

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (LM108/LM208/LM308): As specified in the datasheet (Note 5).
• Input Voltage Range: Differential input voltage should not exceed supply voltage.
• Operating Temperature Range: Commercial (0°C to +70°C), Industrial (-25°C to +85°C), and Military (-55°C to +125°C) grades available.
• Storage Temperature Range: -65°C to +150°C.

Key Performance Parameters

• Input Bias Current: 3.0 nA (max) over temperature.
• Input Offset Current: 400 pA (max) over temperature.
• Supply Current: 300 µA (typical), even in saturation.
• Input Offset Voltage: Typically 0.5 mV, with low drift over temperature.
• Gain Bandwidth Product: Adjustable via compensation capacitor (Cf), proportional to 1/Cf.
• Slew Rate: Proportional to 1/Cf, allowing trade-offs between speed and stability.

Package Information

The LM308N is available in a standard 8-pin DIP (Dual Inline Package) and SOIC (Small Outline Integrated Circuit) package. Key package details include:

• Package Type: 8-pin DIP or SOIC.
• Pin Configuration:
  • Pin 1: Offset Null
  • Pin 2: Inverting Input (-)
  • Pin 3: Non-Inverting Input (+)
  • Pin 4: V- (Negative Supply)
  • Pin 5: Offset Null
  • Pin 6: Output
  • Pin 7: V+ (Positive Supply)
  • Pin 8: NC (No Connection)
• Mounting Style: Through-hole (DIP) or surface-mount (SOIC).
• Material: Molded plastic or ceramic for military-grade variants.

Compensation Techniques

The LM308N supports multiple compensation methods to optimize performance for specific applications:

Standard Compensation

Uses a feedback capacitor (Cf) and resistor (R1) to stabilize the amplifier. Bandwidth and slew rate are inversely proportional to Cf.

Alternate Compensation

Improves power supply noise rejection by a factor of ten. Involves additional components (R2, CO) to enhance stability.

Feedforward Compensation

Further optimizes bandwidth and transient response by introducing a feedforward capacitor (Cs).

Conclusion

The LM308N is a versatile and high-performance operational amplifier, offering precision, low power consumption, and flexible compensation options. Its robust design makes it suitable for a wide range of applications, from medical devices to industrial control systems. Engineers can leverage its advanced features to achieve superior performance in their circuits.