AD820BR: High-Performance J-FET Operational Amplifier - Specs & Applications

The AD820BR from Analog Devices Inc. is a precision J-FET operational amplifier renowned for its low-noise performance and high-speed capabilities. Despite being marked as obsolete, this op-amp remains a favorite among engineers for specialized applications. In this comprehensive guide, we delve into the AD820BR's specifications, features, and use cases to help you understand why it's still relevant today.

AD820BR Overview

Manufacturer and Product Status

• Manufacturer: Analog Devices Inc. (ADI)
• Product Status: Obsolete (but still available through distributors)

While officially obsolete, the AD820BR continues to be used in legacy systems and new designs where its unique combination of features is required. Many engineers stockpile these components for critical applications.

Detailed Technical Specifications

Amplifier Characteristics

• Amplifier Type: J-FET Input
• Number of Circuits: 1
• Output Type: Rail-to-Rail

The J-FET input stage provides ultra-high input impedance (typically >10¹² ) and exceptionally low input bias current, making it ideal for:

• Photodiode amplifiers
• Electrometer circuits
• High-impedance sensor interfaces

Performance Metrics

• Slew Rate: 3V/ s (typical)
• Gain Bandwidth Product: 1.8MHz
• -3dB Bandwidth: 1.9MHz
• Input Voltage Noise: 15nV/ Hz @1kHz

These specifications make the AD820BR suitable for:

• Audio preamplifiers
• Data acquisition systems
• Medical instrumentation
• Test and measurement equipment

Electrical Characteristics

• Input Bias Current: 2pA (max)
• Input Offset Voltage: 300 V (max)
• Supply Current: 700 A (typical)
• Output Current: 20mA

Power Requirements

• Supply Voltage Range: 2.5V to 18V (5V to 36V total)
• Power Supply Rejection Ratio: 100dB (min)

Environmental Specifications

• Operating Temperature Range: -40 C to +85 C
• Storage Temperature Range: -65 C to +150 C

Package Information

• Package Type: 8-SOIC
• Package Dimensions: 4.9mm 3.9mm 1.25mm
• Mounting Style: Surface Mount
• Lead Finish: Matte Tin

Detailed Applications

Precision Instrumentation

The AD820BR excels in precision measurement applications due to its:

• Low input bias current (2pA)
• Low input offset voltage (300 V)
• High open-loop gain (120dB)

Typical uses include:

• Strain gauge amplifiers
• Thermocouple amplifiers
• Bridge transducer amplifiers

Audio Applications

The combination of low noise and wide bandwidth makes the AD820BR suitable for:

• Microphone preamplifiers
• Equalization circuits
• High-end audio processing

Medical Electronics

In medical devices, the AD820BR is used for:

• ECG amplifiers
• EEG signal conditioning
• Patient monitoring systems

Design Considerations

PCB Layout Guidelines

For optimal performance:

1. Use a ground plane
2. Keep input traces short
3. Use proper decoupling capacitors (0.1 F ceramic close to supply pins)
4. Consider guard rings for high-impedance inputs

Thermal Management

While the AD820BR has low power consumption, in high-temperature environments:

• Ensure adequate airflow
• Consider thermal vias under the package
• Monitor junction temperature in critical applications

Alternatives to AD820BR

For designers needing current-production alternatives:

• ADA4610-1 (Higher bandwidth, lower noise)
• AD8641 (Lower power, rail-to-rail)
• OPA140 (Precision, low-noise JFET)

Why Choose AD820BR Today?

Despite being obsolete, the AD820BR offers unique advantages:

1. Unmatched Input Characteristics: The 2pA input bias current is difficult to find in modern op-amps
2. Proven Reliability: Decades of field use demonstrate its robustness
3. Wide Supply Range: Operates from 5V to 36V, accommodating various systems
4. Temperature Stability: Consistent performance across -40 C to +85 C

Purchasing Information

While no longer in production, the AD820BR is available through:

• Authorized distributors with remaining stock
• Specialty electronic component suppliers
• Verified surplus channels

Conclusion

The AD820BR operational amplifier represents Analog Devices' engineering excellence in precision analog design. Its combination of J-FET inputs, low noise, and wide bandwidth continues to make it valuable for demanding applications. For engineers working on legacy systems or requiring its specific performance characteristics, the AD820BR remains an excellent choice.

When designing with the AD820BR, consider:

• Proper PCB layout techniques
• Alternative components for future designs
• Long-term availability planning

For technical support or purchasing assistance with the AD820BR, contact our team of analog specialists today.