JAJSLE9A March   2023  – April 2024 OPA928

PRODUCTION DATA  

  1.   1
  2. 1 特長
  3. 2 アプリケーション
  4. 3 概要
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics: 4.5V ≤ VS < 8V
    6. 5.6 Electrical Characteristics: 8V ≤ VS ≤ 16V
    7. 5.7 Electrical Characteristics: 16V < VS ≤ 36V
    8. 5.8 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Guard Buffer
      2. 6.3.2 Input Protection
      3. 6.3.3 Thermal Protection
      4. 6.3.4 Capacitive Load and Stability
      5. 6.3.5 EMI Rejection
      6. 6.3.6 Common-Mode Voltage Range
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Contamination Considerations
      2. 7.1.2 Guarding Considerations
      3. 7.1.3 Single-Supply Considerations
      4. 7.1.4 Humidity Considerations
      5. 7.1.5 Dielectric Relaxation
      6. 7.1.6 Shielding
    2. 7.2 Typical Applications
      1. 7.2.1 High-Impedance Amplifier
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curve
      2. 7.2.2 Transimpedance Amplifier
        1. 7.2.2.1 Design Requirements
        2. 7.2.2.2 Detailed Design Procedure
          1. 7.2.2.2.1 Input Bias
          2. 7.2.2.2.2 Offset Voltage
          3. 7.2.2.2.3 Stability
          4. 7.2.2.2.4 Noise
      3. 7.2.3 Improved Diode Limiter
      4. 7.2.4 Instrumentation Amplifier
    3. 7.3 Power-Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Examples
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Development Support
        1. 8.1.1.1 PSpice® for TI
        2. 8.1.1.2 TINA-TI™シミュレーション・ソフトウェア (無償ダウンロード)
        3. 8.1.1.3 TI のリファレンス・デザイン
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 ドキュメントの更新通知を受け取る方法
    4. 8.4 サポート・リソース
    5. 8.5 Trademarks
    6. 8.6 静電気放電に関する注意事項
    7. 8.7 用語集
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）
サーマルパッド・メカニカル・データ
発注情報
7.2.2.2.4 Noise

There are three primary sources of noise to consider in transimpedance amplifiers: the feedback resistor, the op amp (with both current and voltage noise), and the photodiode.

All resistors are sources of thermal noise and the feedback resistor contributes to the total noise of the circuit. In very high transimpedance gain configurations, the photodiode shunt resistance can have a significant affect on the total noise of the circuit. Equation 4 shows the input-referred resistor noise density equation, which can be simplified to show that the input-referred resistor noise of the transimpedance amplifier is given by the thermal noise of RF divided by the square root of the noise gain. The output-referred resistor noise is given by multiplying Equation 4 by the noise gain. Thus, the output-referred resistor noise increases with the square root of the noise gain and the square root of the resistor value, while the signal gain increases directly with the resistor value. Therefore, increasing RF provides a signal-to-noise ratio benefit as long as the current noise does not become a dominant source of noise.

Equation 4. e n _ R =   4 k T R F R P D R P D +   R F

Figure 7-14 shows the output voltage noise of OPA928 in a transimpedance amplifier configuration with RF = 1TΩ and CF = 0.15pF. The OPA928 current noise does not significantly contribute to the noise performance of the transimpedance amplifier.

OPA928 OPA928 Transimpedance Amplifier Output Voltage Noise With a 1TΩ Resistor Figure 7-14 OPA928 Transimpedance Amplifier Output Voltage Noise With a 1TΩ Resistor

Comparing the input current noise of the OPA928 to the current noise of RF can be useful to find the dominant noise source. A 1TΩ resistor, for example, is equivalent to 0.1287fA/√Hz of current noise which is well above the measured 0.07fA/√Hz of the OPA928. The OPA928 current noise is equivalent to a 3.34TΩ resistor. For more information on current noise, see the Impact of Current Noise in CMOS and JFET Amplifiers application report.

The total output-referred noise of the transimpedance amplifier is given by Equation 5, where en is the amplifier voltage noise (including the 1/f and broadband regions), in is the amplifier current noise, ipd is the photodiode current noise, f–3dB is the transimpedance bandwidth, G is the dc noise gain, and G(f) is the frequency dependent noise gain.

Equation 5. E n _ t o t a l = ∫ f l f h e n G ( f ) 2 d f +   i n R F 2 +   i p d R F 2 + 4 k T R F G × 1.57 f - 3 d B

The total output-referred voltage noise calculation requires a complicated analysis of the frequency dependent noise gain and voltage noise density, and is not covered here. Unlike the current and resistor noise which are bandwidth limited by the transimpedance bandwidth given by Equation 3, the op amp voltage noise is only limited by the gain bandwidth of the amplifier. Limit the noise bandwidth with an output filter to reduce the voltage noise contribution.

The ultra-low current noise of the OPA928 is not a significant contributor of noise in most applications, and an output low-pass filter makes the en term in Equation 5 negligible. Equation 6 provides a straight-forward calculation for the total output-referred noise for a filtered transimpedance amplifier.

Equation 6. E n _ t o t a l = i p d R F 2 + 4 k T R F G × 1.57 f - 3 d B

Figure 7-15 shows the simulated noise of the OPA928 in a transimpedance configuration with RF = 10GΩ, RPD = 5GΩ, CF = 1pF, CPD = 35pF.

OPA928 Transimpedance Amplifier RC-Filter Noise Comparison Figure 7-15 Transimpedance Amplifier RC-Filter Noise Comparison