JAJSQO6A August   2023  – December 2024 LOG200

PRODMIX  

  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
    6. 5.6 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 High Speed, Logarithmic Current-to-Voltage Conversion
      2. 6.3.2 Voltage and Current References
      3. 6.3.3 Adaptive Photodiode Bias
      4. 6.3.4 Auxiliary Operational Amplifier
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Logarithmic Transfer Function
        1. 7.1.1.1 Logarithmic Conformity Error
        2. 7.1.1.2 Error Analysis Example
    2. 7.2 Typical Application
      1. 7.2.1 Optical Current Sensing
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 サード・パーティ製品に関する免責事項
    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

デバイスごとのパッケージ図は、PDF版データシートをご参照ください。

メカニカル・データ（パッケージ｜ピン）
  • RGT|16
サーマルパッド・メカニカル・データ
発注情報

7.1.1 Logarithmic Transfer Function

The LOG200 uses a differential amplifier to compare the voltage outputs of two logarithmic amplifiers. Logarithmic amplifiers rely on the feedback transistor relation of the base-emitter voltage (VBE) to the collector current IC, according to the principle:

Equation 1. VBE=kTqln⁡ICIS

where

  • k = the Boltzmann constant, 1.381 × 10-23J/K
  • T = absolute temperature in kelvins (K)
  • q = the elementary charge, 1.602 × 10-19C
  • IS = the transistor reverse saturation current

For the basic logarithmic amplifier implementation shown in Figure 7-1, the following expression holds:

Equation 2. VOUT=-VBE=-kTqln⁡IINIS
LOG200 Basic Logarithmic Amplifier Figure 7-1 Basic Logarithmic Amplifier

When a difference amplifier with reference voltage VREF is implemented to compare the outputs of two logarithmic amplifiers with input currents I1 and I2,

Equation 3. V O U T2 - V O U T1 = k T q ln ⁡ I 1 I S1 - k T q ln ⁡ I 2 I S2

As IS1 is approximately equivalent to IS2 by design, this equation is equivalent to:

Equation 4. V O U T2 - V O U T1 = k T q ln ⁡ I 1 I 2 = k T 0.434q log 10 ⁡ I 1 I 2
LOG200 LOG200 Difference Amplifier Figure 7-2 LOG200 Difference Amplifier

In the LOG200, the internal input resistors of the difference amplifier have a positive temperature coefficient to compensate for the temperature dependence of the above expression. The difference amplifier also gains up the nominal output, such that the output of the LOG200 is:

Equation 5. V L O G O U T = K × log 10 ⁡ I 1 I 2 + V R E F

where K is the device scaling factor, nominally 250mV/decade. Thus, for each decade or order of magnitude shift in the difference of I1 and I2, the device output is correspondingly shifted by 250mV (such as by 250mV for I1 = 10µA and I2 = 1µA, or by –500mV for I1 = 10nA and I2 = 1µA).