SBOS301C May   2004  – December 2025 LOG114

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration
  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 (±5V)
    6. 5.6 Electrical Characteristics (5V)
    7. 5.7 Typical Characteristics: VS = ±5V
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Logarithmic and Difference Amplifier
      2. 6.3.2 COM Voltage Range
      3. 6.3.3 VCM IN
      4. 6.3.4 Auxiliary Operational Amplifier
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Applications Information
      1. 7.1.1 Transfer Function
      2. 7.1.2 Input Current Range
      3. 7.1.3 Setting the Reference Current
      4. 7.1.4 Negative Input Currents
      5. 7.1.5 Voltage Inputs
      6. 7.1.6 High-Current Linearity Correction
      7. 7.1.7 Error Sources
        1. 7.1.7.1 Accuracy
        2. 7.1.7.2 Total Error
        3. 7.1.7.3 Errors RTO and RTI
        4. 7.1.7.4 Log Conformity
        5. 7.1.7.5 Individual Error Components
    2. 7.2 Typical Applications
      1. 7.2.1 Design Example for Dual-Supply Configuration
      2. 7.2.2 Design Example for Single-Supply Configuration
      3. 7.2.3 Advantages of Dual−Supply Operation
      4. 7.2.4 Log Ratio
      5. 7.2.5 Data Compression
      6. 7.2.6 3.3V Operation
      7. 7.2.7 Erbium-Doped Fiber Optic Amplifier (EDFA)
    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 Device Nomenclature
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
      2. 8.2.2 PSpice® for TI
      3. 8.2.3 TINA-TI™ (Free Software Download)
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.1.7.5 Individual Error Components

The ideal transfer function with current input is:

Equation 8. V L O G O U T   ( I D E A L ) = 0 . 375 × log I 1 I 2

The actual transfer function with the major components of error is:

Equation 9. V L O G O U T   ( a c t u a l ) = 0 . 375 × ( 1 ± Δ K ) × log ( I 1 I 2 ) ± 2 N m ± V O S O

where:

  • ΔK = gain error (0.4%, typical, as specified in the Electrical Characteristics table)
  • IB1 = bias current of A1 (5pA, typical)
  • IB2 = bias current of A2 (5pA, typical)
  • m = number of decades over which the log conformity error is specified
  • N = log conformity error (0.1%, typical for m = 5 decades; 0.9% typical for m = 7.5 decades)
  • VOSO = output offset voltage (11mV, typical for ±5V supplies; 14mV, typical for +5V supplies)

To determine the typical error resulting from these error components, first compute the ideal output. Then calculate the output again, this time including the individual error components. Then use Equation 10 to determine the error in percent:

Equation 10. % e r r o r = V L O G O U T   ( i d e a l ) - V L O G O U T   ( t y p i c a l ) V L O G O U T   ( i d e a l ) × 100 %

For example, in a system configured for measurement of five decades, with I1 = 1mA, and I2 = 10μA:

Equation 11. V L O G O U T   ( i d e a l ) = 0 . 375 × log ( 10 - 3 10 - 5 ) = 0 . 75 V
Equation 12. V L O G O U T   ( t y p i c a l ) = 0 . 375 ( 1 ± 0 . 004 ) × log ( 10 - 3 - 5 × 10 - 12 10 - 5 - 5 × 10 - 12 ) ± 2 ( 0 . 001 ) ( 5 ) ± 0 . 011

Using the positive error components (+ΔK, +2Nm, and +VOSO) to calculate the maximum typical output:

Equation 13. V L O G O U T   ( t y p i c a l ) = 0 . 774 V

Therefore, the error in percent is:

Equation 14. % e r r o r = 0 . 75   -   0 . 774 0 . 75 × 100 % = 3 . 2 %