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

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.1.3 Setting the Reference Current

When the LOG114 is used to compute logarithms, either I1 or I2 can be held constant to become the reference current (IREF) to which the other is compared. As shown in Figure 7-1, I2 is used as IREF and is generated using the on-chip 2.5V VREF pin.

LOG114 Example of Setting IREFFigure 7-1 Example of Setting IREF

An accurate IREF at lower current values (<20nA) can be difficult to achieve. Rather than choosing an IREF value to be equal to ISIGNAL (min), higher accuracy can be achieved by selecting IREF to be in the center of the full signal range as shown in Equation 3.

Equation 3. IREF=ISIGNAL (min)×ISIGNAL (max)ISIGNAL (min)

For example, for a signal range of 1nA to 1mA, after plugging in the values into Equation 3, IREF = 1μA. Using a 1μA DC current level for the reference current provides higher precision (DC accuracy, temperature stability, and lower noise) than using 1nA level as the reference current.

The reference current can be derived from a voltage source with one or more resistors. When a single resistor is used, the value can be large depending on IREF. If IREF is 10nA and 2.5V is used:

Equation 4. RREF=VSOURCEIREF=2.5V10nA=250MΩ

A voltage divider T-Network circuit can be used to reduce the value of the resistor, as shown in Figure 7-2. When using this method, consider the possible errors caused by the amplifier input offset voltage. The input offset voltage of amplifier A1 has a maximum value of 4mV in a ±5V supply system, and a maximum value of 7mV in a 5V supply system. Consider resistor temperature stability and noise contributions, as well.

LOG114 T-Network for Reference CurrentFigure 7-2 T-Network for Reference Current

VREF can be an external precision voltage reference, or the on-chip 2.5V voltage reference of the LOG114.

IREF can be derived from an external current source, such as that shown in Figure 7-3.

LOG114 Temperature-Compensated Current SourceFigure 7-3 Temperature-Compensated Current Source