SBOSA51 December   2020 THS4567

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics: Differential TIA Mode, ICM loop enabled
    6. 6.6 Electrical Characteristics: FDA operation, ICM loop disabled
    7. 6.7 Typical Characteristics: (VS+) – (VS–) = 5 V
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Main Amplifier
      2. 7.3.2 Output Common-Mode Control
      3. 7.3.3 Input Common-Mode Control
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Differential Transimpedance Amplifier Mode
      3. 7.4.3 Fully Differential Amplifier (FDA) Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Noise Analysis
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure (THS4567 in TIA Mode)
        1. 8.2.2.1 OPA Mode Configuration
      3. 8.2.3 Application Curves
    3. 8.3 Differential TIA with 0-V Biased Photodiode
    4. 8.4 Differential AC Coupled TIA
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Board Layout Recommendations
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8.2.2 Detailed Design Procedure (THS4567 in TIA Mode)

The output current from each photodiode is shown in Figure 8-4. A detailed procedure on how to set the various bias voltages and select the optimal value of transimpedance gain follows.

  • Set VS+ = 5.25 V and VS– = –0.25 V to allow the THS4567 to swing 10 VPP (differential) without introducing distortion due to limited headroom.
  • Set ICM_EN = logic high to enable the THS4567 TIA mode of operation.
  • Set VOCM to 2.5 V to match the ADC input common-mode range.
  • With the photodiodes (PDs) configured with a cathode bias as shown in Figure 8-2 both PDs will source current when light is incident on them. To maximize the reverse bias across the PD, VBIAS is typically set to the amplifiers positive supply voltage or the highest available positive supply voltage.
  • The maximum output current from the PD is the sum of the ambient light current and the maximum signal current.
    Equation 2. ITOTAL = IAMBIENT + ISIGNAL = 10 µA + 5 µA = 15 µA
  • In the TIA mode, VICM is set to its minimum input common-mode compliance limit (1.25 V) to maximize the reverse bias across the PDs thereby reducing the PD capacitance.
    Equation 3. Reverse bias across the photodiodes = (5.25 V - 1.25 V) = 4 V
  • In the TIA mode, the ICM loop cancels the common-mode input current due to ambient light (10 µA) at the amplifier's input pin and only the differential signal current flows through the feedback resistors RF and RF'. The maximum TIA gain is therefore the ratio of the maximum differential output swing and the maximum differential signal current as shown in Equation 4.
    Equation 4. Maximum TIA gain = (10 VPP/ 10 µA) = 1 MΩ
  • Once the feedback resistor value is set, select the value of feedback capacitance as described in Transimpedance Considerations for High-Speed Amplifiers.
GUID-7F79916D-FC55-4F6A-8BC1-693AD1620F3C-low.gif Figure 8-4 Photodiode Differential Output Current