SBOS925A December   2020  – January 2021 OPA391

PRODUCTION DATA  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  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
    6. 6.6 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Low Input Bias Current
      2. 7.3.2 Input Differential Voltage
      3. 7.3.3 Capacitive Load Drive
      4. 7.3.4 EMI Rejection
    4. 7.4 Device Functional Modes
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Three-Terminal CO Gas Sensor
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curve
      2. 8.2.2 4-mA to 20-mA Loop Design
        1. 8.2.2.1 Application Curve
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 TINA-TI™ Simulation Software (Free Download)
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Support Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

Detailed Design Procedure

First, determine the VREF voltage. This voltage is a compromise between maximum headroom and resolution, as well as allowance for the minimum swing on the CE terminal because the CE terminal generally goes negative in relation to the RE potential as the concentration (sensor current) increases. Bench measurements found the difference between CE and RE to be 180 mV at 300 ppm for this particular sensor. To allow for negative CE swing, footroom, and voltage drop across the 10-kΩ resistor, 300 mV is chosen for VREF.

Equation 2. VZERO = VREF = 300 mV

where

  • VREF is the reference voltage (300 mV).
  • VZERO is the concentration voltage (300 mV).

Next, calculate the maximum sensor current at highest expected concentration:

Equation 3. ISENSMAX = IPERPPM * ppmMAX = 69 nA * 300 ppm = 20.7 µA

where

  • ISENSMAX is the maximum expected sensor current.
  • IPERPPM is the manufacturer specified sensor current in amperes per ppm.
  • ppmMAX is the maximum required ppm reading.

Then, find the available output swing range greater than the reference voltage available for the measurement:

Equation 4. VSWING = VOUTMAX – VZERO = 2.5 V – 0.3 V = 2.2 V

where

  • VSWING is the expected change in output voltage.
  • VOUTMAX is the maximum amplifier output swing.

Finally, calculate the transimpedance resistor (RF) value using the maximum swing and the maximum sensor current:

Equation 5. RF = VSWING / ISENSMAX = 2.2 V / 20.7 µA = 106.28 kΩ (use 110 kΩ for a common value)