SBOS622A July   2018  – October 2018 OPA855


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      High-Speed Time-of-Flight Receiver
      2.      Photodiode Capacitance vs Bandwidth and Noise
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
    1.     Pin Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Parameter Measurement Information
    1. 8.1 Parameter Measurement Information
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Input and ESD Protection
      2. 9.3.2 Feedback Pin
      3. 9.3.3 Wide Gain-Bandwidth Product
      4. 9.3.4 Slew Rate and Output Stage
    4. 9.4 Device Functional Modes
      1. 9.4.1 Split-Supply and Single-Supply Operation
      2. 9.4.2 Power-Down Mode
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
      3. 10.2.3 Application Curves
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Development Support
    2. 13.2 Documentation Support
      1. 13.2.1 Related Documentation
    3. 13.3 Receiving Notification of Documentation Updates
    4. 13.4 Community Resources
    5. 13.5 Trademarks
    6. 13.6 Electrostatic Discharge Caution
    7. 13.7 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

Wide Gain-Bandwidth Product

Figure 10 shows the open-loop magnitude and phase response of the OPA855. Calculate the gain bandwidth product of any op amp by determining the frequency at which the AOL is 40 dB and multiplying that frequency by a factor of 100. The open-loop response shows the OPA855 to have approximately 62° of phase-margin in a noise gain of 7 V/V. The second pole in the AOL response occurs before the magnitude crosses 0 dB, and the resultant phase margin is less than 0°. This indicates instability at a gain of 0 dB (1 V/V). Amplifiers that are not unity-gain stable are known as decompensated amplifiers. Decompensated amplifiers typically have higher gain-bandwidth product, higher slew rate, and lower voltage noise, compared to a unity-gain stable amplifier with the same amount of quiescent power consumption.

Figure 50 shows the open-loop magnitude (AOL) of the OPA855 as a function of temperature. The results show approximately 5° of phase-margin variation over the entire temperature range in a noise gain of 7 V/V. Semiconductor process variation is the naturally occurring variation in the attributes of a transistor (Early-voltage, β, channel-length and width) and other passive elements (resistors and capacitors) when fabricated into an integrated circuit. The process variation can occur across devices on a single wafer, or, across devices over multiple wafer lots over time. Typically, the variation across a single wafer is tightly controlled. Figure 51 shows the AOL magnitude of the OPA855 as a function of process variation over time. The results show the AOL curve for the nominal process corner and the variation one standard deviation from the nominal. The simulated results show less than 2° of phase-margin difference within a standard deviation of process variation in a noise gain of 7 V/V.

One of the primary applications for the OPA855 is as a high-speed transimpedance amplifier (TIA). The low-frequency noise gain of a TIA is 0 dB (1 V/V). At high frequencies the ratio of the total input capacitance and the feedback capacitance set the noise gain. To maximize the TIA closed-loop bandwidth, the feedback capacitance is typically smaller than the input capacitance, which implies that the high-frequency noise gain is greater than 0 dB. As a result, op amps configured as TIAs are not required to be unity-gain stable, which makes a decompensated amplifier a viable option for a TIA. What You Need To Know About Transimpedance Amplifiers – Part 1 and What You Need To Know About Transimpedance Amplifiers – Part 2 describe transimpedance amplifier compensation in greater detail.

OPA855 D604_SBOS622.gifFigure 50. Open-Loop Gain vs Temperature
OPA855 D605_SBOS622.gifFigure 51. Open-Loop Gain vs Process Variation