SBOSA87A December   2021  – November 2022 LMH5485-SEP

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 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: Vs+ – Vs- = 5 V
    6. 7.6 Electrical Characteristics: Vs+ – Vs- = 3 V
    7. 7.7 Typical Characteristics: 5 V Single Supply
    8. 7.8 Typical Characteristics: 3 V Single Supply
    9. 7.9 Typical Characteristics: 3 V to 5 V Supply Range
  8. 8 Parameter Measurement Information
    1. 8.1 Example Characterization Circuits
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Differential I/O
      2. 9.3.2 Power-Down Control Pin (PD)
        1. 9.3.2.1 Operating the Power Shutdown Feature
      3. 9.3.3 Input Overdrive Operation
    4. 9.4 Device Functional Modes
      1. 9.4.1 Operation from Single-Ended Sources to Differential Outputs
        1. 9.4.1.1 AC-Coupled Signal Path Considerations for Single-Ended Input to Differential Output Conversion
        2. 9.4.1.2 DC-Coupled Input Signal Path Considerations for Single-Ended to Differential Conversion
      2. 9.4.2 Differential-Input to Differential-Output Operation
        1. 9.4.2.1 AC-Coupled, Differential-Input to Differential-Output Design Issues
        2. 9.4.2.2 DC-Coupled, Differential-Input to Differential-Output Design Issues
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Applications
      1. 10.2.1 Designing Attenuators
        1. 10.2.1.1 Design Requirements
        2. 10.2.1.2 Detailed Design Procedure
        3. 10.2.1.3 Application Curve
      2. 10.2.2 Interfacing to High-Performance ADCs
        1. 10.2.2.1 Design Requirements
        2. 10.2.2.2 Detailed Design Procedure
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
  13. 13Device and Documentation Support
    1. 13.1 Documentation Support
      1. 13.1.1 Related Documentation
    2. 13.2 Receiving Notification of Documentation Updates
    3. 13.3 Support Resources
    4. 13.4 Trademarks
    5. 13.5 Electrostatic Discharge Caution
    6. 13.6 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

8.1 Example Characterization Circuits

The LMH5485-SEP offers the advantages of a fully differential amplifier (FDA) design, with the trimmed input offset voltage of a precision op amp. The FDA is an extremely flexible device that provides a purely differential output signal centered on a settable output common-mode level. The primary options revolve around the choices of single-ended or differential inputs, AC-coupled or DC-coupled signal paths, gain targets, and resistor Value selections. Differential sources can certainly be supported and are often simpler to both implement and analyze. Examples of both AC and DC coupled single-ended to differential circuits is shown in Figure 8-1 and Figure 8-2.

Because most lab equipment is single-ended, the characterization circuits typically operate with a single-ended, matched, 50 Ω input termination to a differential output at the FDA output pins. That output is then translated back to single-ended through a variety of baluns (or transformers) depending on the test and frequency range. DC-coupled, step-response testing uses two 50 Ω scope inputs with trace math.

Figure 8-1 AC-Coupled, Single-Ended Source to a Differential Gain of a 2 V/V Test Circuit
Figure 8-2 DC-Coupled, Single-Ended-to-Differential, Basic Test Circuit Set for a Gain of 5 V/V

Figure 8-1 shows how most characterization plots fix the Rf value at 402 Ω. This value is completely flexible in application, but the 402 Ω provides a good compromise for the issues linked to this value, specifically:

  • Added output loading. The FDA appears like an inverting op amp design with both feedback resistors as an added load across the outputs (approximate total differential load in Figure 8-1 is 500 Ω || 804 Ω = 308 Ω).
  • Noise contributions because of the resistor values. The resistors contribute both a 4kTR term and provide gain for the input current noise.
  • Parasitic feedback pole at the input summing nodes. This pole created by the feedback R value and the
    differential input capacitance (as well as any board layout parasitic) introduces a zero in the noise gain, decreasing the phase margin in most situations. This effect must be managed for best frequency response flatness or step response overshoot. The 402 Ω value selected does degrade the phase margin slightly over a lower value, but does not decrease the loading significantly from the nominal 500 Ω value across the output pins.