SBOSA33A September   2021  – December 2021 LMH5485-SP

ADVANCE INFORMATION  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and 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: VS+ – VS– = 5 V
    6. 7.6  Electrical Characteristics: VS+ – VS– = 3 V
    7. 7.7  Quality Conformance Inspection
    8. 7.8  Typical Characteristics: 5 V Single Supply
    9. 7.9  Typical Characteristics: 3 V Single Supply
    10. 7.10 Typical Characteristics: 3 V to 5 V Supply Range
  8. Parameter Measurement Information
    1. 8.1 Example Characterization Circuits
  9. Detailed Description
    1. 9.1 Overview
      1. 9.1.1 Terminology and Application Assumptions
    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
        3. 9.4.1.3 Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA
        4. 9.4.1.4 Input Impedance for the Single-Ended to Differential FDA Configuration
      2. 9.4.2 Differential-Input to Differential-Output Operation
        1. 9.4.2.1 AC-Coupled, Differential-Input to Differential-Output Design Issues
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Support Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information
    1. 13.1 Tube Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • HKX|8
Thermal pad, mechanical data (Package|Pins)
Orderable Information

AC-Coupled Signal Path Considerations for Single-Ended Input to Differential Output Conversion

When the signal path can be AC-coupled, the DC biasing for the LMH5485-SP becomes a relatively simple task. In all designs, start by defining the output common-mode voltage. The AC-coupling issue can be separated for the input and output sides of an FDA design. The input can be AC coupled and the output DC coupled, or the output can be AC coupled and the input DC coupled, or they can both be AC coupled. One situation where the output might be DC coupled (for an AC-coupled input), is when driving directly into an ADC where the Vocm control voltage uses the ADC common-mode reference to directly bias the FDA output common-mode to the required ADC input common-mode. In any case, the design starts by setting the desired Vocm. When an AC-coupled path follows the output pins, the best linearity is achieved by operating Vocm at mid-supply. The Vocm voltage must be within the linear range for the common-mode loop, as specified in the headroom specifications (approximately 0.91 V greater than the negative supply and 1.1 V less than the positive supply). If the output path is also AC coupled, simply letting the Vocm control pin float is usually preferred in order to get a mid-supply default Vocm bias with minimal elements. To limit noise, place a 0.1 µF decoupling capacitor on the Vocm pin to ground.

After Vocm is defined, check the target output voltage swing to ensure that the Vocm plus the positive or negative output swing on each side does not clip into the supplies. If the desired output differential swing is defined as Vopp, divide by 4 to obtain the ±Vp swing around Vocm at each of the two output pins (each pin operates 180° out of phase with the other). Check that Vocm ±Vp does not exceed the absolute supply rails for this rail-to-rail output (RRO) device.

Going to the device input pins side, because both the source and balancing resistor on the nonsignal input side are DC blocked (see Figure 8-1), no common-mode current flows from the output common-mode voltage, thus setting the input common-mode equal to the output common-mode voltage.

This input headroom also sets a limit for higher Vocm voltages. Because the input Vicm is the output Vocm for AC-coupled sources, the 1.2 V minimum headroom for the input pins to the positive supply overrides the 1.1 V headroom limit for the output Vocm. Also, the input signal moves this input Vicm around the DC bias point, as described in the Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA section.