SLOS217I July   1998  – December 2024 THS3001

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Recommended Feedback and Gain Resistor Values
      2. 7.1.2 Noise Calculations
      3. 7.1.3 Slew Rate
      4. 7.1.4 Offset Voltage
    2. 7.2 Typical Applications
      1. 7.2.1 General Configurations
      2. 7.2.2 Driving a Capacitive Load
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 PCB Design Considerations
        2. 7.4.1.2 Thermal Considerations
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Evaluation Board
    2. 8.2 Receiving Notification of Documentation Updates
    3. 8.3 Support Resources
    4. 8.4 Trademarks
    5. 8.5 Electrostatic Discharge Caution
    6. 8.6 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.1.2 Noise Calculations

Noise can cause errors on small signals. This problem is especially true for amplifying small signals coming over a transmission line or an antenna. The noise model for current-feedback (CFB) amplifiers is the same as for voltage-feedback (VFB) amplifiers. The only difference between CFB and VFB amplifiers is that CFB amplifiers generally specify different current-noise parameters for each input, whereas VFB amplifiers usually only specify one noise-current parameter. Figure 7-1 shows the noise model. This model includes all of the noise sources as follows:

  • en = Amplifier internal voltage noise (nV/√Hz)
  • IN+ = Noninverting current noise (pA/√Hz)
  • IN– = Inverting current noise (pA/√Hz)
  • eRx = Thermal voltage noise associated with each resistor (eRx = 4 kTRx)
THS3001 Noise Model Figure 7-1 Noise Model

The total equivalent input noise density (eni) is calculated by using the following equation:

Equation 1. THS3001

To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (eni) by the overall amplifier gain (AV).

Equation 2. THS3001

The previous equations show that to keep noise at a minimum, use small-value resistors. As the closed-loop gain is increased (by reducing RG), the input noise is reduced considerably because of the parallel resistance term. This result leads to the general conclusion that the most dominant noise sources are the source resistor (RS) and the internal amplifier noise voltage (en). Noise is summed in a root-mean-squares method; therefore, noise sources smaller than 25% of the largest noise source can be effectively ignored. This threshold can greatly simplify the formula and make noise calculations much easier.