SNVS420D November   2008  – May 2018 LM7705

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 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 3.3-V Electrical Characteristics
    6. 6.6 5-V Electrical Characteristics
    7. 6.7 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Supply Voltage
      2. 7.3.2 Output Voltage and Line Regulation
      3. 7.3.3 Output Current and Load Regulation
      4. 7.3.4 Quiescent Current
    4. 7.4 Device Functional Modes
      1. 7.4.1 General Amplifier Application
        1. 7.4.1.1 One-Stage, Single-Supply True Zero Amplifier
        2. 7.4.1.2 Two-Stage, Single-Supply True Zero Amplifier
        3. 7.4.1.3 Dual-Supply, True Zero Amplifiers
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Functional Description
      2. 8.1.2 Technical Description
      3. 8.1.3 Charge Pump Theory
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Basic Setup
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Examples
  11. 11Device and Documentation Support
    1. 11.1 Community Resources
    2. 11.2 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

7.4.1.1 One-Stage, Single-Supply True Zero Amplifier

This application shows a sensor with a DC output signal, amplified by a single supply op amp. The output voltage of the op amp is converted to the digital domain using an Analog to Digital Converter (ADC). Figure 24 shows the basic set-up of this application.

LM7705 20173035.gifFigure 24. Sensor With DC Output and a Single-Supply Op Amp

The sensor has a DC output signal that is amplified by the op amp. For an optimal signal-to-noise ratio, the output voltage swing of the op amp must be matched to the input voltage range of the Analog to Digital Converter (ADC). For the high side of the range this can be done by adjusting the gain of the op amp. However, the low side of the range cannot be adjusted and is affected by the output swing of the op amp.

Example:

Assume the output voltage range of the sensor is 0 to 90 mV. The available op amp is a LMP7701, using a 0/+5-V supply voltage, having an output drive of 50 mV from both rails. This results in an output range of 50 mV to 4.95V.

Select two resistors values for RG1 and RF1 that result in a gain of 50x. The output of the LMP7701 must swing from 0 mV to 4.5 V. The higher value is no problem, however the lower swing is limited by the output of the LM7701 and won’t go below 50 mV instead of the desired 0 V, causing a non-linearity in the sensor reading. When using a 12-bit ADC, and a reference voltage of 5 V (having an ADC step size of approximate 1.2 mV), the output saturation results in a loss of the lower 40 quantization levels of the ADCs dynamic range.