SNOSDB6C December   2020  – March 2022 LMP7704-SP

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and 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 Electrical Characteristics: VS = 5 V
    6. 6.6 Electrical Characteristics: VS = 10 V
    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 Radiation Hardened Performance
      2. 7.3.2 Engineering Model (Devices With /EM Suffix)
      3. 7.3.3 Capacitive Load
      4. 7.3.4 Input Capacitance
      5. 7.3.5 Diodes Between the Inputs
    4. 7.4 Device Functional Modes
      1. 7.4.1 Precision Current Source
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Low Input Voltage Noise
      2. 8.1.2 Total Noise Contribution
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Receiving Notification of Documentation Updates
    2. 11.2 Support Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2 Detailed Design Procedure

Many sensors have high source impedances that may range up to 10 MΩ. The output signal of sensors must often be amplified or otherwise conditioned by means of an amplifier. The input bias current of this amplifier can load the sensor output and cause a voltage drop across the source resistance, as shown in Figure 8-4, where VIN+ = VS – IBIAS * RS.

GUID-996A78BA-9E8A-489F-9B8B-110C01A56735-low.gifFigure 8-4 Offset Error Due to IBIAS

The last term, IBIAS * RS, shows the voltage drop across RS. To prevent errors introduced to the system due to this voltage, an op amp with very low input bias current must be used with high impedance sensors. An amplifier with low input bias also has low input current noise, further improving the accuracy of systems with high source resistance.

Figure 8-3 shows one channel of the LMP7704-SP configured for a gain of 25. A high source impedance is placed between the input signal and the noninverting input of the amplifier to represent the output impedance of the sensor.

With the ultra-low input bias current of the LMP7704-SP, even with a signal source that has high output impedance, the system output maintains very good linearity to the ideal output voltage (that is, the output of an ideal amplifier in the same configuration). Figure 8-5 shows the output voltage vs input voltage of the LMP7704-SP with a 10-MΩ source impedance. Figure 8-6 shows the output voltage vs input voltage for an ideal amplifier with no input bias current. Comparing the two graphs shows that the LMP7704-SP maintains high accuracy even with a large source impedance connected to an input.