SBOSAI7B November   2023  – July 2024 OPA2891 , OPA891

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 - OPA891
    5. 5.5 Thermal Information - OPA2891
    6. 5.6 Electrical Characteristics - RL = 150Ω
    7. 5.7 Electrical Characteristics - RL = 1kΩ
    8. 5.8 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagrams
    3. 6.3 Feature Description
      1. 6.3.1 Offset Nulling
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Driving a Capacitive Load
      2. 7.1.2 Low-Pass Filter Configurations
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1 Selection of Multiplexer
        2. 7.2.2.2 Signal Source
        3. 7.2.2.3 Driving Amplifier
        4. 7.2.2.4 Driving Amplifier Bandwidth Restriction
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 General PowerPAD™ Integrated Circuit Package Design Considerations
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Documentation Support
      1. 8.1.1 Related Documentation
    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 Low-Pass Filter Configurations

When receiving low-level signals, limiting the bandwidth of the incoming signals into the system is often required. Figure 7-2 shows how the simplest way to accomplish this limiting is to place an RC filter at the noninverting pin of the amplifier.

OPA891 OPA2891 Single-Pole Low-Pass Filter Figure 7-2 Single-Pole Low-Pass Filter
Equation 1. V O U T V I N = 1 + R F R G × 1 1 + s R 1 C 1

If more attenuation at higher frequencies is required, a multiple-pole filter is required. Figure 7-3 shows a common implementation of a second-order filter called a Sallen-Key filter. When designing this type of filter, choose an amplifier with a bandwidth that is approximately an order of magnitude larger than the desired filter bandwidth. See Active Low-pass Filter Design for more detailed active-filter design information.

Assuming R1 = R2 = R and C1 = C2 = C, use Equation 2 to set the bandwidth of the filter.

Equation 2. f 3 d B = 1 2 π R C

The Q-factor of a filter controls the amount of peaking of the small-signal frequency response and the settling time of the pulse response. Set Q to 0.707 to provide a Butterworth response with a maximally-flat pass-band. Choose the ratio of RF and RG to obtain the desired Q value as shown in Equation 3.

Equation 3. R F R G = 2 - 1 Q
OPA891 OPA2891 Two-Pole Low-Pass Sallen-Key Filter Figure 7-3 Two-Pole Low-Pass Sallen-Key Filter