SLVSIS7 October   2026 OPA2986

ADVMIX  

  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 for OPA986
    5. 5.5 Thermal Information for OPA2986
    6. 5.6 Thermal Information for OPA4986
    7. 5.7 Electrical Characteristics for VS = 8V to 48V
    8. 5.8 Electrical Characteristics for VS = 4.5V to 8V
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Mux-Friendly Inputs
      2. 6.3.2 Thermal Protection
      3. 6.3.3 Overload Recovery
      4. 6.3.4 Slew Rate
      5. 6.3.5 Full-Power Bandwidth Improved
      6. 6.3.6 e-Trim
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Applications
      1. 7.2.1 2nd Order Low Pass Filter
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curves
      2. 7.2.2 High-Side Current Sense
      3. 7.2.3 MDAC Output Gain and Buffer
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Receiving Notification of Documentation Updates
    2. 8.2 Support Resources
    3. 8.3 Trademarks
    4. 8.4 Electrostatic Discharge Caution
    5. 8.5 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.2.1.2 Detailed Design Procedure

OPAx986 is an excellent choice for active filter design. The OPAx986 offers a few advantages over existing amplifiers, such as ample gain bandwidth and slew rate, low noise, rail-to-rail inputs, low input bias current, and low input offset voltage.

A multiple feedback filter (MFB) topology is used for the topology's improved high frequency performance compared to other topologies. There are different filter response options to select from, but a Bessel response is chosen here to preserve phase linearity. The MFB topology provides a −40dB/decade roll off and an inverting gain. The transfer function is given by Equation 3.

Equation 3. H ( s ) = - R 2 R 1 × ( 1 s 2 ( R 2 C 1 R 3 C 2 ) + s ( R 2 C 2 + R 3 C 2 + R 2 R 3 C 2 R 1 ) + 1 )

Where the ratio of R2 and R1 set the inverting gain of the filter. The frequency response is determined by the combination of the resistors and the capacitors in the filter. Converting to the standard form of the transfer function given by Equation 4 provides for a more straight forward analysis of the filter response.

Equation 4. H ( s ) = ( K ω 0 2 s 2 + s ω 0 Q + ω 0 2 )

The standard form of the transfer function yields the following equations.

Equation 5. K = - R 2 R 1
Equation 6. Q = R 2 C 1 R 3 C 2 R 2 C 2 + R 3 C 2 + R 2 R 3 C 2 R 1
Equation 7. ω 0 = 1 R 2 C 1 R 3 C 2

The best approach is to select the capacitor values in such a way that the roots of the transfer function yield real resistor values. Using Equation 5, Equation 6, Equation 7, and the quadratic formula, an inequality for C1 can be derived. Selecting a standard capacitor value for C2, the capacitor, C1, is calculated using Equation 8.

Equation 8. C 1 ≥ C 2 × 4 Q 2 ( 1 + K )

Setting C2 to 1nF, means that C1 must be ≥ 2.66nF. A standard 4.7nF capacitor is chosen for C2. Recall that 2nd order Bessel filter response is achieved with a quality factor (Q) of 0.577. With C1, C2, Q, and K fixed, the resistor R2 can be found using Equation 9.

Equation 9. R 2 = 1 4 π f c Q C 2 ( 1 + K ) × ( 1 + 1 - 4 C 2 Q 2 ( 1 + K ) C 1 )

Substituting the known values into Equation 9 yields a resistor value of 1.143kΩ. A standard value of 1.14kΩ is chosen. Note that R1 = R2 for an inverting gain of 1. To solve for R3, use Equation 7 and the component values found in previous steps. The calculated value for R3 is 471Ω so a standard 470Ω resistor is used.

The circuit is then verified using TINA-Spice for TI with the gain and transient response are shown in Figure 7-2 and Figure 7-3, respectively. The cut-off frequency is calculated to be approximately 100.3kHz with the calculated component values and matches well with simulated curve.