SBOA571 august   2023 OPA2387 , OPA387 , OPA4387

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Example Considerations
  6. 3Introducing a Preconditioning Circuit
    1. 3.1 First Order Shelving Filter
    2. 3.2 Second Order Shelving Filter
    3. 3.3 Noise Contribution of the Second Order Shelving Filter
    4. 3.4 DC and AC Gain
  7. 4Design Procedure for the Second Order Shelving Filter
    1. 4.1 Definition of Boundary Conditions
    2. 4.2 Calculation of Component Values
  8. 5Influence of Component Tolerances
  9. 6Summary
  10. 7References

1 Introduction

When measuring current, often shunt-based designs are used. In grid systems, the AC content is 50/60 Hz based and contains the fundamental plus harmonic distortion. For example, in power quality analysis or DC/AC power conversion, the harmonics are analyzed individually to monitor and minimize distortion. The AC carries the main power but an unwanted DC content can be present in addition which must be eliminated by the control loop or detected as an error. Therefore, measuring the DC value at the same time is important. For example, a DC/AC system can generate a 20-A AC current sine wave. At the same time, a 20-mA DC current must be measured. This requirement increases the dynamic range of the system and challenges the analog to digital conversion. Furthermore, the voltage across the shunt can be small such that an external gain stage can be beneficial.