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

6 Summary

The second order shelving filter offers the option to individually set the DC gain and the AC gain of a preconditioning circuit in front of an ADC when measuring current with shunts. The small voltages can be increased to improve the DC value amplitude and at the same time to adjust the AC value to full scale range. This increase reduces the dynamic range needed in the system. Due to the second order system, a much smaller group delay for the DC range can be achieved and the time to detect a DC fault reduced. For lowest error and minimal noise contribution, an auto-zero op amp must be selected like OPA387. With no 1/f noise contribution of such auto-zero op amp, the SNR within the DC energy bin can be improved.