SBOA436 October   2020 OPA192

 

  1.   Design Goals
  2.   Design Description
  3.   Design Notes
  4.   Design Steps
  5.   Design Simulations
  6.   DC Simulation Results
  7.   Design References
  8.   Design Featured Op Amp
  9.   Design Alternate Op Amp

Design Notes

  1. Ensure common-mode voltages at the inputs (Vcm nodes) of the op amp are within the Vcm range listed under Electrical Characteristics in the data sheet of the op amp.

  2. Refer to the typical “Output Voltage Swing vs. Output Current” graphs in the data sheet of the op amp to account for output swing from rails (Vout node).

  3. Resistor mismatch will contribute gain error and degrade CMRR of the circuit.

  4. Error in final results can be expected due to Ifeedback current. Placing high-value resistors will limit the effect of this current, but will add thermal noise to the circuit. Possible bandwidth limitations and stability issues caused by large resistances and parasitic capacitances in the circuit also become more prevalent.

  5. In an ideal “Improved” Howland current pump, resistor R4 is usually set equal to R2-Rs, which slightly alters the feedback network but results in the expected Iload value. Accuracy of these resistors will limit the effectiveness of the technique at reducing errors.

  6. Special precautions should be taken when driving reactive loads.

  7. A typical design procedure first calculates the gain for a known output current and shunt resistor; then sets R1 and scales R2 through R4 accordingly. This can be an iterative process.