The “Improved” Howland current pump is a circuit that uses a difference amplifier to impose a voltage across a shunt resistor (Rs), creating a voltage-controlled bipolar (source or sink) current source capable of driving a wide range of load resistance. See the AN-1515 A Comprehensive Study of the Howland Current Pump Application Report for more information on the functionality of the “Improved” Howland current pump.

1. Ensure common-mode voltages at the inputs (V_cm nodes) of the op amp are within the V_cm 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 (V_out node).

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

4. Error in final results can be expected due to I_feedback 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 I_load 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.