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For the improved Howland current pump application, the output voltage (Vout) that is attainable at the load
is further constrained by the voltage drop developed across Rhcp. The 20-mA load current flows through
the resistor, and causes a voltage drop. Therefore, with a fixed 20-mA output current flowing through RL,
there is a limit as to how far Vout can swing. For this example, the limit occurs when Vout reaches
approximately 57.4 V, and RL is 2.87 kΩ. This limit can be seen in
from 0 Ω to 2.87 kΩ, and the output will remain within the circuit linear compliance range. However, if the
value of RL is increased further, the current pump operation is forced outside the compliance range, and
the output current will no longer remain constant. Thus, increasing the value of RL to greater than 2.87 kΩ
defeats the goal of providing a constant current.
Figure 2. Improved Howland Current Pump Output-Voltage Compliance Range
The improved Howland current pump can be used to generate an ac waveform with a peak current that
corresponds to a maximum specified dc value. A 20-mA dc current source using the OPA462 has been
demonstrated, and will now be used to show how a 20-mA peak ac-current waveform can be realized.
SBOU175 – October 2019
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Application Circuits
Figure
2. Resistor RL can be any value
OPA462EVM
7
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