Load in the Loop Dual Howland Current Source for Wide Frequency Bandwidth and Wide Load Range Bioimpedance Measurements

The Howland current source is widely recognized as a high-efficiency voltage-controlled current source (VCCS) because it delivers a precise and stable current with a high output impedance over a wide frequency bandwidth. The dual Howland (DH) is distinguished by a relatively high output impedance in comparison to the enhanced Howland circuit (EH) has therefore the ability to supply higher loads. Nevertheless, at high frequencies, stray capacitances occurring mainly at the input of the operational amplifier and the circuit’s output strongly affect its behavior. The output impedance strongly decreases, especially at frequencies higher than 100 kHz, and the flatness of the output current in dependence on the load is strongly affected. To overcome these challenges, we propose a novel DH current source with load in the loop (DH-LL) completely isolating the load from the ground and maintaining the simplicity of the circuit. A simulation study shows that the output impedance (
$Z_{\mathrm{ out}}$
) of the DH-LL remains constant independent of the input stray capacitances. The influence of the output stray capacitance on the relative deviation of the output current decreases from 92% in the case of DH to 2.7% in the case of DH-LL. Experimental results show that the DH-LL reaches a frequency bandwidth of up to 5 MHz with a deviation of only 2.32% for a varying load of up to 10
$\text{k}\Omega $
. Thereby, the relative deviation of the current amplitude, e.g., at 1 MHz, decreases from 6.98% for DH to 1.44% for DH-LL.