Bandwidth and Common Mode Optimization for Current and Voltage Sources in Bioimpedance Spectroscopy

Area of Science:

• Biomedical Engineering
• Electrical Engineering

Background:

• Bioimpedance measurements require high-bandwidth current or voltage sources (1 kHz to 1 MHz) for injecting excitation signals.
• Patient safety necessitates current limitation and low common mode signals in these sources.
• Existing circuits like enhanced Howland current sources (EHCS) and voltage sources (VS) have limitations in bandwidth and common mode rejection.

Purpose of the Study:

• To compare the performance of a symmetric enhanced Howland current source (EHCS) and a symmetric voltage source (VS) for bioimpedance spectroscopy.
• To evaluate the effectiveness of common mode reduction circuits in both source types.
• To determine which source offers superior performance for bioimpedance measurements between 1 kHz and 1 MHz.

Main Methods:

• Simulations were used to optimize the bandwidth and output impedance of both EHCS and VS circuits.
• A common mode reduction circuit was implemented in both source designs.
• Laboratory measurements were conducted to assess output impedance, bandwidth, and common mode rejection ratio (CMRR).

Main Results:

• The EHCS exhibited a -3 dB bandwidth at 400 kHz, while the VS reached a +3 dB bandwidth at 600 kHz.
• The VS achieved a higher average common mode rejection of -71.8 dB compared to -57.7 dB for the EHCS over the observed frequency range.
• Both modified circuits achieved low common mode signals without complex additions like general impedance converters.

Conclusions:

• Voltage sources (VS) are superior to enhanced Howland current sources (EHCS) for bioimpedance spectroscopy due to their higher bandwidth.
• The performance advantage of VS is contingent upon the ability to accurately measure the injected current.
• The implemented modifications enhance the performance of both EHCS and VS for bioimpedance applications.