An instrumentation amplifier as a front-end for a four-electrode bioimpedance measurement.
The performance of a monolithic instrumentation amplifier used as an interface for a four-electrode bioimpedance measurement is examined with a commercially available impedance meter based on an auto-balancing bridge. The errors due to particularities in the input stage of the impedance meter, when used without a front-end, were several orders of magnitude higher than the measured quantity. The analysis was performed on an electrical circuit model of the skin and electrodes over a frequency range of 20 Hz to 1 MHz. The achieved accuracy with balanced electrode impedances for the frequencies up to 100 kHz can be below 0.2% for impedance magnitude and 0.1 degrees for impedance phase, which is within the specified basic accuracy range of the LCR-meter used for the measurements. At frequencies above 100 kHz the errors are increasing and are higher than the LCR-meter's basic accuracy. This study indicates that use of an instrumentation amplifier as a front-end with the particular LCR-meter can significantly improve the measurement accuracy of the four-electrode bioimpedance measurement at low frequencies.