Three-Electrode Double-Differential Biopotential Amplifier for Surface EMG Measurements

In this work, a novel double-differential (DD) amplifier for electromyogram (EMG) measurements is presented, which requires only three electrodes, a reduced number of components, and is focused on wearable devices of reduced size. The proposed topology is dual-mode since it can also be used as a conventional single-differential (SD) EMG amplifier. The front end sets the measurement center node at a reference potential and uses single-ended (SE) amplifiers to form the DD signal at the common-mode output. To be able to measure with negligible electromagnetic interference (EMI), it is necessary to achieve a sufficiently large isolation impedance. A simple circuit with discrete components was implemented, which allowed validating the operation of the proposed topology. Experimental measurements were made for its characterization, and the results are: 3-dB bandwidth from 5 to 460 Hz, DD gain of 34 dB in the passband, differential-mode rejection ratio (DMRR) greater than 40 dB in the passband, and total noise referred to the input of 2.1
$\mu \text{V}_{\mathrm {RMS}}$
. Also, the measurement of DD signals from the amplifier was verified by comparison with an additional acquisition device. Finally, the circuit was tested inside a small wearable device to measure surface EMG (sEMG). The results of this work validate that the proposed amplifier measures the DD sEMG signals with only three electrodes and is suitable for use in portable and wearable systems. This topology requires one less electrode than usual and has an extremely simple design with a reduced number of components.