In the first stage of this project, I will focus on a basic circuit which will demonstrate rudimentary signal acquisition with front-end amplification and filtering. In the first stage of the circuit, 3 electrodes are placed on the body, two in close proximity on the medial portion of the bicep and the other on the elbow. The two electrodes on the bicep act as a differential pair, whereby signals common to both electrodes will be greatly reduced at the output of the differential amplifier (depending on the CMRR). This technique removes noise common to both sites, usually consisting of (in Australia) 50 Hz mains hum and possibly noise caused by the electrode-skin interface. The reference electrode is placed somewhere unrelated to the activity of the muscle being studied: The elbow is a good location since it is relatively far from the bicep muscle.
The signal at the first and second bicep electrode can be represented as (s1 + n) and (s2 + n) respectively. At the output of the differential amplifier, we have (ideally)
effectively removing the common noise from the signal and leaving behind the difference between the two EMG signals.
After the differential amplifier has subtracted the noise, a high pass filter is implemented with a cut off at 15 Hz. Variations in the electrode skin interface can cause unwanted signals in the 0 - 20 Hz range, so the high pass filter is set to cut out most of these. Next a low pass filter is implemented with a cutoff at 450 Hz which is nearing the higher end of useful signal bandwidth.
The filtered analog signal is then converted to digital via a microcontroller ADC and fed to a PC application like matlab via USB serial for analysis.
Roberto Merletti, Philip Parker, Electromyography: Physiology, Engineering, and Noninvasive Applications, 2004, John Wiley & Sons Inc.
Carlo J. De Luca, SURFACE ELECTROMYOGRAPHY: DETECTION AND RECORDING, 2002, DelSys Inc, Link: https://www.delsys.com/Attachments_pdf/WP_SEMGintro.pdf