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Biopotentials and Electrophysiology Measurement
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
Environmental electric interference is always present, especially in urban hospital environments. It is desirable to eliminate interference before it enters the amplifier, for example, by proper shielding of the subject, leads, and the instrument and by grounding the subject and the instrument. Sources of interference include induced signals from power lines and electric wiring; RF from transmitters, electric motors, and other appliances; and magnetically induced currents in lead wires [13]. Interference induced on the body common to the biopotential sensing electrodes is called the common-mode interference (as distinguished from the biopotential that is differential to the sensing electrodes). If the induced current is id and the resistance to ground is R0, then the common-mode interference potential is Vc = idR0. The common-mode interference is principally rejected by a differential or instrumentation amplifier with a high CMRR. Further improvement is possible by use of the “driven right leg circuit.” The right leg lead, by standard convention, is used as the ground or the circuit reference. The driven right leg circuit employs the clever idea of negative feedback of the common-mode signal into this lead. The common-mode signal is sensed from the first stage of the instrumentation amplifier, amplified and inverted, and fed back into the right leg lead (Figure 64.6a). At this stage the common-mode signal is reduced to (idR0)/(l + 2R2/R1). Thus, the common-mode interference is greatly reduced at its source. The driven right leg circuit along with a high CMRR of the amplifier and filtering permit very high-quality biopotential measurements.
Quantification of error between the heartbeat intervals measured form photoplethysmogram and electrocardiogram by synchronisation
Published in Journal of Medical Engineering & Technology, 2018
Srinivas Kuntamalla, Ram Gopal Reddy Lekkala
Ag/AgCl disposable electrodes are used for recording standard Lead I ECG signal. A custom-designed circuit consisting of an instrumentation amplifier and a third-order Butterworth bandpass filter and right leg drive circuit is used for ECG recording. The circuit diagram of the ECG measurement system is shown in Figure 5. The cut off frequencies for the bandpass filter are fixed as 0.5 and 25 Hz, to maximise the QRS complex energy and avoid the baseline drift and reduce noise due to electrical interference. The common mode voltage on the body sensed by two averaging resistors is inverted, amplified and low pass filtered. This voltage is given as feedback to the right leg, which acts as a negative feedback and drives the common mode voltage to a low value. This driven right leg circuit also provides some safety, any high voltage appears between the patient and ground, in which case the operational amplifier connected to Right Leg saturates and effectively ungrounds the patient. The circuit also shows two diodes connected back to back at the input of instrumentation amplifier that provides protection from high voltage transients.