Differential biopotential amplifiers

The output of a real biopotential amplifier will always consist of the desired output component owing a differential biosignal, an undesired component because of incomplete rejection of common mode... 

Biopotential amplifiers used for EMG monitoring are fairly simple since standard instrumentation amplifiers or even inexpensive operational amplifiers can be used in a differential mode. An electrode placement and differential amplifier setup us to measure the EMG is shown in Rg. 17.41. 

Figure 8.14 Differential preamplifier well adapted for serving as biopotential preamplifier. Input equipped with three-wire connections for two PU electrodes and one Ind electrode. The indifferent electrode is coupled to the amplifier reference wire. The purpose is to equipotentialize...

The use of differential amplifiers is common in biopotential measurements because of a greater ability to reject environmental interferraice compared with ground-referenced single-ended amplifiers. Differential amplifiers subtract the electric potential present at one place on the body from that of anoth. Both potentials are measured with respect to a Aird body location that serves as a common point of reference. 

Differential amplifiers are useful because biopotentials generated within the body vary over the body surface, but line-coupled noise does not For example, subtraction of the heart electric potential at two points on the chest surface will produce a res ting potential since the local biopotential amplitudes and wave shapes at each electrode are different. ivironniental electric fields from the power line are more remote and couple such that they are present unifonnly over the body. This is partly due to the distributed nature of capacitive coupling. It is also because die low 50- to 60-Hz line frequencies have electric field wavelengths so long (hundreds of meters) that a person s body can be considered to be, in some sense, an antenna in the uniform near field of an electric field source. 

FIGURE 1131 Diagram of the placement of differential electrodes on a subject s body. The capacitively coupled potential is common mode to the amplifier and sums with Ae biopotential. 

Measurement of the ECG is performed using a differential amplifier. Two limb electrodes at a time are selected for the input to the differential amplifier stage. ECG amplifiers typically have gains of about 1000 and so increase the nominally 1-mV biopotential to about 1 V for driving a strip-chart recorder, cathode-ray-tube display, or computer data-acquisition card. Chart speeds in ECG recording have been standardized at 25 and 50 mm/s. 

The instrumentation amplifier is a circuit configuration that potentially combines the best features desirable for biopotential measurements [8], namely, high differential gain, low... 

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