Abstract
Differential input amplifiers are commonly used in the measurement of biopotential signals. This is because of their ability to reject common-mode signals, for example power-line interference. Rejection is further enhanced by referencing the subject to the instrumentation system. One such referencing technique uses a third subject electrode connected either to circuit common or to ground. However, these techniques are rarely explained or quantified in the literature. Four recording configurations are examined, isolated and non-isolated amplifiers with two and with three electrodes, and existing models are extended. From a brief assessment of power-line interference sources, it is concluded that only displacement currents induced into the leads and the body need be considered. The effect of these on the four configurations is analysed using SPICE. Results show that interference from displacement currents induced into the leads and into the body is dependent upon the recording configuration used and is significantly lower in three-electrode than in two-electrode amplifiers. Isolation also reduces levels of interference. It is concluded that the choice of referencing may affect interference levels on the biopotential signal significantly. Experimental and further analytical work is planned to confirm this.
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Abbreviations
- B :
-
magnetic flux density
- C b :
-
capacitive coupling of power line to subject
- C d1 ,C d2 :
-
capacitive coupling of power line to input leads
- C e :
-
stray capacitance between circuit common and ground
- C g :
-
capacitive coupling of subject to ground
- C m :
-
capacitive coupling of power line to chassis/circuit common
- C 1,C 2,C 3 :
-
cable capacitance:C 1,2 screen to core,C 3 core to core
- C 4,C 5,C 6 :
-
capacitive coupling of power line to cable shields
- CMRR :
-
common-mode rejection ratio
- ECG:
-
electrocardiogram
- EMG:
-
electromyogram
- E1, E2 :
-
input electrodes
- f :
-
signal frequency
- I b :
-
displacement current coupled into body
- I d1 ,I d2 ,I d :
-
displacement currents coupled into input leads
- IMRR :
-
isolation-mode rejection ratio
- R1, R2 :
-
current-limiting resistors in amplifier inputs
- ref:
-
reference (third) electrode
- S :
-
area of conductive loop
- SW1 :
-
switch to determine either two or three electrode system
- SW2 :
-
switch to determine either isolated or non-isolated amplifier
- V B :
-
common-mode body potential with respect to ground
- V CM :
-
common-mode body potential with respect to circuit common
- V 1 :
-
isolation-mode potential
- V n :
-
electrical interference
- Z CM1 ,Z CM2 :
-
amplifier common-mode input impedance
- Z DM :
-
amplifier differential-mode input impedance
- Z e1 ,Z e2 ,Z r :
-
skin-electrode impedances
- Z s :
-
isolation impedance; barrier resistance and capacitance,R s ‖C s
- Z t ,Z t1 ,Z t2 :
-
internal body impedances between el electrode
- ΔZ e :
-
skin-electrode impedance imbalance,Z e1 -Z e2
References
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Wood, D.E., Ewins, D.J. & Balachandran, W. Comparative analysis of power-line interference between two- or three-electrode biopotential amplifiers. Med. Biol. Eng. Comput. 33, 63–68 (1995). https://doi.org/10.1007/BF02522948
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DOI: https://doi.org/10.1007/BF02522948