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Sunday, 9 December 2018

How does EMI affect the bio-medical sensors

EMI to biomedical sensors:

We know that the role of sensors in biomedical field is to measure the very weak bioelectric signals from the human body. These biomedical signals such as ECG, EEG, EMG etc are very weak signals in the order of millivolt or microvolt range. So the EMI effect to the biomedical sensor can be very serious because the EMI can superimpose on the bioelectric signals and can cause measurement errors. Most biomedical sensors are immune to EMI because of the typically very low signal levels produced by those sensors. There are three main transmission paths for EMI into a sensor system. They are

1. Penetration
2. Leakage
3. Conduction

a. Penetration 

Here the Electromagnetic wave cuts across the sensor and due to this RF current are produced in the sensor circuitry. This type of EMI tends to interfere with the reception of desired signals or that produces undesirable responses in the electronic system. This can affect the normal reading of sensors. The schematic representation is shown in figure.
Penetration EMI
b. Leakage

It occurs due to breakage in the shielding. Even though we use shields, the EMI will enter the sensor circuit through breaks in the shielding. So if the shielding is not perfect, even the shielded devices are not immune from EMI. So the leakage can produce radiated EMI, which is caused by induction without the contact with the conductor. EMI leakage between the mating surfaces at the joints and seams is a primary issue connected with the EMI shield designers.
Leakage EMI

c. Conduction 

Conduction EMI occurs when a signal induces current into the exposed cable. So EMI can travel through this cable and can cause conducted EMI. It mainly occurs due to the breaks in insulation layer of cables. In this type of interference, The EMI signal entered inside the shielded cabinet can cause RF currents which in turn lead to measurement imperfections. For low frequencies, EMI is caused due to conduction. Due to conduction EMI, the EM field around the conductor is no longer evenly circulated and causes skin effect. The conducted noise is further divided into differential noise and common mode noise. Differential mode noise flows in both directions along the line and neutral wires whereas common mode noise flows between one line and the ground. The conducted EMI can have an effect on the function of operational amplifiers even when the frequency is well above the working frequency range of amplifier. So advanced techniques such as computerized conducted EMI filter design are used widely to prevent its effect on biomedical sensors.
Conduction EMI

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