3 Sources of Noise in Biomedical Measurement Systems

Biomedical measurements involve low-level signals which are susceptible to noise interference.

We have 3 common sources of noise in biomedical measurements which are:

1. Improper grounding
2. Electrostatic Coupling to AC signals
3. Electromagnetic coupling to AC signals

Improper Grounding

Grounding means a low impedance metallic connection to a properly designed ground grid, located in the earth. Placing more than one ground on a signal circuit produces a ground loop which may generate so much noise that it may completely obscure the useful signal.

Stable impedance is needed to attain effective shielding of low-level circuits (i.e. biomedical measurement circuits) to provide a stable reference for making voltage measurements and to establish a solid base for the rejection of unwanted common-mode signals.

Usually we have two grounding systems i.e. a systems and a signal ground. All low-level biomedical measurements and recording systems should be provided with a stable system ground to ensure that electronic enclosures and the external body operating in an electromagnetic environment are maintained at zero potential.

Largely, third copper conductors in all electrical circuits, that is firmly tied to both electric power ground (the building ground & water ground) will provide satisfactory system ground, in contrast; in a signal ground, it is essential to ensure a low noise signal reference to the ground. This ground should be a low-impedance path to wet earth to minimize the introduction of spurious voltages into the signal circuitry and the signal circuit should be grounded at one point only.

Two separate grounds are hardly at the same absolute voltage. If we connect more than one ground to the same circuit, an unwanted current will flow in the ground loop thus created. This current combines itself with useful signals.

In addition, there is a second ground loop through the signal cable-shield from the signal source to the amplifier. The current in the shield is coupled to the signal pair through the distributed capacitance in the signal cable. This current then flows through the output impedance of the signal source and back to the ground, adding a second source of noise to the useful signal. Each one of these ground loops generates a noise signal that is larger than a typical millivolt useful signal.

These ground loops are eliminated by the floating lower input terminal to the amplifier.

The amplifier enclosure remains solidly grounded to the second ground but this will not create a ground loop; since the amplifier enclosure is insulated from the signal circuit. The ground loop through the signal cable is removed by grounding the shield only at the signal source which is the proper configuration for minimum noise pick-up.

Electrostatic Coupling to AC signals

The distributed capacitance between the signal conductors and from the signal conductors to the ground provides a low impedance AC path resulting in a signal contamination from external sources like transformers and power lines.

Low-level signals like those in biomedical measurements are sensitive to external contamination especially in the case of high source impedance.

To avoid pick-up from electrostatic fields, low-level signal conductors are covered by an effective shield. This is normally a woven metal braid around the signal pair, which is placed under an outside layer of insulation. A more effective shielding is provided by a special type signal cable, which has lapped foil shields, plus a low resistance drain wire instead of the conventional braided wire shield.

Electromagnetic Coupling to AC signals

Alternating magnetic flux from the adjacent power line wires induces voltages in the signal loop which is proportional to the rate of change of the disturbing current, the magnitude of the disturbing current and the areas enclosed by the signal loop. This induced voltage is inversely proportional to the distance from the disturbing wire to the signal circuit. Unequal distances of the two signal carrying conductors from the disturbing current wire result in unequal mutual inductances which causes the magnetic field to produce a noise voltage across the amplifier input terminals.

To protect a cable against external electromagnetic interferences, have the circuit conductors twisted closely together to cancel electrically the effect of an external magnetic field. The shorter the lay of the twist, the greater the noise rejection, in addition electromagnetic interferences are prevented by shielding the cables and proper grounding which provides a balanced pair with satisfactory noise rejection characteristics.