Signal amplification and conditioning. Reduction of noise, страница 2

Since the impedance of these elements depends upon the frequency of the signal the amplification depends upon the frequency. The amplifier can be connected to the transducer in two ways. It can be directly coupled (DC coupled), Fig.8.3a, or it can be coupled using a series capacitor C_c (AC coupled), Fig.8.3b. With DC coupling all of the signals from the transducer are presented to the amplifier whereas with AC coupling only the alternating signals appear at the input because C_c has infinite impedance for DCsignals. The amplifying stages in an amplifier can also be connected directly together, DC coupled, or with a coupling capacitor, AC coupled. The frequency dependence of the amplification of a DC and an AC coupled amplifier is shown in Fig.8.4.

(a)                                                                         (b)

Fig.8.4.The frequency dependence of the amplification for: (a) a DC coupled amplifier, and (b) an AC coupled amplifier

The gain of the AC coupled amplifier is zero at low frequencies because of the infinite impedance of C_c. The amplification of both types of amplifier decreases at high frequency because of stray capacitance, C_s, which 'shunts' the signal. The operating frequency range of an amplifier is described by the upper and lower cut-off frequencies f_u and f₁. These are the frequencies at which the amplification is 0.707 times that in the plateau region. The bandwidth Вof an amplifier is the frequency range between f_u and f₁, i.e. В =f_u –f₁. The amplification in the normal operating range of an amplifier is indicated as A_op.

Biomeasurement systems must be capable of reproducing pulses as well as DC and alternating signals. Good reproduction of a pulse requires DC coupling and an infinite bandwidth. In many cases it is necessary to employ AC coupling (to block unwanted changes in DC level). The coupling capacitor C_c introduces a time constant τ' into the measurement system. The value of the time constant, τ' = R_inC_c, should be large compared to the duration of the pulse. If τ' is about the same as the pulse duration the output voltage will be distorted.

Spurious information is introduced into the signal by the transducer, through the connecting leads and by the amplifier is called noise. There are basically two types of noise:

-  The low frequency noise which is caused by the pick-up of low frequency and mains (50 Hz) voltages in the biosystem and connecting leads,

-  The high frequency noise, which is generated in the transistors and resistors.

Since many of the signals of biological origin are very small it is necessary to reduce the noise to a minimum.

Reduction of noise

Pick-up can be avoided by placing the biosystem and the signal conditioner in earthed metal containers. This is called shielding. Shielded connecting leads should also be used. High frequency noise is due to the random fluctuation of the flow of electrons. High frequency noise can be eliminated by purposely introducing a shunt capacitor C_s (Fig.8.3) into the measurement system. The effect of a shunt capacitor on a noisy waveform is shown in Fig. 8.5.

Fig.8.5. (a) A noisy waveform. (b) Elimination of noise using a shunt capacitor.

(c) A balanced amplifier

Low frequency noise can be eliminated, by using a balanced amplifier. This type of amplifier, which is widely used in biomeasurement systems, is shown in Fig.8.5c. It consists of an amplifier with two live inputs a and b instead of one live and one earthed input as in the amplifier shown in Fig.8.3. The output V_out of the amplifier is given by V_oul = А_V(V_a - V_b)where A_V is the amplification. If the centre of the bio-system is fixed at earth potential (0V) by a third electrode the biopotential Е_Bwill cause the potential on the upper electrode to rise to E_B/2 and that on the lower electrode to fall to -E_B/2. The noise potential E_N picked-up by the leads is the same for each input and adds to the biosignal. Thus V_a = Е_B/2 + E_N and V_b = -Е_ъ/2 + E_N so that V_out = A_VE_B. The noise signal has been eliminated.