CHAPTER 3 ANALOG SIGNAL CONDITIONING

Transcription

1 19 CHAPTER 3 ANALOG SIGNAL CONDITIONING

2 CHAPTER 3 ANALOG SIGNAL CONDITIONING 3.1 INTRODUCTION Amplifiers and Filters are the essential signal processing blocks of any analog signal conditioner. The characteristics that are required for an amplifier depends upon the specific application. These are specified in terms of the following most important parameters: i) Input Impedance ii) iii) iv) Output Impedance Gain and Frequency response Noise Level Low voltage amplitude and high output impedance are the characteristics of all bioelectric signals. Before digitisation of these signals can be attempted,certain amount of amplification and filtering are to be done. Since most of the bioelectric signals are bipolar signals, the common mode rejection ratio of the amplifier must be made as high as possible. Bandwidth requirements of most of the bioelectric signals are very low and vary from a few hertz to a few kilohertz. The basic characteristics of the three types of bioelectric signals under discussion are listed in Table 3.1 [2]. TABLE 3.1 CHARACTERISTICS OF BIOELECTRIC SIGNALS SIGNAL VOLTAGE RANGE IN pv OUTPUT IMPEDANCE IN Mn BW in Hz ECG 10 to to to 85 EEG 10 to to 10 DC to 100 EMG 20 to to 5 10 to

3 21 Based on the signal characteristics the specifications of Amplifiers and filters can be arrived at. The bioelectric signals are always associated with muscle artefacts and electromagnetic interference. A band-pass and a band-rejection (notch) characteristics are incorporated in the signal conditioner for eliminating the unwanted noise and mains disturbances. A band-pass filter is usually realised by cascading a low-pass filter and a high-pass filtei [7]. The signal conditioner selected for the present requirement consists of an Instrumentation amplifier, a double low-pass filter a high-pass filter and a notch-filter. The recommendations promulgated by the council on Physical Medicine of the American Medical Association (AMA) and the American Heart Association (AHA) for the bioelectric potential recorders are reproduced in Table INSTRUMENTATION AMPLIFIER The schematic diagram of an Instrumentation amplifier is shown in Fig,3.1(a)[7]. The important features of the Instrumentation amplifiers are : i) selectable gain with high accuracy and linearity ii) iii) differential input capability with high CMRR high stability of gain with low temperature coefficient iv) low dc offset and drift errors and v) low output impedance The analysis of the circuit shown in Fig.3.1(a) results in the following equations.

4 22 TABLE 3.2.SPECIFICATIONS OF BIOELECTRIC POTENTIAL RECORDERS CHARCTERISTICS BIOELECTRIC SIGNALS ECG EEG EMG Input voltage Range 0 to 10 mv 0 to 200pV - Input Impedance a0.5 Mn >0.5 Mn s 0.5Mn,s25pF Amplifier Gain 5to20mm/mV lmm/lpv(max) lmm/5pv(max) Bandwidth 0.14to50Hz 1 to 65Hz 40to3000Hz CMRR 1000:1 1000:1 2000:1 Calibration Voltagp 1 mv 100 pv - Noise Level rslo pv <4 pv - Time constant(min) 3.2 s 0.3s 5ms Temperature range L0 Cto50 C 10 Cto50 C 10 Cto50 C Minimum paper speeds 25&50mm/s 15,30&60mm/s - Minimum sensitivity 5 mm/mv 1 mm/ pv 0.2 mm / p v

5 = ( 1 * VRi > el " ( R2/Rl ) e0 + e m l cm 23 = ( 1 + r2/ri ) e2 - ( r2/r1 ) el + ecm Out put is given by e5 = ( e3-e4 ) ( - R4/R3) = ( 1+2 R2/RP (ex - e2) (-R4/R3) Differential Gain = 65/6^-62 = ( 1+2 R2/R1 ) ( R4/R3) A general purpose Instrumentation amplifier capable of handling all the three bio-signals (ECG,EEG,EMG) has been designed with the following specifications using jua 308 operational amplifier ICs. i) Gain = 100 to 1000 ii) Input Impedance = 10 Mn iii) CMRR = 1000 iv) Bandwidth = 0.05 to 5000 Hz The transfer characteristics of the amplifier is shown in Fig.3.2(a). 3.3 BAND-PASS FILTER Configuration of the filter Band-pass filters can be realised either with a single operational amplifier module or by cascading a low-pass filter and a high-pass filter. In the latter method, the cut-off frequencies on either side of the pass band can be adjusted independently and the design process is also much simpler compared to the single operational amplifier circuit. Hence cascaded second-order Butterworth filters are chosen for the analog conditioner implementation.

6 Low-pass filter For the second-order Butterworth low-pass active filter circuit shown in Fig.3.1(b), the transfer function is given by [8]: Eo (s) _ 2/R1R2 (s) s^ + s/r^f l/ri^2 The normalised Butterworth (3.1) approximation for a second-order low-pass filter is given by : H (s) = (3.2) s2+ /2s + 1 The filter design is completed by comparing the coefficients and assuming a cut-off frequency of 1 rad/s. By changing the value of capacitors suitably the cut-off frequency of the circuit is adjusted to 200 rad/s (fc=100hz). By assuming a scaling factor the component values are adjusted to realise the circuit with convenient values of R's and C's. Detailed design procedure is given in Appendix-B. The frequency response of the designed low-pass filter with fc=100 Hz is shown in Fig.3.2(b). Two such low-pass filters are cascaded to obtain a higher slewrate using 747 dual operational amplifier ICs High Pass Filter A high-pass filter with a cut-off frequency of 0.1 Hz is designed using the capacitance to conductance and conductance to capacitance (CG : GC) transformation technique [9]. The high-pass filter Transfer function is converted into a low-pass Transfer function by replacing

7 25 's' by 1/s.Then the low-pass filter is designed and at the final step of design the capacitors are replaced by resistors and the resistors are replaced by capacitors to arrive at the high-pass filter configuration. The cut-off frequency of the high-pass filter is assumed as 0.1 Hz and type 741 operational amplifier IC is used for the circuit implementation. Detailed design steps are included in the Appendix-B. 3.4 NOTCH-FILTER The 50 Hz mains frequency interference is a major problem in all bioelectric potential recorders. To effectively suppress the same, a triple 'T' section notch-filter is employed. The circuit diagram of the notch filter is shown in Fig.3.1(c). The component values are chosen via the coefficient matching technique, as already discussed for low-pass filter design. The frequency reponse of the notch-filter is shown in Fig.3.2(c). The complete circuit diagram of the analog signal conditioner is shown in Fig.3.3. The higher cut-off frequency of the low-pass filter is varied by changing the capacitances (c*) connected in the circuit. The value of capacitances for the cut-off frequencies 50,100 and 5000 Hz are given in Fig CONCLUSION The analog signal conditioner has been fabricated on a single printed circuit board reducing the noise

8 26 disturbance to quite a low level. The performance of the signal conditioner was found to meet fully the requirements for the ECG and EMG signals. For the EEG signal however an additional single-ended amplifier stage with a gain of 10 was found to obtain more satisfactory results. Plate 3.1 shows tha analog conditioner module constructed on a single printed circuit board.

9 27 (a) Instrumentation Amplifier c Fig.3.1 ANALOG SIGNAL CONDITIONER.

10 28 y response of low-pass filter Gain in db o a> X) c n > output - 3 i i i K Frequency in Hz - 'i" (c) Frequency response of notch-filter Fig.3.2 PERFORMANCE CHARACTERISTICS OF ANALOG CONDITIONER

11 29 i U V l l 2/1. *c in Hz * \ u "* c 10 K 10 K J T - C = > o in -I H r 2 2.SK X * e x 01v*> - d l r C 0.1 /JF to 90K I0K 90K 64nF 100 I O v% INSTRUMENTATION AMPLIFIER GAIN : 100 TO 1000 LOW PASS FILTERS f c : 100 Hz JJA tt,3 K - j t~ HIGH PASS FILTER NOTCH FI1JER f, :S 0 H z ZH 10 - I RESISTOR VALUES IN OHMS Fig. 3.3 ANALOG SIGNAL CONDITIONER CIRCUIT t-o OUTPUT 10K 6AnF JJA7 41 drtooi jr ffo, P ~ JJA 30 8 ItOOK T > 22.6K, X, C r tot vrf M001 2K 100 K 100 K 20K 4 = } 20K 11.3 K t 10 K 10K 1/2JJA 747 < 1 io d N I

12 PLATE 3.1 ANALOG CONDITIONER