A SIGNAL-SHAPING NETWORK

Abstract: A signal-shaping network comprising a differential amplifier (G) having an output and an inverting (A) and a non-inverting (B) input between which a signal source (S) is connected. When a differential amplifier is used particularly for the measuring of very weak biosignals, interference signals may easily make the measuring completely impossible. In the signal-shaping network according to the invention, interferences have been eliminated in such a way that an integrating feedback circuit (1a; 1b) is connected between the output and one input of the differential amplifier (G) and/or a derivative feedback circuit (2a) is connected between the output and the other input of the differential amplifier. In this way an adaptive differential amplifier is achieved which attenuates slow interference signals and/or rapid interference signals.

Description: A signal-shaping network The present invention relates to a signal- shaping network comprising a differential amplifier having an output and an inverting and a non-inverting input between which a signal source is connected. Figure 1 illustrates the principle of a typical differential amplifier of the above kind, applied e.g. in the measuring of bioelectric phenomena. The signal between input terminals A and B is obtained amplified from the output of- an amplifier G, i.e. an output signal y = Gs + c, wherein G = an amplifying coeffi¬ cient and c represents an error term caused by inter¬ ferences. The value of this factor depends on the mag- nitude of a common mode interference signal acting at the input terminals of the differential amplifier, the CMRR (common mode rejection ratio) factor of the am * plifier / the impedance of the signal. source (e.g. the moistness of skin), and the ratio between the resis- tances between the input terminals and the earth, i.e. how close their values are to each other. In practice, the interference signals acting at the input terminals of the amplifier are unequal in most cases, whereby not only the desired signal but also the difference signal of this interference will be amplified G-fold at the output of the amplifier. This usually results in a forceful interference in the output signal y, G being typically 100...1000. Biosignal amplifiers are usually intended for ac signals, so that in principle it would be possible to attenuate slower (interference) potential changes by connecting series capacitors to both input ter¬ minals. This, however, would deteriorate the matching between the impedances of the input terminals and, on the other hand, a strong interference might easily overdrive the amplifier into an inoperative state for a longer period of time due to the charging of the ca¬ pacitors. For this reason differential amplifiers are nearly without exception dc-connected, whereby they, however, are very sensitive to disturbances the energy of which is concentrated at the lower end of the fre¬ quency spectrum (e.g. movements and bumps) . The easiest way of attenuating rapid, periodical dis¬ turbances is to connect low pass filters having the marginal frequency clearly above the net interference frequency (50 Hz) e.g. directly to the input ter¬ minals. The attenuation of common mode interferences can be improved to some extent (approx. 10...20 dS) in a known manner shown in Figure 2 by connecting an interference applied from the input terminals of the differential amplifier (a common mode interference) amplified and with a phase shift of -180° back as active earth to which an earth connection electrode is connected. However, prior signal-shaping .networks based on a differential amplifier, described in Figures 1 and 2, are practically unsuitable for measurings to be carried out in conditions liable to disturbances, such as for the real-time measurement of the profile of a muscle tonus (EMG signal) occurring during free move¬ ment performance. Thus the object of " the present in¬ vention is to provide a signal-shaping network suited for purposes mentioned above, giving extremely reli- able and clear information on the magnitude to be measured without that the interpretation of the measuring results would be affected by interference signals. The above object is achieved by means of a signal-shaping network according to the invention, which is characterized in that an integrating feedback circuit is connected between the output and one input of the differential amplifier and/or a derivative feedback circuit is connected between the output and the other input of the differential amplifier. The integrating feedback circuit is preferably connected to the non-inverting input of the differential am¬ plifier with a phase shift of -180° or to the invert¬ ing input thereof with a phase shift of 0°. Corre- spondingly, the derivative feedback circuit is prefer¬ ably connected to the non-inverting input of the differential amplifier with a phase shift of -180° or to the inverting input thereof with a phase shift of 0°. By means of the feedbacks of the above kind, an adaptive differential amplifier is achieved which effects an adaptive attenuation of an interference signal at low and/or high frequencies, thus enabling a stable and substantially interference-free measuring of even very weak biosignals. In the following the signal-shaping network ac¬ cording to the invention will be described in more de¬ tail with reference to the attached drawing, wherein Figure 1 illustrates the connecting principle of a conventional differential amplifier of the prior art, Figure 2 illustrates the connecting principle of another conventional differential amplifier of the prior art, Figure 3a illustrates the connecting principle of a first differential amplifier according to the in¬ vention , Figure 3b illustrates the connecting principle of a second differential amplifier according to the invention, Figure 4 illustrates in more detail the realiz¬ ation of the connecting principle of Figure 3a, Figure 5 illustrates the connecting principle of a third dif erential amplifier according to the in- vention, and Figure 6 illustrates in more detail the realiz¬ ation of the connecting principle of the third embodi¬ ment of the invention shown in Figure 5. Figures 3a and 3b show a block diagram of an adaptive differential amplifier according to the in¬ vention particularly suited for a stable and sub¬ stantially interference-free measuring of very weak (1...100/uV) biosignals. In the amplifier of Figures 3a and 3b, an integral signal z is counted from an output signal y, whereby the integral signal obtained is directly proportional to the amplitude and duration of an input signal S. In the embodiment of Figure 3a,. the integral signal z is applied back to the input terminal B with a phase shift of -180° (-l/Tfydt.) • & the embodiment of Figure 3b, the integral signal (l/'Z ydt) is applied back to the input terminal A with a phase shift of 0°. Figure 4 shows a more detailed circuit diagram of the realization of the block diagram- of Figure 3a. Herein an integrating feedback la is effected by means of a conventional inverting integrator connection formed by an operational amplifier OP and a resis¬ tance R and a capacitor C. In this connection, the re¬ sistance R is connected to the output of the differen- tial amplifier G for receiving an output signal y thereof, and one terminal of the resistance R is con¬ nected to the negative terminal of the operational am¬ plifier OP, the positive terminal thereof being earthed. The capacitor C is arranged as a feedback component between the negative input terminal and the output terminal of the operational amplifier OP. An output signal z of the operational amplifier is applied through a resistance Rg to a non-inverting in¬ put B of the differential amplifier G. A resistance Rg equal in magnitude is connected between one input A of the differential amplifier and an earth connection. By giving the integration time constant (T = RC) a suit¬ able value, it is possible to determine the range of the rate of change of signals (inter erences) such that signals slower than . these start to attenuate rapidly. In other words, the slower the interference, the more efficient the attenuation. On the other hand, an increase in the impedance of the signal source automatically shifts the border of the attenuation range towards higher rates of signal change and vice versa, i.e. the amplifier adapts itself to such changes. This kind of adaptive differential amplifier also retains the good CMRR properties thereof, for the input terminal impedances remain resistive and nearly equal in magnitude. This is based on the fact that the output impedance of the operational amplifier OP is very small, so that the effect of the feedback loop on the input terminal impedance is practically insigni¬ ficant. In cases where the internal impedance of the input signal source S is very low, it is possible to connect a resistance RO in series with . the signal source S between the input terminals A and B, said resistance being chosen so that it is considerably lower than the resistance Rg of the input terminals but clearly higher than the internal impedance of the signal source S. One important field of application of an adapt¬ ive differential amplifier of the above type is a real-time measuring of the profile of a muscle tonus (EMG signal) during free movement performance. The re¬ gistration of these phenomena is necessary e.g. in the training of dif erent sports as well as in physical and rehabilitative treatments. Previously the measure- ment of such kinesiological phenomena has not been possible in any greater degree. Figure 5 shows a block diagram of the third em¬ bodiment of the adaptive differential amplifier accor¬ ding to the invention, in which the attenuation pro- perties increase with an increase in the rates of sig¬ nal change, so that the operation thereof is a mirror image of the embodiment shown in Figure 3a. In this embodiment, a derivative of the output signal y of the differential amplifier G is formed instead of the integral thereof, which derivative is summed to the non-inverting input B of the differential amplifier G with a phase shift of -180° (-C* dy/dt). According to the principle shown in Figure 3b, this derivative feedback 2a could also be connected between the ou put of the differential amplifier and the inverting input A thereof, whereby it should have a phase shift of 0°. The structure of the embodiment of the inven¬ tion shown in Figure 5 is illustrated in more detail in Figure 6, wherein the derivative feedback circuit 2a is effected by means of the operational amplifier OP and the resistance R and the capacitor C. One terminal of the capacitor C is connected to the output of the differential amplifier G, the other terminal thereof being connected both to the resistance R and the inverting input of the operational amplifier OP. The non-inverting input of the operational amplifier OP is connected to the earth. The resistance R is connected across the operational amplifier OP to form a feedback resistance, whereby one terminal thereof is connected to the output of the operational amplifier. This output is through the resistance Rg connected to the non-inverting input terminal B of the differential amplifier G. By means of the adaptive differential amplifier according to the invention it is possible to effect the attenuation of either slow interferences (the em¬ bodiments of Figures 3a and 3b) or rapid interferences (the embodiment of Figure 5), or the attenuation of both slow and rapid changes by combining the embodi- ments of Figures 3 and 5, whereby the differential amplifier comprises both a derivative and an integrat¬ ing feedback. The invention has been described in detail merely by means of some specific embodiments and it is to be understood that the structure of the feedback circuit essential to the invention, either derivative or integrating, may differ considerably from the above-described simplest realization, thereof. The feedback circuit of the differential amplifier accord- ing to the invention may consist of nearly any known connection effecting the derivation or integration.

Claims: Claims : 1. A signal-shaping network comprising a diffe¬ rential amplifier (G) having an output and an invert- ing (A) and a non-inverting (B) input between which a signal source (S) is connected, c h a r a c t e r¬ i z e d in that an integrating feedback circuit (la; lb) is connected between the output and one input of the differential amplifier (G) and/or a derivative feedback circuit (2a) is connected between the output and the other input of the differential amplifier. 2. A signal-shaping network according to claim 1, c h a r a c t e r i z e d in that the integrating feedback circuit is connected to the non-inverting input (B) of the differential amplifier with a phase shift (la) of -180° or to the inverting input (A) thereof with a phase shift of 0°. 3. A signal-shaping network according to claim 1, c h a r a c t e r i z e d in that the derivative feedback circuit is connected to the non-inverting input (B) of the differential amplifier with a phase shift (2a) of -180° or to the inverting input (A) thereof with a phase shift of 0°. 4. A signal-shaping network according to any of the preceding claims, c h a r a c t e r i z e d in that a resistance (Rg) is connected between the feed¬ back circuit (la; lb; 2a) and the input (A; B) of the differential amplifier, a resistance (Rg) equal in magnitude being connected between the other input thereof and the earth/another feedback circuit.

Inventors: ESKELINEN PAAVO, ESKELINEN, PAAVO
Applicants: NORAXON OY
Application year: 1987
Main classification: H03F1
IPC codes: A61B5, H03F1, H03F3, H03H11