Abstract:
The invention provides an electric potential sensor, comprising at least one detection electrode arranged for capacitive coupling with a sample under test and for generating a measurement signal, and a sensor amplifier adapted to receive the measurement signal as input and to supply an amplified detection signal as output. Input impedance enhancing means provide a high input impedance to the sensor amplifier for increasing the sensitivity of the electrode to reduced electric potentials, and feedback means apply a coherent feedback signal to the input of the sensor amplifier for enhancing the signal to noise ratio of the sensor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention concerns electric potential sensors for use for the measurement of potentials in a variety of applications, including for example the fields of medical applications and microscope applications, such as microscopic imaging and spectrum analysis, as well as nuclear magnetic resonance (NMR) applications, such as NMR imaging and spectroscopy. 
       BACKGROUND TO THE INVENTION 
       [0002]    In order to create a sensitive electrodynamic measuring device, it is customary to provide a high input impedance and thereby reduce the power of the input signal required to operate the device. However, electronic circuits with a very high input impedance tend to be unstable, and so practical devices are usually a compromise between achieving the necessary degree of sensitivity, providing the desired input impedance and ensuring an acceptable degree of stability. 
         [0003]    In International Patent Application No. WO 03/048789, an electrodynamic sensor is disclosed in which a number of different circuit techniques are combined to achieve several orders of magnitude improvement in sensitivity, by comparison with previously known electrodynamic sensors, whilst still maintaining sufficient stability to permit a relatively unskilled operator to make measurements in everyday conditions. According to this earlier application, an electrodynamic sensor is provided comprising a high input impedance electrometer, which is adapted to measure small electrical potentials originating from an object under test and which employs at least one input probe having no direct electrical contact with the object. The circuit arrangement of the electrometer of this invention comprises an amplifier, which includes a combination of ancillary circuits providing feedback from the output of the amplifier and arranged cumulatively to increase the sensitivity of the electrometer to the small electrical potentials whilst not perturbing the electrical field associated therewith, the ancillary circuits serving to provide at least two of: guarding, bootstrapping, neutralisation, supply rail drift correction, supply modulation and offset correction for said sensor. 
         [0004]    Whilst these features assist in providing a sensor with high input impedance and a relatively stable operation, nevertheless, in situations where there may be weak capacitive coupling to, or a signal of small amplitude generated by, a source or sample under test, noise problems may still remain and may inhibit or prevent accurate signal measurement. This is particularly the case in certain medical and microscopic applications in which there is only a weak capacitive coupling and yet highly accurate signal measurement is essential, for example in a remote off-body mode of sensing in which the or each probe has no physical contact with the human body and typically the weak capacitive coupling would be &lt;1 pF. 
         [0005]    More particularly, in applications where there is a weak coupling between a sample under test and the sensor electrode, the capacitive coupling to the sample may be comparable with or much smaller than the input capacitance of the sensor. In this case, the measurement signal received by the sensor is attenuated by the capacitive potential divider formed by the coupling capacitance and the input capacitance and may be difficult to capture. 
         [0006]    Furthermore, the use of the output signal from the amplifier as the feedback signal has the disadvantage that such a signal is a broadband signal, which may have a poor signal to noise ratio. Hence, the noise is then fed back to the amplifier input with the feedback signal, causing further degradation of the signal to noise ratio. 
         [0007]    There is thus a significant need for an electric potential sensor in which the possibility for accurate signal measurement is enhanced in cases of weak capacitive coupling to a sample under test. 
         [0008]    Such a need is especially pronounced in cases where accuracy of signal measurement is critical. 
         [0009]    There is also a significant need for an electric potential sensor in which the signal to noise ratio is substantially improved. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention seeks to overcome the problems described above and to provide a novel electric potential sensor, which is capable of highly accurate signal measurement. 
         [0011]    The present invention, at least in the preferred embodiments described below, also seeks to provide an electric potential sensor in which the signal to noise ratio is significantly enhanced. 
         [0012]    The present invention further seeks to provide various techniques, and combinations of techniques, for enhancing the signal to noise ratio in an electric potential sensor. 
         [0013]    More especially, at least in the preferred embodiments described below, the present invention seeks to provide various techniques for improvement of the signal to noise ratio in an electric potential sensor using a coherent narrowband feedback signal. 
         [0014]    According to the present invention, there is provided an electric potential sensor comprising:
       at least one detection electrode arranged for capacitive coupling with a sample under test and for generating a measurement signal;   a sensor amplifier adapted to receive the measurement signal as input and to supply an amplified detection signal as output;   input impedance enhancing means for providing a high input impedance to the sensor amplifier for increasing the sensitivity of the electrode to reduced electric potentials; and   feedback means for applying a coherent feedback signal to the input of the sensor amplifier for enhancing the signal to noise ratio of the sensor.       
 
         [0019]    The present invention is thus concerned with improving the signal to noise ratio by applying a coherent feedback signal. 
         [0020]    In contrast with the usual practice for electrodynamic sensors, the feedback signal is not a broadband signal derived directly from the output of the sensor but is a coherent signal made available for feedback, and this significantly improves the signal to noise ratio. 
         [0021]    According to one possibility, the feedback signal comprises the simplest case of a single frequency and the feedback means are arranged such that the input impedance of the sensor is only enhanced at the exact frequency and phase of the feedback signal. In other words, the sensor is arranged differentially to amplify the measurement signal thus increasing the signal to noise ratio. In this case, the sensor becomes tuned to the feedback signal frequency, and rejects all other frequencies due to the lower sensitivity of the sensor in the absence of an effective feedback signal at other frequencies. 
         [0022]    Advantageously, the coherent feedback signal may be used to provide bootstrapping, guarding and neutralisation, as desired. 
         [0023]    The invention is particularly applicable in situations where a periodic signal is to be detected from a sample for generating a measurement signal for supply to the amplifier for amplification and output. 
         [0024]    In one such embodiment of the invention, the electric potential sensor comprises an external source for providing a drive signal for exciting the sample being measured, and the coherent feedback signal is derived from this external source. The use of such an external source of excitation is a common situation in analysis applications, such as microscopic imaging of dielectric properties of materials. In this instance, the excitation signal from the external source of excitation may be suitably attenuated to provide a reference signal for use as the feedback signal. 
         [0025]    In another such embodiment of the invention, the sample being measured may be self-exciting, in which case no external reference signal is available. An example of such a self-exciting sample might be an electronic circuit undergoing self-oscillation. In this case, a phase locked loop arrangement may be provided for deriving the coherent feedback signal from the output of the sensor amplifier. A considerable improvement in the overall signal to noise ratio is still possible in this instance, because of the restricted bandwidth in which the phase locked loop operates. 
         [0026]    The invention is also applicable in situations where the sensor is designed to drive or excite a sample being measured for generating a measurement signal for supply to the amplifier for amplification and output, and where it is desirable to eliminate charging of the sample and maintain a minimum signal on the sample. An example of this would be in microscopic applications in which a large electric field could damage or destroy a small semi-conducting device, or the surface of a sample, being measured. 
         [0027]    Such a sensor may be described as a zero voltage mode sensor, and the sensor in this instance advantageously comprises an external source for providing a drive signal for exciting the sample being measured, a feedback loop from the output of the sensor amplifier to the input, and a voltage summer arranged in the feedback loop so that both the fedback detection signal and the excitation signal from the external source of excitation are fed to the voltage summer for supply to the sample. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
           [0029]      FIG. 1  is a circuit diagram of an electrodynamic sensor according to the prior art; 
           [0030]      FIGS. 2 and 2   a  are block diagrams of a first embodiment of electrodynamic sensor according to the present invention, employing neutralisation, and a modification thereof; 
           [0031]      FIG. 3  is a circuit diagram of a modification to a detail of the  FIG. 2  circuit, employing guarding and bootstrapping in addition to neutralisation; 
           [0032]      FIG. 4  is a block diagram of a second embodiment of electrodynamic sensor according to the present invention, employing neutralisation and a phase lockedloop; and 
           [0033]      FIG. 5  is a circuit diagram of a third embodiment of electrodynamic sensor according to the present invention, comprising a zero voltage mode sensor. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Prior Art 
       [0034]    Referring to  FIG. 1 , an electrodynamic sensor as disclosed in International Patent Application No. WO 03/048789 will first be described. 
         [0035]    As shown in  FIG. 1 , an eletrodynamic sensor  10  according International Patent Application number WO 03/048789 comprises a detection electrode  12  connected to the non-inverting input of a sensor amplifier  14 . In use, the detection electrode  12  supplies a measurement signal as input to the sensor amplifier  14 , which supplies an amplified detection signal as output. 
         [0036]    The detection electrode  12  includes an electrode disc  16  mounted on a conductive stem  18 , the electrode disc  16  comprising a surface oxide layer  20  on a substrate  22 . The sensor amplifier  14  has a fixed input resistance  24 , provided by two resistors  26 ,  28 , connected between the electrode  12  and the non-inverting input of the amplifier  14 , to provide a steady input bias current to the amplifier  14 . In practice, the input resistor  24  will generally have a high resistance of the order of 100 GΩ or greater. The sensor amplifier  14  also has a guard  30  physically surrounding the input circuitry including the electrode  12  and the resistor  26  and providing a shield driven by the output of the amplifier  14 . Stray capacitance is thus alleviated by means of this positive feedback technique by maintaining the same potential on the guard or shield  30  as on the input detection electrode  12 . 
         [0037]    In addition to the guard  30 , further circuit components are provided for bootstrapping, comprising a capacitor  32  arranged to apply the output voltage of the amplifier  14  to the mid point of the resistance  24 , which occurs between the two resistors  26 ,  28 , as well as for neutralisation, comprising another feedback arrangement including a capacitor  34  connected to the non-inverting terminal of the amplifier  14 . Additional resistors  36 ,  38  and a potentiometer  40  are provided to set the neutralisation to a desired level, as described in International Patent Application number WO 03/048789. 
       Driven Neutralisation—Reference Source 
       [0038]    Referring now to  FIG. 2 , a first embodiment of the invention will be described. Although neutralisation may be used to increase dramatically the input impedance of the electrodynamic sensor of  FIG. 1 , as described in International Patent Application number WO 03/048789, the signal to noise ratio is not enhanced by this technique since the noise present at the sensor output is fed back to the sensor input. 
         [0039]    In many situations where neutralisation is important, for example in microscope applications, the sample is excited by an externally applied signal. In these cases, a reference signal is available from the oscillator providing the drive signal for the sample, and this reference may be used to provide a neutralisation signal for the sensor as shown in  FIG. 2 . 
         [0040]    The electrodynamic sensor of  FIG. 2  includes some of the same elements as the  FIG. 1  sensor. Accordingly, like parts are designated by the same reference numerals and will not be described further. As shown, the detection electrode  12  of the sensor is represented by the input V in  and is coupled to a sample  42  being measured by way of a capacitor  44  representing the coupling capacitance to the sample. The sample is driven or excited by a reference oscillator  46 , and hence an AC measurement signal is produced at the sensor input V in , which is connected to the non-inverting input of the operational amplifier  14 . As a result, the amplifier  14  supplies an amplified AC detection signal at its output V out1 . The output V out1  is connected by way of resistors  48 ,  50  to ground and by way of a root mean square (RMS) converter circuit  52  and a low pass filter  54  to an attenuator  56 , from which a reference signal V n  is taken for feedback. These circuits serve for level control of the reference signal V n  which is fed back by way of capacitor  34  to the non-inverting input of the amplifier  14  to provide neutralisation. The gain of this positive feedback loop may be so controlled such that maximum neutralisation is achieved within the limit of stable operation. 
         [0041]    The embodiment of  FIG. 2  thus includes an automatic gain control (AGC) feedback loop to control the amplitude of the neutralisation signal V n  in order to prevent oscillation, with the amplitude of the feedback signal being controlled by the amplitude of the sensor output signal at a control output V out2 . The feedback loop here includes the RMS converter circuit  52 , the low pass filter  54  and the attenuator  56 . The control output of the sensor V out2  is taken from the AGC feedback line between the low pass filter  54  and the attenuator  56  and, since the reference and measurement signal frequencies are the same, the AGC control voltage V out2  will be a quasi-DC control signal providing information about the sample  42  derived from the amplitude of the AC measurement signal supplied to the amplifier  14 . 
         [0042]    This arrangement has the effect of enhancing significantly the input impedance of the sensor, but only at the frequency of the applied signal and only for components of constant phase, hence providing an improved signal to noise ratio. 
         [0043]    An extension of the technique described with reference to  FIG. 2  allows the frequency of the reference oscillator  46  to be swept under the control of a sweep control arrangement  58  shown in  FIG. 2   a . In practice, the sweep control arrangement  58  for sweeping the frequency of the oscillator may be either a digital control circuit connected to the oscillator  46  (as shown) for controlling the frequency digitally via a suitable interface or an arrangement (not shown) for frequency modulating the source via an appropriate FM input or a circuit (not shown) using an analogue quasi-DC level derived independently to set the operating frequency via a swept input. As already stated, since the reference and signal frequencies are the same, the AGC control voltage at the control output V out2  will be a quasi-DC signal. This will vary with the amplitude of the measured signal and as a function of the frequency, and so may be used to provide a spectral plot as the frequency of the reference oscillator is swept. This arrangement resembles a spectrum analyser in operation. 
       Fully Driven Sensor—Reference Source 
       [0044]    The embodiment of  FIG. 2 , in which the external reference source  46  is used to provide a neutralisation signal V n , may be extended also to provide signals V g , V b , suitable respectively for guarding and bootstrapping, with commensurate additional improvements in the signal to noise ratio. This variation is shown in  FIG. 3 , in which the reference signal from the oscillator  46  and attenuator  56  is fed back as a guard signal V g  to the shield or guard  30  surrounding the electrode  12 . In addition, the reference signal is fed back by way of the capacitor  32  and two resistors  26 ,  28  to the non-inverting input of the amplifier  14  as a bootstrapping signal V b . The individual relative signal levels required for each feedback technique are obtained from a set of independent potential dividers  60 , as shown in  FIG. 3 , driven by the external reference source or oscillator  46 . The overall level of the feedback signals is set globally based on the amplitude of the output signal V out2  through the use of the AGC loop as described in relation to  FIG. 2 . 
       Driven Neutralisation—No Reference 
       [0045]    Another variation on the embodiment of  FIGS. 2 and 3  employs a phase lock loop oscillator to derive the drive and neutralisation signals, as shown in  FIG. 4 . In this case, the sample is self-exciting, and thus the drive oscillator  46  is not present and the local reference signal from the drive oscillator  46  is not available. However, an enhanced signal to noise ratio may still be achieved by the introduction of an oscillator into the sensor which is phase locked to the detection signal output by the amplifier  14 . 
         [0046]    The embodiment of  FIG. 4  comprises a phase locked loop oscillator  61  which is connected to the output V out1  of the amplifier  14  and which comprises a frequency multiplier  62  arranged to receive the signal V out1  and an output signal from an oscillator  64 . An amplifier  66  feeds back the output from the frequency multiplier  62  to the oscillator  64  in order to frequency modulate the signal output by the oscillator  64 . As a result, the amplified detection signal is mixed with the output from the oscillator  64  to form the phase locked loop. 
         [0047]    In operation, the oscillator  64  sweeps in frequency until a beat is found with the measurement signal, at which point the sweep will be frozen. Because there is no constant phase relationship between the sample and the oscillator  64 , the beat will take the form of a low frequency waveform, which will become DC when the phase lock is achieved. This DC signal is output as the overall output V out2  of the sensor and is also used as an AGC signal whose amplitude controls the amplitude of the feedback signal used for neutralisation. For this purpose, the DC signal is fed back by way of the attenuator  56  and the capacitor  34  to the non-inverting input of the amplifier  14  to provide the neutralisation signal. In use, the phase locked loop oscillator  61  will sweep in operation until a measurement signal is acquired and will then feed back a phase locked signal to enhance the input impedance of the sensor at this frequency only. This serves to increase the input impedance at the signal frequency without broadband noise being fed back to the amplifier input. 
         [0048]    In a first variation of the  FIG. 4  embodiment, guarding and bootstrap signals may also be derived from the output of the phase locked loop oscillator  61 , using similar circuit features to those shown in  FIG. 3 , with a commensurate improvement in the signal to noise ratio. 
         [0049]    In a further variation of the  FIG. 4  embodiment, the oscillator  64  may in addition supply a drive output for exciting the sample, as in the case of the  FIG. 2  embodiment. 
       Zero Voltage Mode Sensor 
       [0050]    Turning now to  FIG. 5 , a further embodiment of the invention will be described for use in the cases where for example charging of the sample needs to be eliminated. One example of this is where problems caused by relative motion of the sample and the sensor need to be minimised. Another important example is in microscopic applications where a large electric field may damage the surface of the sample. Again, this embodiment employs some of the same circuit features as the previous embodiments and like parts will be designated by the same reference numerals and will not be described further. 
         [0051]    According to this embodiment, the sample  42  is excited by an oscillator  70  by way of a voltage summer  72 , which also receives the detection signal fed back from the output V out1  of the amplifier  14 . By closing the feedback loop with the voltage summer  72 , it is possible to ensure that only a small error signal appears on the sample  42 . The detection signal at the output V out1  is fed back to the voltage summer  72  by way of a further amplifier  74 , which is used to set the loop gain and time constant for optimum operation. The signal to noise is thus enhanced by removing artefacts which may be caused by the presence of large signals on the sample. 
         [0052]    In this embodiment, the oscillator  70  is again used to provide a reference signal for feedback, as shown in  FIG. 2 . For simplicity, only the features required for neutralisation have been shown in  FIG. 5  but, of course, guarding and bootstrapping may also be employed as described with reference to  FIG. 3 . In all these cases, the guarding, bootstrapping and neutralisation signals are derived from the coherent source  70 , and this enhances the signal to noise ratio as described previously. 
         [0053]    Use of the  FIG. 5  embodiment is restricted to the case where the sample  42  is excited with an external signal.