Patent Description:
Biopotentials are usually recorded using electrodes attached to the body, such as wet (gel) electrodes, or dry electrodes. The electrodes are used to measure biopotentials, which typically have a magnitude in the range 1µV to 10mV.

Active electrodes have been employed in biopotential acquisition systems, in which the electrodes are integrated with amplifiers for the suppression of interference picked up from cables. The active electrode based system is robust to cable motion artifacts and interferences, which makes it suitable for dry electrode applications.

An ideal active electrode comprises a passive electrode and a pre-amplifier that are integrated within the same package or board, which can be placed very close to the skin to extract the low-level biopotential signals. In this way, the signal path length between the electrode and the pre-amplifier is minimized, maintaining the highest possible input impedance of the amplifier and lowest possible noise pick-up from electromagnetic fields. Furthermore, the output of the active electrode forms a low-impedance node and the interference and motion artifacts obtained by cable movement and electromagnetic fields in the vicinity can both be reduced when compared to a conventional passive electrode interface.

<FIG> shows the basic active electrode system. A reference active electrode <NUM> comprises an amplifier <NUM>. Contact is made with the patient at a first contact <NUM> which provides an input to the system. A first signal active electrode <NUM> comprises an amplifier <NUM>. Contact is made with the patient at a second contact <NUM>. The gain of the reference active electrode amplifier is shown as A and the gain of the signal active electrode amplifier is shown as A+ΔA. Thus, there is a gain mismatch between the active electrodes. Note that for a digitally interfaced system, analogue to digital converter units may be implemented as part of the electrodes <NUM>,<NUM> or they may be implemented downstream from the electrodes.

Common mode interference is one of the major problems in such active electrode biopotential signal acquisition systems. For example, biopotential signals can be affected by interference currents derived from the mains power supply lines, known as "common mode aggressors". The mains frequency generally falls within the frequency range of interest of biomedical signals, which makes such common mode signals a particular problem. For example, an ECG signal has its main frequency components in the range <NUM> to <NUM>, but signal information up to around <NUM> or even <NUM> is desired.

The signals generated by the patient side are shown in region <NUM>. The common mode interference is represented by the signal source <NUM> in <FIG>. The patient generates the signal ExG to be monitored between the reference electrode and the signal electrode. The notation ExG is used to indicate any of EEG, ECG, EMG or EOG signals.

The interference signals can have larger amplitudes than the biopotential signals to be measured. This requires biopotential amplifier to have very high common mode rejection ratio (CMRR) in order to prevent the common mode interference signals from converting into differential signals, appearing at the output of the amplifier, and thereby reject the <NUM> or <NUM> common mode interferences whilst extracting the biopotential signals. When a high common mode rejection ratio is not possible, notch filtering is normally required to remove the common mode interfering signal.

A particular problem is that the common mode interference at the inputs of the active electrodes can be converted to a differential mode error at the outputs of active electrode pairs due to the voltage gain mismatch between the active electrode pairs, as well as the contact-impedance mismatch. The output error can have significant amplitude when compared to the amplitude of the biopotential signals. The CMRR of active electrode systems is usually limited by this voltage gain mismatch (VCM *ΔA) between the signal active electrode and the reference active electrode, which in return is due to the process variation and component mismatch. The process variation and component mismatch make it difficult to achieve well-matched voltage gain. Without compensation circuits or trimming, achieving more than 60dB CMRR is very difficult because of the component mismatch, while medical grade biopotential systems typically require more than 80dB CMRR.

A large common mode input interference may also saturate the active electrode, especially when the amplifier has a large voltage gain, and the other input is connected to a reference voltage. To avoid saturation of the active electrode, its voltage gain is usually limited to accommodate large input common mode dynamic range.

The article "<NPL> discloses the use of a feedforward approach for cancellation of a common mode interferer. The common mode feedforward scheme is implemented inside each of the active electrodes. There are limitations to the use of the circuit, in particular it is only suitable for certain amplifier designs.

Another known system for the measurement of physiological signals using active electrodes is described in <CIT>, which includes a sensing element for sensing a biological signal generated in a subject's body, the biological signal including a noise component and an information carrying component: a filter module separates the noise component and the information carrying component of the biological signal from each other, the filter module generating a filtered signal from the biological signal input into the filter module; an amplifier has an inverting input and a non-inverting input, the filter module being connected to one of the inputs of the amplifier for receiving the filtered signal and the sensed biological signal being input to the other of the inputs so at least the noise component of the biological signal is attenuated relative to the information carrying component.

According to the invention, there is provided a biopotential signal acquisition system as in claim <NUM>.

According to an exemplary embodiment, the integrated amplifier of each active electrode (which functions as a pre-amplifier, as there will generally be further amplification in the signal processing path) is connected to a feedforward common mode voltage provided by the unity gain voltage buffer. This buffered signal is approximately equal to the input common mode signal, which can be used to bias the other input of the amplifier. As a result, the effective common-mode input voltage which appears at the input of each amplifier is significantly reduced. This reduces the common-mode gain of the two active electrodes, and thus improves their common mode rejection ratio. It also avoids the saturation of the active electrodes, but without affecting the differential amplification of the signal of interest. The system may be applied to a single channel system (with two active electrodes) or to a multiple channel system (with more than two active electrodes). The system can be used with different types of amplifier, including inverting and non-inverting amplifiers.

The first and second inputs are skin contact points at which electrode pads are applied to the skin of the patient being monitored. They may be wet (gel) or dry contacts, although as described above systems using dry electrode contacts are preferred by patients for reasons of ease of use.

The unity gain buffer may be for receiving a signal from an associated input which is additional to the first and second inputs. Alternatively, the unity gain buffer may be for receiving a signal from the first or second input. In this case, no additional contact electrodes are needed to those already required for the active electrodes. The unity gain buffer may for example be implemented as an operational amplifier with full negative feedback, i.e. the output connected directly to the inverting input, and the input signal from the skin contact provided to the non-inverting input.

There may be at least a third active electrode for receiving a signal from a third input and comprising a third integrated amplifier. Three active electrodes enable a two channel system. The unity gain buffer may then also provide the buffered output to the third integrated amplifier as a reference signal. Thus, the unity gain buffer may be shared between two or more active electrodes. There may be one unity gain buffer even for many active electrodes, but equally, there may be more than one unity gain buffer, each providing a reference signal to an associated set of active electrodes.

In one set of examples, the output of the unity gain buffer may be provided to the respective integrated amplifiers through decoupling capacitors. These prevent a dc signal from the unity gain buffer from saturating the active electrode amplifiers. The unity gain buffer output may be provided to inverting inputs of the respective integrated amplifiers through the decoupling capacitors, and the respective integrated amplifiers may then also comprise a negative feedback path. This feedback path may be used to stabilize the DC operating points of the active electrode amplifiers. This provides an implementation using inverting amplification of the input signals and decoupling of the buffer signal.

In another set of examples, the output of the unity gain buffer is provided to non-inverting inputs of the respective integrated amplifiers. The first and second inputs may then be provided to the respective integrated amplifiers through decoupling capacitors, and the respective integrated amplifiers may again comprise a negative feedback path. This provides an implementation using non-inverting amplification of the input signals, and decoupling of the input signals.

The first active electrode may comprise a reference electrode, and the second active electrode may comprise one of a set of active electrode. As a minimum, the set may comprise only one active electrode (for a one channel implementation). However, the set may for example comprise between <NUM> and <NUM> active electrodes (for <NUM> to <NUM> channels respectively).

The invention also provides a biopotential signal acquisition method as in claim <NUM>.

This method provides a common mode feedforward approach in which a common mode signal may be fed forward from a unity gain buffer, which may be external to the active electrodes, to any number of active electrodes, in order to improve the common mode rejection ratio. The first, second and common mode biopotential signals may each be received from a respective input or else the first and second biopotential signals may each be received from a respective input and the common mode biopotential signal comprises one of the first and second biopotential signals.

Third or further biopotential signals may also be captured using a third or further active electrode comprising an associated integrated amplifier. The buffered output may also be provided to the third or further integrated amplifier as a reference common mode signal. In this way, the concept can be applied to single channel or multiple channel systems.

The buffered common mode biopotential signal or the first and second biopotential signals may be provided to the respective integrated amplifiers through decoupling capacitors to suppress DC signals between the inputs. A negative feedback path may be used to stabilize the operating points of the amplifiers.

Examples of the invention will now be described in detail, with reference to the accompanying drawings, in which:.

The invention provides a biopotential signal acquisition system, comprising at least two active electrodes each for receiving a signal from a respective input. The input may for example be a dry electrode applied to the skin of a patient being monitored.

A unity gain buffer is used to feed forward a signal from an associated input. The buffered output is provided to the amplifiers of the active electrodes as a reference signal. The buffered signal is approximately equal to the input common mode signal so that the effective common mode input voltage appearing at the inputs of each amplifier is significantly reduced. This reduces the common mode gain of the active electrodes and thus improves their common mode rejection ratio while also avoiding saturation of the active electrodes.

<FIG> shows a first example of signal acquisition system of the invention. The same reference numbers are used as in <FIG> for the same components.

Instead of connecting one input of the amplifiers <NUM>, <NUM> to a reference voltage as in <FIG>, a third electrode <NUM> and a unity gain voltage buffer <NUM> generate a feedforward voltage (VCMFF in <FIG>) approximately equal to the input CM signal (Vcm), which can be used to bias one input of the active electrode amplifiers <NUM>,<NUM>.

As a result, the effective common-mode input voltage appearing at the inputs of each amplifier is significantly reduced. This method reduces the common-mode gain of the two active electrodes, and thus improves their CMRR and avoids the saturation of the active electrodes, while the differential amplification of the signal of interest ExG is not affected.

The signal levels are shown in <FIG>. These examples of signal levels are based on the common mode feedforward signal being applied to the inverting input of the amplifiers. For example, for the active electrode <NUM>, the input signals are Vcm+ExG to the non-inverting input, and αVcm to the inverting input. The output is (A±ΔA)(Vcm+ExG- αVcm). The output signal resulting from the common mode interference is thus (A±ΔA)(<NUM> - α)Vcm.

For the reference electrode <NUM>, the signal levels shown are based on input signals Vcm to the non-inverting input, and αVcm to the inverting input. The output is (A)(Vcm - αVcm). The output signal all results from the common mode interference because this is the reference electrode, and the output resulting from the common mode interference is thus A(<NUM> - α)Vcm. As will be clear from examples below, the common mode feedforward signal may instead be applied to the non-inverting input of the amplifiers, and the input signals applied to the inverting inputs. The final differential output signal remains the same.

This approach involves feeding forward a buffered common mode input signal to the other inputs of all amplifiers. Therefore, the inputs of each amplifier see a reduced common mode swing (of (<NUM>-α)*Vcm), instead of a full common mode swing of Vcm as in <FIG>. In this way, most of the input common mode interference is compensated before amplification.

The gain of the differential amplifier to the common mode input is reduced by a factor of <NUM>/(<NUM>- α). As α approaches <NUM>, this reduction in gain approaches infinity, corresponding to an infinite common mode rejection ratio. The large common mode signal is compensated at the inputs of each amplifier, and this means that amplifier saturation is avoided.

<FIG> shows a system which has an input in the form of a skin contact <NUM> dedicated to the common mode buffer <NUM>. However, the feedforward signal may be derived from one of the active electrode skin contacts, as shown in <FIG>. The contact <NUM> for the reference electrode is used to derive the common mode feedforward signal.

<FIG> and <FIG> show single channel systems, with a reference active electrode and a single further active electrode. The approach can be extended to any number of active electrodes.

<FIG> shows the system extended with a third active electrode <NUM> with it associated amplifier <NUM>. It receives a signal from a third input, again in the form of a patient contact <NUM>. This is a two channel system, and the two biopotential signals being monitored are shown as ExG1 and ExG2. The common mode feedforward signal is applied to the inverting input of the amplifier of each of the three active electrodes. Both channels experience the same common mode rejection ratio if the value of ΔA is the same. Of course, for different values of ΔA for the two signal electrodes, the output signals resulting from the common mode interference will differ accordingly in dependence on the mismatch between the gain of the amplifier in the reference electrode <NUM> and in the signal electrodes <NUM>, <NUM>.

<FIG> shows the signal levels at different points in the circuit which result from the common mode interfering signal Vcm. For simplicity, the signal of interest is not shown. In each case, it comprises an additional signal (A±ΔA)ExG at the output of the active signal electrode (or (A±ΔA)ExG1 and (A±ΔA)ExG2 for the two active signal electrodes in <FIG>).

One possible drawback of the scheme in <FIG> is that a DC signal which exists between the contacts is also fed forward to the active electrodes. This means, in practice that the electrode offset between leads can reach to ±300mV as shown in <FIG>. In <FIG>, there is a DC offset between the contact <NUM> at which the common mode signal is derived and the contacts <NUM>, <NUM>. A large DC offset between the inputs of an active electrode amplifier can saturate the output, and cause the failure of ExG recording. In this case, the signals <NUM> become saturated.

<FIG> shows an example modification to the circuit of <FIG> in which the output from the buffer <NUM> is supplied to the inverting inputs of the amplifiers <NUM>,<NUM> through respective DC blocking capacitors <NUM>,<NUM>.

<FIG> shows different DC voltage levels Vdc1, Vdc2 at the patient contacts <NUM>, <NUM> relative to the common mode voltage Vcm.

By capacitively coupling the fed forward common mode signal to each amplifier input, any DC voltage offset is cancelled so that only the AC components are coupled to the amplifier (such as <NUM> or <NUM> mains interference). The DC operating point after the capacitor <NUM>, <NUM> is defined by a negative feedback loop <NUM>, <NUM> together with capacitor <NUM>, <NUM>. This feedback loop with capacitor <NUM>, <NUM> behaves as a low-pass filter, which only feeds back the DC and low frequency components to the inverting input of the amplifier. As a result, the DC voltage of the amplifier inverting input always follows its positive input. The electrode offset between leads is in this way blocked.

The DC voltage level at both inputs of the amplifier <NUM> is thus Vdc1 and the DC voltage level at both inputs of the amplifier <NUM> is Vdc2. The output signals resulting from the common mode interferer are the same as in <FIG>. The blocking capacitors <NUM>,<NUM> behave as a short circuit for the common mode signal. As discussed above, the effective common mode swing to each amplifier is reduced, which leads to improved CMRR of both active electrodes and avoid amplifier saturation.

<FIG> shows an implementation in which the input signals are provided to the inverting inputs of the integrated amplifiers through the decoupling capacitors <NUM>, <NUM>, and the buffered common mode signal is provided to the non-inverting inputs. The same reference numbers are used as in <FIG>. The feedback paths are shown as implemented by capacitors <NUM>, <NUM>. Note that the common mode voltage across the input decoupling capacitors is almost zero so that there is no input common mode current. Thus, the common mode gain is reduced and the common mode rejection ratio is boosted.

In practice, the fed forward common mode signal will be connected to each active electrode through a cable which may be long. The buffered output is a low impedance node, which reduces the noise and interference pick-up from environment by the cable. Compared to a conventional common mode feedback scheme, the system has better stability and faster settling time because of the elimination of the feedback loop. There is also improved power/bandwidth tradeoff, as it is possible to use a low power analog buffer to achieve sufficient CMRR bandwidth (<NUM>-<NUM>). In a conventional common mode feedback scheme, the feedback amplifier suffers from the tradeoff between bandwidth, power and stability. The power consumption of the feedback amplifier is usually high.

In a common mode feedback scheme, if one of the active electrodes is disconnected off, the whole feedback scheme does not work anymore. The approach described above allows each electrode to function regardless of the state of the others. The feedforward scheme can continuously run in the background without interrupting ExG signal recording.

The common mode feedforward approach has been verified by hardware measurement of a <NUM>-channel digital active electrode system for EEG recording, and shows more than 30dB CMRR improvement and significant <NUM>/<NUM> interference reduction.

The invention is applicable generally to single or multi-channel ExG systems, especially for (digital) active electrode systems for healthcare monitoring. The invention does not require any particular active electrode design and can be applied regardless of the detailed architecture and implementation of the active electrodes.

Claim 1:
A biopotential signal acquisition system for measuring biopotential signals (ExG) generated by a patient, comprising:
a first active electrode (<NUM>) comprising a first integrated amplifier (<NUM>); the first integrated amplifier (<NUM>) comprising a first input and a first buffered signal input, wherein the first input is connected to a first contact (<NUM>);
a second active electrode (<NUM>) comprising a second integrated amplifier (<NUM>); the second integrated amplifier (<NUM>) comprising a second input and a second buffered signal input, wherein the second input is connected to a second contact (<NUM>); and
a unity gain buffer (<NUM>) having an input connected to the first, the second or a third contact (<NUM>, <NUM>, <NUM>) and an output connected to the first buffered signal input of the first integrated amplifier (<NUM>) and to the second buffered signal input of the second integrated amplifier (<NUM>); and is characterized in that said biopotential signal acquisition system is configured such that, when the contacts are applied to the skin of the patient, said first and said second buffered signal input received by the first and the second integrated amplifier (<NUM>,<NUM>) contains a biopotential signal (ExG) generated by the patient.