Technical Field

The present invention relates generally to tunable electronic filters, in particular, to a tunable multiple feedback filter.

Background Art

Filters are an essential component in signal processing where it is desired to remove unwanted frequency components from a signal and to extract a particular frequency component or band of frequency components, and are ubiquitous across fields such as telecommunications, audio-visual processing, healthcare, automotive, and so on. Filters can be implemented in a variety of manners, including analogue and digital forms, in order to achieve a particular frequency response appropriate to a particular application. In many applications, however, it is desirable to be able to control or vary frequency responses in order to enable dynamic filtering, or to adapt to particular incoming signals or filtering requirements. In the case of analogue filters, which are

advantageous for their simplicity, tuning of a frequency response is typically achieved by altering the configuration of an RC network, composed of resistors and capacitors, which enables filter cut-off frequencies as well as filter types (e.g. high-pass, low-pass, band-pass and band-stop) to be specified and controlled. For applications which require dynamic frequency responses, the RC layout as well as their values would need to be altered every time a different frequency response is needed. One conventional way to solve this problem in a multi-channel filter is to implement a number of different filters in parallel, which requires components for all the required frequency responses to be arranged on the same circuit board, which often needs a large number of components, conflicting with space and cost restrictions. In an analogue filter, the RC network is typically used in conjunction with an amplifier, such as an operational amplifier, and so arranging many amplifiers on the same circuit board is particularly disadvantageous due to the particular size, power consumption and cost associated with the amplifier.

Embodiments of the present invention aim to provide an efficient architecture or topology in which filter flexibility and scalability can be achieved in a manner which reduces space requirements, and also reduces costs by reducing the number of required components. In addition, embodiments of the present invention aim to reduce power consumption . By reducing the number of components, the present invention also reduces the space occupied on a printed circuit board (PCB), which opens the possibility of new applications, where smaller circuits are required.

Sum m ary o f Inve ntio n

According to an aspect of the present invention, there is provided a tunable electronic filter comprising: an input stage coupled to a tuning stage; an amplifier with an inverting input coupled to an output of the tuning stage, and a non-inverting input coupled to ground and an output; and an output stage coupled to the amplifier output; wherein the tuning stage comprises N RC networks and a multiplexer, wherein the N RC networks are connected in parallel between the input stage and respective ones of the N inputs of the multiplexer; wherein the multiplexer is configured to selectively connect one of the N RC networks to the inverting input of the amplifier, such that a second order multiple feedback filter is formed between the input stage and the output stage, wherein selection of one of the N RC networks tunes the electronic filter to have a respective one of N frequency responses, for N > 2.

Each of the N frequency responses may be associated with a respective cut-off frequency.

The N frequency responses may represent at least one of a low-pass filter, a high-pass filter, a band-pass filter and a band-stop filter. Gains of the N RC networks may be configured by setting impedance values of the N RC networks.

The multiplexer may be an A: 1 multiplexer comprising N inputs, each connected with a respective one of the N RC networks.

The multiplexer may be an N+1A multiplexer comprising N+ l inputs; and wherein the tuning stage may further comprise a resistor network; wherein the resistor network comprises a first resistor connected between the input stage of the filter and a respective input of the A+ l: l multiplexer and a second resistor connected between the respective input of the A+ l: l multiplexer and the output stage of the filter; and wherein the A+ l: l multiplexer may be configured to selectively connect one of the N RC networks or the resistor network to the inverting input of the amplifier, wherein selection of the resistor network introduces a gain defined by the first and second resistors to the filter. The filter apparatus may further comprise two or more tunable electronic filters of any one of claims 1 to 5 arranged in series, in which the output stage of a tunable electronic filter is connected to the input stage of a successive tunable electronic filter.

The filter apparatus may comprise a group of first M tunable electronic filters arranged in series, in which each first tunable electronic filter is of a first type; a group of second M tunable electronic filters arranged in series, in which each second tunable electronic filter is of a second type; wherein either: the first type is a high-pass filter and the second type is a low-pass filter, or the first type is a low-pass filter and the second type is a high-pass filter; for M > 2, wherein the groups of first and second M tunable electronic filters may be connected in series such that the filter apparatus is an Mx 2 order band-pass filter.

The filter apparatus may comprise M groups of tunable electronic filters arranged in series, in which each of the M groups comprises a first and a second tunable electronic filter of respective first and second types; wherein : either: the first type is a high-pass filter and the second type is a low-pass filter, or the first type is a low-pass filter and the second type is a high-pass filter; for M > 2, such that the filter apparatus is an Mx 2 order band-pass filter. The filter apparatus may comprise M band-pass filters connected in series, such that the filter apparatus is an Mx 2 order band-pass filter.

According to another aspect of the present invention, there is provided an Mx 2 band- stop filter apparatus comprising an Mx 2 order band-pass filter apparatus as described above; and an output amplifier having an inverting and a non-inverting input; wherein the non-inverting input of the output amplifier is coupled to an input of the Mx 2 order band-pass filter, and the inverting input of the amplifier is coupled to an output of the Mx 2 order band-pass filter. The Mx 2 order band-stop filter apparatus may comprise an output amplifier having an inverting and a non-inverting input; wherein the non-inverting input of the output amplifier is coupled to an input of the Mx 2 order band-pass filter, and the inverting input of the amplifier is coupled to an output of the Mx 2 order band-pass filter, for even

M. The Mx 2 band-stop filter apparatus may further comprise a gain amplifier connected between the input stage to the Mx 2 order band-pass filter and the non-inverting input of the output amplifier, having gain which is substantially equal to the gain provided by the Mx 2 order band-pass filter. According to another aspect of the present invention, there is provided an Mx 2 band- stop filter apparatus comprising an Mx 2 order band-pass filter apparatus; an output amplifier having an inverting and a non-inverting input; wherein the non-inverting input of the output amplifier is coupled to an input of the Mx 2 order band-pass filter, and the inverting input of the amplifier is coupled to an output of the Mx 2 order band- pass filter; the Mx 2 band-stop filter apparatus may further comprise an inverting gain amplifier connected between the input stage to the Mx 2 order band-pass filter and the non-inverting input of the output amplifier, having gain which is substantially equal to the gain provided by the Mx 2 order band-pass filter, for odd M. According to another aspect of the present invention, there is provided a filter apparatus comprising a plurality of stages, in which at least one of the stages comprises a first tunable electronic filter as described above, and at least one of the stages comprises a second tunable electronic filter, the second tunable electronic filter comprising: an input stage coupled to a tuning stage; an amplifier with a non-inverting input coupled to an output of the tuning stage, and an inverting input and an output; and an output stage coupled to the amplifier output; wherein the tuning stage comprises N RC networks and a multiplexer, wherein the N RC networks are connected in parallel between the input stage and respective ones of the N inputs of the multiplexer; wherein the multiplexer is configured to selectively connect one of the N RC networks to the non-inverting input of the amplifier, such that a second order Sallen -Key filter is formed between the input stage and the output stage, wherein selection of one of the N RC networks tunes the electronic filter to have a respective one of N frequency responses, for N > 2. Brief Des criptio n o f Drawings

Embodiments of the present invention will be described by way of example only, with reference to the accompanying drawings, in which :

Figure 1 is a system diagram of a tunable electronic fdter according to embodiments of the present invention ;

Figure 2 illustrates a fundamental second order MFB fdter;

Figure 3 illustrates a second order tunable MFB fdter according to an embodiment of the present invention ;

Figure 4 illustrates a second order tunable MFB fdter with an additional resistor network according to an embodiment of the present invention ;

Figure 5A, 5B and 5C are block diagrams of tunable band-pass fdters (BPFs) comprising cascaded second order MFB fdters according to embodiments of the present invention ;

Figure 6A, 6B and 6C are block diagrams of tunable band-stop fdters with cascaded tunable second order MFB fdters according to embodiments of the present invention ; Figure 7A and 7B are block diagrams of tunable band-stop fdters with cascaded tunable second order MFB fdters configured to have a gain according to embodiments of the present invention ;

Figure 8 is a block diagram of a tunable band-stop fdter with cascaded tunable second order MFB fdters configured to have a gain according to embodiments of the present invention ;

Figure 9A and 9B are block diagrams of tunable band-stop fdters with Mx 2 order MFB band-pass fdters according to embodiments of the present invention ; and

Figure 10 illustrates a fundamental second order Sallen-Key fdter topology.

Detaile d D es criptio n

Figure 1 illustrates a system diagram of a tunable electronic fdter 100 according to embodiments of the present invention. The tunable electronic fdter 100 comprises an input stage 101 coupled to a tuning stage 102, an amplifier 103 coupled to an output of the tuning stage, and an output of the amplifier coupled to an output stage 104. The input stage 101 of the tunable electronic fdter 100 comprises a terminal or circuitry suitable for interfacing with and receiving a signal to be filtered, and the output stage 104 of the tunable electronic fdter comprises a terminal or circuity suitable for interfacing with and providing the filtered signal to any downstream system

components (not shown). Tunability is achieved by the tuning stage which comprises a multiplexer (MUX) 106 and N passive RC networks 105, also referred to herein as Filter Forming Circuitries FFCs 1-N, according to embodiments of the present invention, for integer N > 2. The multiplexer in the tuning stage is a multiplexer having N inputs and a single output. The N RC networks 105 share the same input signal from the input stage 101 and the multiplexer 106 operates such that the output of only one out of the N RC networks 105 is available at the output of the multiplexer 106.

N may be freely defined by the user in the design of the architecture shown in Figure 1, but it is advantageous if N = 2 ⁿ , since this facilitates addressing of the RC networks using n binary control lines (not shown) to the multiplexer. For example, eight RC networks can be addressed by three control lines, 32 networks can be addressed by five control fines, 128 networks can be addressed by seven control lines, 512 networks can be addressed by nine control lines, and so on.

The selection of one of the N RC networks 105 by the multiplexer 106 tunes the electronic filter 100 to have a respective one of N frequency responses, associated with the configuration of the resistors and capacitors in the RC networks 105. The N frequency responses may differ in parameters including any, or combinations of any of cut-off frequency, roll-off and filter type (high-pass, low-pass, band-pass, band-stop).

An active tunable electronic filter is achieved by coupling, via the multiplexer, the passive RC networks to an amplifier, in order to circumvent any signal attenuation caused by the load impedance of the RC network, and to optimise the frequency response. The amplifier used in the embodiment illustrated in Figure 1 is an operational amplifier (op-amp), having high input impedance, low output impedance, and a power supply (not shown). The output of the multiplexer is connected to the input of the amplifier. In embodiments of the present invention, the RC networks are advantageously configured such that when connected to the op-amp by the multiplexer, the resultant circuit forms a second order multiple feedback filter. Multiple feedback filters represent an efficient design, having high performance, such that design of the tunable electronic filter of embodiments of the present invention can be achieved effectively, and replacement, expansion or inclusion of redundancy for individual RC networks for the filter can be performed without difficulty. Figure 2 illustrates a fundamental second order MFB fdter 200. A second order MFB filter 200 is well known in the art, and is used in a variety of active filter applications, including low-pass, high-pass, band-pass and band-stop (notch) functions. A MFB filter uses an operational amplifier 203 with inverting 209 and non-inverting 210 inputs and an output 211 as an integrator as shown in Figure 2.

First Zl 204 and fourth Z4 207 impedances are connected in series between the input 201 of the filter and the inverting input 209 of the amplifier. Second impedance Z2 205 is connected between a junction between Zl 204 and Z4 207 and the output of the amplifier 211 in a feedback loop. Third impedance Z3 206 is connected between a junction between Zl 204 and Z4 207 and ground. Fifth impedance Z5 208 is connected between the inverting input 209 of the amplifier 203 and the output 211 of the amplifier 203 via the feedback loop, such that the feedback loop effectively has two“branches” from the amplifier input, i.e. through impedances Z2 205 and Z5 208. The non inverting input 210 of the amplifier is grounded.

For a low-pass filter, Zl 204, Z2 205 and Z4 207 are resistors, Z3 206 and Z5 208 are capacitors. For a high-pass filter, Zl 204, Z2 205 and Z4 207 are capacitors, Z3 206 and Z5 208 are resistors. The second order MFB filter 200 is capable of forming a band pass/ band-stop filter with one amplifier and one multiplexer. For the band-pass configuration, Zl 204, Z3 206 and Z5 207 are resistors, Z2 205 and Z4 207 are capacitors. The band-pass MFB filter 200 can be inverted to form a band-stop MFB filter, as described in more detail below.

It is conventional to tune both the resistors and the capacitors in a MFB filter in order to tune the overall filter response, but in embodiments of the present invention, dynamic tuning of an individual RC network is not required. Figure 3 shows a configuration of a second order tunable MFB filter 300 according to embodiments of the present invention. Each RC network 305 comprises resistors and capacitors arranged to implement different filter functions based on the principles illustrated with respect to Figure 2, so different RC networks have different frequency responses when connected to the inverting input 306 of the amplifier 304 via the multiplexer 303. The RC network selected by the MUX determines the bandwidth of the second order tunable MFB filter 300 and its Q factor in the selected configuration . The outputs of the feedback loops introduced by the RC networks are directly connected to the output stage 302 of the fdter without changing the transfer function of the system. The system response of the second order tunable MFB fdter approximates the response of a second order MFB topology that uses the selected RC network for impedances Zl 204, Z2 205, Z3 206, Z4 207 and Z5 208 of Figure 2. The N RC networks can be customised to have different gains by setting the appropriate impedance values for each RC network. The second order tunable MFB fdter topologies according to embodiments of the present invention are designed with the minimum number of required components, the amount of operating current is reduced, and as a result, the total power dissipation is minimized. A single amplifier is required to create an active fdter using any of the N passive RC networks.

Additionally, embodiments of the present invention offer the ability to use a mixture of different types of fdter in one device. For example, the user is able to select, via multiplexer control signals, between different types of filtering (low-pass, high-pass, band-pass, band-stop), which are input branches connected to the multiplexer.

Furthermore, the embodiments of the invention allow for multiple cut-off frequencies to be provided using a single device, either with, or without filter-type variability. In other words, if the application requires only one type of fdter (for example, either low- pass or high-pass), the multi-type fdter device is easily configured as a single-type device having multiple cut-off frequencies, but the multiple cut-off frequencies can also be used in combination with multiple fdter types.

Figure 4 illustrates a second order tunable MFB fdter 400 with an additional resistor network 407 according to embodiments of the present invention. The resistor network 407, also known as non-filter path, connects the input stage 401 of the fdter to a respective one input of an N+ 1: 1 multiplexer in parallel with the N RC networks. The resistor network 407 further connects the respective one input of the A+ l: l multiplexer 403 to the output stage of the fdter 402. The resistor network 407 comprises a first resistor 408 connected between the input stage of the fdter 401 and the respective input of the A+ l: l multiplexer 403 and a second resistor 409 connected between the respective input of the A+ l: l multiplexer 403 and the output stage 402 of the fdter. The A+ l: l multiplexer 403 is configured to selectively connect one of the N RC networks or the resistor network to the inverting input of the amplifier, wherein selection of the resistor network introduces a gain defined by the first 408 and second 409 resistors to the filter. The selection of the resistor network bypasses the N RC networks which enables conversion of a signal recording system from AC-coupled to DC-coupled to capture low- frequency (close to DC) signals according to embodiments of the present invention.

It will be appreciated that the use of the MUX and the amplifier does not impose a limit on the number of available frequency responses according to embodiments of the present invention. In other words, a user is free to decide on the number of RC networks/ frequency responses depending on the application requirements (e.g. signals to be filtered, bandwidth, power consumption, area, redundancy to cater for component failure etc.).

In alternative embodiments of the present invention, a filter apparatus comprises two or more instances of the second order tunable MFB filters of the type shown in Figure 3 arranged in series, for realisation of filters with different frequency responses. For instance, it may be possible to arrange M stages of tunable MFB filters in series, for integer M > 2. If the filter apparatus is made up of M second order tunable MFB filters of the same type (for example, all high-pass or all low-pass), the filter order increases by two.

The resultant filter apparatus achieved through such a cascaded system may be low- pass, high-pass, band-pass (BPF) or band-stop (BSF). As mentioned above, in some embodiments, the second order tunable MFB filter can realise a BPF or a BSF on its own. In alternative embodiments, to realise a second order BPF, one second order MFB high-pass stage of the type shown in Figure 3 (in which all N RC networks represent high-pass filter circuitry) and one second order MFB low-pass stage of the type shown in Figure 3 (in which all A RC networks represent low-pass filter circuitry) are connected in series, the two stages connected in either order. In further embodiments, it is possible to“nest” different filter types within each stage, as described below.

Figure 5A illustrates a cascaded Mx 2 order MFB BPF 500 , in which there are M groups of a BPF, each group 502, 503 containing a pair of a HPF 505, 507 of the type shown in Figure 3 (in which all N RC networks represent high-pass filter circuitry) and a LPF 506, 508 of the type shown in Figure 3 (in which all N RC networks represent low-pass filter circuitry) connected in series. M groups of the HPF and LPF pairs 502, 503 are cascaded to realise an Mx 2 order BPF. In such a system, there are 2 M amplifiers and 2 M multiplexers, leading to N ^2M combinations of RC networks in the signal path.

According to embodiments of the present invention, the M groups of the HPF and LPF pairs 502, 503 may all have unity gain, such that the Mx 2 order BPF 500 has unity gain. In another embodiment of the present invention, one or more of the M groups of the HPF and LPF pairs 502, 503 may introduce a gain, such that the Mx 2 order BPF 500 provides an overall gain G. In some embodiments, each of the M groups 502, 503 of the HPF and LPF pairs may have substantially equal gains.

In alternative embodiments, a band-pass filter response can be realised by having a group 513 of first M tunable electronic filters of a first same type 515, 516 (high-pass or low-pass) arranged in series, cascaded with a group 514 of second M tunable electronic filters of a second type 517, 518 (high-pass or low-pass) arranged in series as illustrated in Figure 5B, in which the second type is different from the first type.

In other words, to realise an Mx 2 BPF as a stage 1 512, an Mx 2 order HPF 513 is cascaded with an Mx 2 order LPF 514. Multiple instances of cascaded arrangement (stage 1) may be further cascaded in order to generate yet higher order filters. For example, K instances of a single HPF group with a single LPF group as shown in Figure 6B results in a BPF of order K x M x 2 order BPF is formed by cascading K instances of the stage 1.

As described above with respect to Figure 5A, each of the HPF group and the LPF group may have unity gain, such that the Mx 2 order BPF provides an overall unity gain, or one or more of the HPF group and the LPF group may introduce a gain, such that the Mx 2 order BPF provides an overall gain G.

Figure 5C illustrates the block diagram of cascaded M single amplifier tunable second order BP MFB filters 519 according to embodiments of the present invention. Each BPF 521, 522 in Figure 5C is a BPF of the type shown in Figure 3 , in which all N RC networks represent band-pass filter circuitry. The realisation of a tunable Mx 2 order BPF 519 with MFB filters in Figure 5C uses a single amplifier in each BPF thus for the same frequency response requirement, saves more space than the ones in Figure 5A or 5B.

As described above with respect to Figure 5A, each of the BPF may have unity gain, such that the Mx 2 order BPF provides an overall unity gain, or one or more of the BPF may introduce a gain, such that the Mx 2 order BPF provides an overall gain G.

A band-stop or notch fdter is an inverted or complimented form of a standard band pass fdter. In embodiments of the present invention, a Mx 2 order band-stop fdter may be realised by inverting an Mx 2 order BPF, such as those shown in Figure 5A

500 , Figure 5B 509 and Figure 5C 519 using an output amplifier 602, 608 , 617 as shown in Figure 6A, 6B and 6C. This can be achieved by having the input to the band-pass fdter 60 1, 606, 613 coupled to the non-inverting input 605, 611, 616 of the output amplifier 602, 608 , 617 and the output of the band-pass fdter connected to the inverting input 604, 610 , 615 of the output amplifier 602, 608 , 617 as shown

respectively in Figures 6A, 6B and 6C.

According to embodiments of the present invention, the unity gain band-stop fdter apparatus as shown in Figure 6C is configured to be used when M is even only. When M is odd, an inverting gain amplifier is arranged in the topology, this is discussed in more detail with respect to Figure 8 and 9.

Figure 7A and 7B are block diagrams of tunable band-stop filters with cascaded tunable second order MFB filters configured to have a gain according to embodiments of the present invention.

The cascaded tunable second order MFB filters, for example, can be the Mx 2 order band-pass filters illustrated in Figure 5A and 5B according to embodiments of the present invention. Whereas the Mx 2 order band-pass filters shown in Figure 6A and 6B may be configured to have unity gain, in some embodiments of the present invention the Mx 2 order band-pass filters have a gain G, as described above with respect to Figure 5A and 5B. In addition to the output amplifiers 602 and 608 as shown in Figure 6, the filter apparatus further comprises a gain amplifier 703 having a second inverting 705 and a second non-inverting 704 input and a second output 711 as shown in Figure 7A and 7B corresponding to the band-pass filters illustrated in Figure 6A and 6B. The second output 711 of the gain amplifier is coupled to the first non-inverting input 709 of the output amplifier 712 in Figure 7A and 7B, and the input 701 to the cascaded tunable MFB filters is coupled to an input 716 of a switch 713. A first output 715 of the switch 713 is directly coupled to the non-inverting input 709 of the output amplifier 712. A second output 714 of the switch 713 is coupled to the non-inverting input 704 of the gain amplifier 703.

According to embodiments of the present invention, a first resistor 706 is connected between the inverting input 705 of the gain amplifier 703 and ground and the variable resistor 707, which may be a voltage-controlled resistor (VCR), is connected between the inverting input 705 of the gain amplifier 703 and the output 711 of the gain amplifier 703. When no gain is required, the input signal may be directly connected through the switch 713 to the non-inverting input 709 of the output amplifier thus bypassing the gain amplifier. When gain is required, the input signal may be connected through the switch 713 to the non-inverting input 704 of the gain amplifier. The operation of the switch 713 is controlled via a control signal A _o . The gain amplifier 703, together with the first resistor 706 and the variable resistor 707, is configured to provide a gain which is substantially the same as the gain G provided by the Mx 2 order band-pass filters. The output of the cascaded tunable MFB filters is coupled to the inverting input 711 of the output amplifier 712, which may be an instrumentation amplifier or a differential amplifier. The band-pass filter section (which is the cascaded tunable second order MFB filters) provides the gain G to the inverting input 711 of the output amplifier 712, and the gain amplifier 703 provides substantially the same gain G to the non-inverting input 709 of the output amplifier 712, such that the filter apparatus is an Mx 2 order band-stop filter 700 ,716 with an overall gain which is substantially equal to the gain G.

The gain matching may be achieved by controlling the value of the variable resistor 707, through I2C/ SPI communication protocols according to embodiments of the present inventions.

According to embodiments of the present invention, when gain is required for the band-stop filter apparatus as shown in Figure 6C, an inverting gain amplifier 767, 768 is arranged to set the gain of each BPF 753, 754 as shown in Figure 8. In this topology, the multiplexers included in the BPF 753, 754 are configured to share the same control lin es (Ao, Ai,.. A _p ) as gain multiplexers 759, 760 of the inverting gain amplifiers 767, 768. As a result, the controlling code that activates the first RC network of the tunable BPF1 MFB filter 753 also activates the first resistor 763 connected with the gain multiplexer 759 of the inverting amplifier 767, which introduces the same gain with the first RC network of the tunable BPF1 MFB filter 753. Similarly, the controlling code that activates the 77 ^th RC network of the tunable BPF1 MFB filter 753 also activates the 77 ^th resistor 765 connected with the gain multiplexer 759 of the inverting gain amplifier 767, which again introduces the same gain with the 77 ^th RC network of the tunable BPF1 MFB filter 759. According to alternative embodiments of the present invention, Figure 9 A and 9B illustrate tunable band-stop filters 800 , 820 using band-pass filters 519 as illustrated in Figure 5C with extra gain. If M is even, the topology of the tunable MFB band-stop filter is effectively the same as the one 612 presented in Figure 6C if no gain is required, since the input signal 801 is directly connected through the switch 8 14 to the non-inverting input 806 of the output amplifier 804 bypassing the non-inverting gain amplifier 807. If gain is required, the configuration is shown in Figure 9A. The input signal 801 is connected through the switch 8 14 to the non-inverting input 808 of the non-inverting gain amplifier 807. The gain of the non-inverting gain amplifier 807 is configured by the values of a first resistor Zl 8 10 and a variable resistor, which may be a voltage- controlled resistor VCR 8 11 to be substantially equal to the gain of the BPFs that exist on the cascaded tunable band-pass filter section which is on the upper branch of the topology. The value of VCR 8 11 may be controlled through I2C/ SPI communication protocols. If M is odd, the topology of the tunable MFB band-stop filter is the one presented in

Figure 9B according to embodiments of the present invention. The first resistor Zl 827 is coupled between the input of the band-stop filter 820 and an inverting input 826 of the inverting gain amplifier 830. A VCR 829 is coupled between an inverting input 826 of the inverting gain amplifier 830 and the non-inverting input 833 of the output amplifier 831. The non-inverting input 825 of the inverting gain amplifier is coupled to ground. The gain of the inverting gain amplifier 830 is configured by the values of Zl 827 and VCR 829 to be substantially equal to the gain of the cascaded BPFs that exist on the upper branch of the topology. This value of the VCR may be controlled through I2C/ SPI communication protocols. According to some embodiments of the present invention, changes in the FFCs may lead to a changing gain of the gain amplifier 807, 830 , but in other embodiments, different FFCs may be associated with the same gain . This tunable band-stop filter 800 , 820 as shown in Figure 9A and 9B according to embodiments of the present invention saves the number of components needed than those shown in Figure 7A and 7B for the same frequency response requirement. For example, for a second order tunable band-stop filter (M= 1), the realisation in Figure 7A and 7B may require two amplifiers and two multiplexers for the cascaded BPF in addition to a gain amplifier and an output amplifier, which gives a total of six major components. On the other hand, only one amplifier and one multiplexer is needed for the BPF realisation in Figure 9 A and 9B for M= 1, in addition to a gain amplifier and an output amplifier, which gives a total of four major components, providing a further saving in space. The second order tunable notch filter according to this embodiment of the present invention also provides higher attenuation in comparison to other hardware notch filters, such as 2 ^nd order Bainter notch filters or all the other 2 ^nd order notch filters which are based on the cascade LPF-HPF topology as described in the other embodiments of the present invention. According to alternative embodiments of the present invention, additional types of filter may be employed in the tunable filter, for example, second order tunable Sallen- Key (SK) filters. Filter apparatuses according to embodiments of the present invention which combine such MFB filters with SK filters are referred to herein as“hybrid filter topologies”.

Figure 10 shows a fundamental second order SK filter 900. A SK filter is a variation on a voltage-controlled voltage-source (VCVS) filter that uses unity-gain amplifier. The circuit comprises an op-amp 901, in the arrangement shown in Figure 2 having non inverting 908 and inverting 909 inputs and an output 912. First and second

impedances Zl 904 and Z2 905 are connected in series between the input 902 of the filter and the non-inverting 908 input of the amplifier 901. A third impedance Z3 906 is connected in the feedback loop between a junction between Zl 904 and Z2 905 and the output 903 of the filter. A fourth impedance Z4 907 is connected between the non inverting input 908 of the amplifier and ground. For a low-pass filter, Zl 904 and Z2 905 are resistors, Z3 906 and Z4 907 are capacitors. For a high-pass filter, Zl 904 and Z2 905 are capacitors, Z3 906 and Z4 907 are resistors. It is conventional to tune both resistors and both capacitors in a SK filter in order to tune the overall filter response, but in embodiments of the present invention, dynamic tuning of an individual RC network is not required.

In the arrangement illustrated in Figure 9 , gain is achieved via impedances Z5 910 and Z6 911 are present and the gain is 1 + Z6/ Z5, where Z5 910 and Z6 911 are resistors. If gain is not needed, the inverting input of the amplifier is connected directly to the output of the filter.

A second order tunable SK filter comprises an input stage coupled to a tuning stage, an amplifier with a non-inverting input coupled to an output of the tuning stage, and an inverting input and an output; and an output stage coupled to the amplifier output. The tuning stage comprises N RC networks and a multiplexer, wherein the N RC networks are connected in parallel between the input stage and respective ones of the inputs of the multiplexer. The multiplexer is configured to selectively connect one of the N RC networks to the non-inverting input of the amplifier, such that a second order Sallen- Key filter is formed between the input stage and the output stage, wherein selection of one of the N RC networks tunes the electronic filter to have a respective one of N frequency responses.

It will be appreciated that, depending upon a particular application, the present invention can be embodied in a number of different ways, based on the principle of selectively connecting multiple RC networks to an amplifier, via a multiplexer.

Depending on the required application, a single device or printed circuit board may contain RC networks enabling high-pass filters and/ or low-pass filters and/ or band pass filters and/ or band-stop filters and/ or notch filters to be selectively employed, with tunability being available for both filter types, and cut-off frequencies, filter order, and roll-off for a given type, using minimal components.

As a comparative example, consider a three-channel configuration having a low-pass channel, a high-pass channel and a band-pass channels on a single hardware device, based on embodiments and cascades of embodiments as described above. This configuration could use as few as six higher-cost components, namely one multiplexer and one amplifier for each of the low-pass, high-pass and band-pass channels. This number of components will apply, regardless of the number of cut-off frequencies to be used for each channel type, whereas a conventional frlter would need to scale up.

Consequently, it can be seen that the area and cost savings increase as the number of channels increases. For the case where there are four cut-off frequencies per channel, the resultant device occupy around 40 % of the space of a conventional three-channel frlter, which will require 12 amplifiers in total, four for each of the high-pass and low- pass channels, and four for the band-pass filters. Furthermore, the efficiency of the arrangement of the present invention is particularly highlighted by the fact that in the case of the band-pass frlter channels, the example arrangement of the present invention will achieve four band-pass frequency bands using only one amplifier in the case of a four-channel system , whereas conventionally, four amplifiers would only enable four frequency bands.

The architecture described above may find particular applicability in multi-channel signal processing systems, such as data acquisition apparatuses. One particular example is in brain monitoring, where signals having a very large number of frequency channels may require rapid and dynamic filtering, which would not be possible if the hardware required changing in order to switch between each channel. The

embodiments of the present invention would add bandwidth, gain and filter-type flexibility in the process of recording potential signals.

The present invention is not limited to the embodiments described above, and it will be appreciated that combinations of compatible features of different embodiments may be permissible within the scope of the invention as defined by the appended claims.