Patent Publication Number: US-2023145384-A1

Title: Bio-impedance measurement using voltage to current conversion

Description:
BACKGROUND 
     Remote and personal health care monitoring is a growing market with new and improved devices being produced. Wearable fitness trackers measure heartbeats, distance traveled, steps and other impact events. New sensors are being developed to support glucose monitoring, blood oxygen levels and other vital signs that can be measured directly or inferred from other sensors. These sensors are combined with wireless interfaces such as Bluetooth, Wi-Fi, and cellular to provide continuous data reports to the patient and health care professionals for In-Patient and Out-Patient monitoring. 
     The Internet-of-Things (IoT) offers low cost, low power, low data rate communications with a wide range of devices on external networks. IoT has generated particular excitement in healthcare because of its singular potential to improve the health, safety, and quality of life for people everywhere. The semiconductor content in clinical-grade smart IoT edge healthcare devices provides smart processing, analysis, and reporting of many different vital signs. 
     Bio-impedance is suggested as a measurement for a variety of different body parameters. One use of bio-impedance is as a part of an electrocardiogram (ECG) patch. The patch is provided as a self-contained small flat device that can be attached to the body with an adhesive much like a self-adhesive bandage. For bio-impedance, an ECG patch uses dry electrodes that are directly applied to or connected to the body skin. A signal is applied across the electrodes and the impedance to the signal is measured. 
     SUMMARY 
     A method and apparatus are described for bio-impedance measurement using voltage to current conversion. In one example, a bio-impedance transducer includes an input stage to receive a bio-impedance signal having an oscillating voltage from two electrodes, the electrodes being coupled to a body, a resistance across the two electrodes to determine an alternating current of the bio-impedance signal, a gain stage coupled to the resistance to amplify the alternating current, a down converter coupled to the gain stage to convert the amplified alternating current to a direct current bio-impedance signal, and an analog-to-digital converter coupled to the gain stage to convert the direct current bio-impedance signal to a digital bio-impedance signal. 
     In some embodiments, the input stage comprises a source-follower circuit coupled to the electrodes at a first input and coupled to the resistance at a first output. In some embodiment, the input stage comprises a feedback loop coupled across the electrodes and to the gain stage. In some embodiments, the feedback loop includes a balanced operational transconductance amplifier. In some embodiments, the feedback loop includes diode-connected transistors between the electrodes and the gain stage. 
     Some embodiments include an excitation current source coupled to the electrodes to generate an alternating current excitation current to be injected into the body through the electrodes. In some embodiments, the down converter comprises a mixer to mix the amplified alternating current with the excitation current. In some embodiments, the analog-to-digital converter comprises a continuous time sigma delta analog-to-digital converter. 
     In some embodiments, the continuous time sigma delta analog-to-digital converter comprises a balanced operational transconductance amplifier coupled to the amplified alternating current as an input and a capacitor coupled across the input and an output of the balanced operational transconductance amplifier. 
     In some embodiments, the gain stage further includes an in-phase gain stage coupled to the resistance to amplify an in-phase component of the alternating current, and a quadrature phase gain stage coupled to the resistance to amplify a quadrature phase component of the alternating current. 
     In some embodiments, the down converter further includes an in-phase down converter coupled to the in-phase gain stage to convert the amplified in-phase component to a direct current in-phase bio-impedance signal, and a quadrature phase down converter coupled to the quadrature phase gain stage to convert the amplified quadrature phase component to a direct current quadrature phase bio-impedance signal 
     In some embodiments, the analog-to-digital converter further includes an in-phase analog-to-digital converter coupled to the in-phase gain stage to convert the direct current in-phase bio-impedance signal to a digital in-phase bio-impedance signal, and a quadrature phase analog-to-digital converter coupled to the quadrature phase down converter to convert the direct current quadrature phase bio-impedance signal to a digital quadrature phase bio-impedance signal; 
     Some embodiments further include a current mirror coupled to the down converter to convert a differential current from the down converter to a single ended current to the analog-to-digital converter. 
     In an embodiment, a method includes receiving a bio-impedance signal at an input stage, bio-impedance signal having an oscillating voltage from two electrodes, the electrodes being coupled to a body, determining an alternating current of the bio-impedance signal through a resistance coupled across the two electrodes, amplifying the alternating current, converting the amplified alternating current to a direct current bio-impedance signal, and converting the direct current bio-impedance signal to a digital bio-impedance signal. 
     Some embodiments further include feeding back the input stage signal across the electrodes to the amplifying of the alternating current. Some embodiments further include generating an alternating current excitation current, and injecting the excitation current into the body through the electrodes. 
     In some embodiments, converting comprises mixing the amplified alternating current with the excitation current. 
     In an embodiment, a health monitor system includes an input stage to receive a bio-impedance signal having an oscillating voltage from two electrodes, the electrodes being coupled to a body, a resistance across the two electrodes to determine an alternating current of the bio-impedance signal, a gain stage coupled to the resistance to amplify the alternating current, a down converter coupled to the gain stage to convert the amplified alternating current to a direct current bio-impedance signal, an analog-to-digital converter coupled to the down converter to convert the direct current bio-impedance signal to a digital bio-impedance signal, and a radio frequency system to send the digital bio-impedance signal to external components. 
     In some embodiments, the input stage is based on a current balancing instrumentation amplifier such that the oscillating voltage from the electrodes appears on the resistance through a source follower input stage. In some embodiments, the input stage is coupled directly to the gain stage with no intermediate stage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of a system for measuring parameters of a body. 
         FIG.  2    is a block diagram of a bio-impedance sensor. 
         FIG.  3    is circuit diagram of portions of a bio-impedance transducer. 
         FIG.  4    is a circuit diagram of portions of an alternative bio-impedance transducer. 
         FIG.  5    is a sequence of graphs of different stages of a bio-impedance signal aligned on the horizontal axis as it is modified through a bio-impedance transducer. 
         FIG.  6    is a simplified circuit diagram of a continuous time current input second order sigma delta analog-to-digital converter. 
         FIG.  7    is a process flow diagram of the operation of a bio-impedance sensor. 
     
    
    
     DETAILED DESCRIPTION 
     It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended drawings could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     A power efficient bio impedance measurement device and method are described herein. The bio-impedance measurement may be used for any of a wide variety of different purposes. The measurement device is described as being incorporated into a patch but may also be used with and for other devices and for in-patient and out-patient monitoring. The described implementation may be part of a complete vital signs analog front end (AFE) for an Internet of things (IoT) or other connected device. 
     Electrodes on a human body tend to pick up noise and interference that the body has received from external electrical and magnetic sources. Common mode noise is a particularly large component of the noise and is received from alternating current (AC) power systems, i.e., AC mains power, such as lamps, motors, wiring, etc. The electrodes not only pick up the desired impedance signal but also pick up the noise. The described analog front end amplifier has a very high common mode rejection and is able to measure the desired signal with low noise. Efficiency is improved by providing a current signal as an input to a continuous time sigma delta analog-to-digital converter (ADC) instead of a voltage signal into a voltage input ADC. The continuous time sigma delta ADC also eliminates any requirement for an anti-aliasing filter. 
       FIG.  1    is a block diagram of a health monitoring system such as a smart patch or other system for measuring parameters of a body. The form factor may be modified to suit different purposes and uses. In some embodiments, the system  100  is 20-40 mm wide and long and less than 5 mm thick with an adhesive strip on one side to attach to the human body in an appropriate place. A strap or band (not shown) may be provided to wrap around an arm or other body part to hold the system  100  in place. The system  100  may have a housing that is moisture and impact resistant with an access port to allow service to batteries and other components. 
     The health monitoring system  100  has one or more sets of electrodes  102  to make electrical contact with the human body. The electrodes  102  are coupled to an impedance sensor  104  referred to as a bio-impedance sensor for measuring an impedance across the electrodes when the electrodes are electrically coupled to the body. The impedance sensor  104  is coupled to a processor  108  for processing the measured impedance for controlling an applied signal across the electrodes and for controlling other components of the system. 
     The electrodes  102  are further optionally coupled to other sensors  106  to measure other vital signs or parameters of the body or the surrounding environment. The other sensors  106  may include environmental temperature, pressure, humidity, and any suitable body parameters. The same electrodes or other electrodes may be used, depending on the particular implementation. The other sensors  106  are further coupled to the processor  108  to provide data which the processor may use and communicate in any suitable way. The processor is coupled to a memory  110  to store data from the sensors and other values determined by the processor or received from other components. 
     The processor and memory are further coupled to a radio frequency (RF) system  112  that provides communication to external components that may include network nodes, a user interface, and further sensors. In one use example, the system  100  communicates with a smart phone (not shown) through an RF interface to provide information to the user and may also send information to a health monitoring facility. In another use example, the system communicates with a cellular or Wi-Fi network node to send information directly to a health care facility. The health care facility may then provide alerts to the user through any desired means. The RF interface may use one or more wireless communications system and protocols including near field communication, Bluetooth, Wi-Fi and cellular. Each of the components are also coupled directly or indirectly to a power supply  114  which may be in the form of a battery, a solar cell, a kinetic energy harvester, a thermal energy harvester or any other suitable power source or combination of sources. 
     The system  100  may be in the form of a single integrated circuit (IC) or some of the components may be combined while other components are not. The multiple components may be combined into a single package for example as a multi-chip module (MCM) or system in a package (SiP) or other physical implementation. Additional components may be added to the system  100  and fewer components may be used. Some functions may be provided on different modules than as described herein. More or fewer modules may be provided than as shown herein. 
       FIG.  2    is a block diagram of a bio-impedance sensor  200  such as that of  FIG.  1    or for use in a different system. The bio-impedance sensor  200  includes or is connected to an excitation current source  202 , which may be a part of the bio-impedance sensor or a separate component as shown. The excitation current source  202  includes a current digital to analog converter (DAC) that generates an excitation current, identified as f bio , as an alternating current (AC). The amplitude of the AC excitation current may have a variety of different amplitude values as determined by an external control input  203 . The control input  203  may be a digital value applied to the DAC to suit different uses for the bio-impedance measurement. The current is injected into the body  208  through electrodes  204 ,  206 . The body  208  is difficult to characterize electrically due to its complexity and variability under external and internal factors. It presents an impedance to an applied voltage. The current generates a potential through the body across the electrodes. The voltage is measured by a bio-impedance channel in the bio-impedance sensor  200 . 
     The electrodes  204 ,  206  are coupled to respective input ports  210 ,  212  of a bio-impedance transducer  232 . The bio-impedance transducer  232  converts the received voltage at its input ports  210 ,  212  to a digital value at its outputs  226 ,  228 . The bio-impedance transducer has an input stage  214  which provides a high input impedance to receive the voltage through the body  208  at very low current. A gain stage  216  amplifies the signal. A first mixer  218  and a second mixer  220  mix the amplified signal with the excitation current frequency to down convert the impedance signal as a current. The first mixer  218  receives an in-phase excitation signal to mix with the amplified signal to generate an in-phase (I) output. The second mixer  220  receives a quadrature phase excitation signal to generate a quadrature phase (Q) output. The impedance signal is coupled to analog-to-digital converters (ADC)  222 ,  224  to generate respective I and Q numerical values to represent the body impedance as indicated by the voltage received from the electrodes  204 ,  206 . In some embodiments, the gain stage converts the received voltage to a current and a current is applied to the ADCs. The ADCs may be in the form of continuous time sigma-delta (CTΣΔ) ADCs. 
       FIG.  3    is a circuit diagram of portions of the bio-impedance transducer  232  of  FIG.  2    including the input stage, and gain stage. An input stage  302  receives the oscillating voltage from the electrodes at an n input  310  and a p input  312 . The n input  310  and the p input are applied to a source-follower input stage  316  including transistor MP 0  and MP 1 . The voltage between the two branches of the input stage is applied across a resistor  314 , labeled Rin, to generate the input current, Iin, across the resistor  314 . As shown, while the input is an oscillating voltage across the n input  310  and the p input  312 , the signal across the resistor  314  is an alternating current. The differential input voltage produces the current in the resistor  314 , so that the input stage produces a very high common mode rejection. 
     The current produced in the resistor  314  is copied via a feedback loop across then input  310  and the p input  312  through a gm stage  318  and two diode-connected P-type Metal Oxide Semiconductor (PMOS) transistors MP 4  and MP 5 . The feedback loop is used for biasing and also to couple out the current. This configuration consumes very little current for the feedback loop. The gm stage may be implemented using a balanced operational transconductance amplifier with one or more stages or using any other suitable transconductance stage, including differential stages. Other types of amplifier architectures may be used instead or in addition. It may be formed using differential amplifiers, cascodes, or other components. 
     The alternating current flowing through the diode connected PMOS transistors MP 4  and MP 5  is copied to the gain stage via current mirrors. The ratio of these current mirror transistors determines the gain of the amplifier. A first current mirror is formed in the gain stage  304  by transistors MP 8  and MP 10  coupled to input stage transistor MP 3  through diode-connected transistor MP 4 . A second current mirror is formed in the gain stage  304  by transistors MP 9  and MP 11  coupled to input stage transistor MP 2  through the diode-connected transistor MP 5 . 
     The gain stage has differential alternating current outputs and the first output is provided to an in-phase mixer  320  to be down converted using the excitation signal fbio (0°). The second output is provided to a quadrature phase mixer  322  to be down converted using an orthogonal complement of the excitation signal fbio (90°). The bio-impedance measurements from the electrodes have both real and imaginary parts. The I-Q mixer formed of the in-phase mixer  320  and quadrature phase mixer  322  is used in the current domain which simplifies down conversion performed by the I-Q mixer. 
     The outputs of the down conversion from the in-phase mixer  320  and the quadrature phase mixer  322  are current outputs at direct current (DC) which are each fed to a respective  2 nd order CTΣΔ ADC, namely an I ADC  306  and a Q ADC  308 , optionally through cascodes  324 . The resulting quantified I-Q result with an I digital output  344  from the I ADC  306  and a Q digital output  346  from the Q ADC  308  is then provided to e.g., the processor for further analysis, storage, communication, and other functions. 
     As shown, the circuit of  FIG.  3    generates an I digital output  344  and a Q digital output  346  without any need for any intermediate gain stage. Eliminating the intermediate gain stage improves power efficiency which may be important for a smart patch or remote monitoring implementation. The gain stage is closely linked to the mixing stage and the combination provides a high common mode rejection ratio (CMRR). The CMRR of the transducer is affected by the precision of the matching at the input stage. This matching may be improved further at the input stage by chopping the input signal to an intermediate frequency and adding that frequency to the down conversion. 
     A current mirror  330 , in this case an N-type Metal Oxide Semiconductor (NMOS) current mirror, is coupled below the CTΣΔ ADC  306  input to convert the differential current from the in-phase mixer  320  serving as a down converter to a single ended current that is measured by the CTΣΔ ADC  306 . The current mirror  330  is coupled to the differential output of the in-phase mixer  320 . The CTΣΔ ADC  306  is coupled to a single node between the in-phase mixer  320  and the current mirror. A mixer  332  coupled above the current mirror  330  improves the matching of the input current with the current mirror  330 . Cascodes  324 ,  334  are coupled to either side of the CTΣΔ ADC input to increase the output impedance of the current mirror  330 . The particular circuit illustrated in  FIG.  2    and the other figures may be modified to suit cost, form factor, precision, and other constraints. Elements may be substituted with other equivalent elements to suit different implementation. 
       FIG.  4    is a circuit diagram of portions of the bio-impedance transducer  232  of  FIG.  2    in an alternative embodiment in which a chopping mixer is used at the input stage. From the electrodes, a first bio-impedance signal is received as an oscillating voltage, Vin n, and a second bio-impedance signal is received as a complementary oscillating voltage, Vin p. The received bio-impedance signal has a frequency that is determined by the excitation current applied from the excitation current source  202  as shown in  FIG.  2   . The bio-impedance signals are applied to a mixer  426  to mix with a chopping frequency, fc,  424  to down convert the bio-impedance signals. The down conversion allows process variations in input source-follower circuits  414  that receive the bio-impedance signals, to be averaged over time. This may allow for higher precision in measuring the impedance of the bio-impedance signal. 
     The input source-follower circuits  414  are coupled across a resistance  412 , such as a resistor, to the down converted bio-impedance signals. The input source-follower circuits  414  are coupled to current mirrors  444  that couple the bio-impedance signal over to gain stages  404 ,  406 . The input source-follower circuits  414  are also coupled to a feedback loop  416 , through a gm stage  418  and transistor diodes MP 4 , MP 5 , to the gate nodes of the current mirrors. Except for the initial chop mixer  426 , a chop frequency down mixer, this is the same configuration described above with respect to  FIG.  3   . 
     At the gain stages, the in-phase component is amplified, down converted, and converted to digital separately from the quadrature component. The two components may be recombined as differential digital components of the bio-impedance signals for further processing and storage. The in-phase gain stage  404  has gain amplifiers MP 8 , MP 9  that couple the in-phase component into a down converter  430 . The down converter uses an input frequency that combines the chop frequency, fc, with the in-phase excitation current frequency fbio(0°). The resulting DC signal, ibio(0°), is provided to a current input ADC  408 , e.g., a CTΣΔ ADC. In a similar way, the quadrature phase gain stage  404  has gain amplifiers MP 10 , MP 11  that couple the quadrature phase component into a down converter  432 . The down converter uses an input frequency that combines the chop frequency, fc, with the quadrature phase excitation current frequency fbio(90°). The resulting DC signal, ibio(90°), is provided to a current input ADC  410 , e.g., a CTΣΔ ADC. By adding the chop frequency, fc, to the excitation current frequency, the effect of the initial chop mixer  426  is compensated for and the ADC outputs are the same or similar to that of  FIG.  2   , but with less distortion due to any imprecision in the input stage  402 . Except for using the chop frequency in the down converters  430 ,  432 , the gain stages and the CTΣΔ ADCs have the same configuration described above with respect to  FIG.  3   . 
       FIG.  5    is a sequence of graphs of different stages of the bio-impedance signal aligned on the horizontal axis as it is modified through the bio-impedance transducer  232 . At  502  a signal fbio has an amplitude of A in amperage from zero on the vertical axis. This signal, fbio, represents the absolute value of the amplitude of the alternating voltage signal applied to the body  208  from the excitation current source  202  by the electrodes  204 ,  206 . 
     At  504 , the signal has been received through the input stage  214  and coupled to the gain stage  216 . The signal still has an amplitude of A in amperes. Noise  524  has been added to the signal through the body and the input stage which may include common mode noise and other noise introduced by the circuits. 
     At  506 , the signal is amplified through the gain stage before being down converted and the amplitude is now G*A above zero on the vertical axis. The noise  526 , of the input stage is also amplified with the same gain factor The signal amplitude  516  is shown higher. At  508 , the signal  518  is a DC signal from the first mixer  218  and the second mixer  220 , which act as downconverters, and is shifted with respect to the noise  528 . The noise  528  is primarily flicker noise. Due to the mixing operation, the flicker noise is moved to higher frequencies at the right side of the graphs as shown in  508 . 
       FIG.  6    is a simplified circuit diagram of a current input second order continuous time sigma delta ADC suitable for use with the described embodiments. A CTΣΔ ADC filters out-of-band signals to virtually eliminate them. It also eliminates any requirement for an anti-aliasing filter. The input signal, Iin, whether the in-phase component or the quadrature phase component is received at an input port  602  of the ADC  600 . The input signal, Iin, is applied as an input to a first OTA  604  together with a reference voltage, Vref, input. The output is applied through a resistance  605  as an input to a second OTA  608  together with the same reference voltage, Vref. 
     A first capacitance  606  is coupled across the input to the first OTA  604  and the output of the first OTA  604  in the form of a capacitor. A second capacitance  610  is coupled across the input to the second OTA  608  and the output of the second OTA  608  in the form of a second capacitor. The capacitances  606 ,  610  absorb and discharge the input current as it is received so that the ADC can operate using a current input and not be subject to the noise of a resistance in a voltage input ADC. This allows for a more precise bio-impedance measurement result. 
     The ADC  600  further includes a latched comparator  612  that operates on an input clock, fclk,  614  to generate a digital data result on an output port  616 . A 1-bit DAC  618  is connected to the output port  616  and the input port  602  as a feedback loop. There may also be filters, interpolators, and a variety of other components to the ADC  600  (not shown). The described configuration is provided as an example and any of a variety of other ADCs may be used instead. 
     Embodiments as described herein provide for bio-impedance measurement using a very high CMRR input amplifier stage, for converting the voltage to current and for using a CTΣΔ ADC which also eliminates a requirement for an anti-aliasing filter. 
       FIG.  7    is a process flow diagram of operations performed by the bio-impedance transducer described herein. Block  702  is receiving a bio-impedance signal at an input stage. The bio-impedance signal has an oscillating voltage from two electrodes. The electrodes are coupled to a body. Block  704  is determining an alternating current of the bio-impedance signal through a resistance coupled across the two electrodes. 
     Block  706  is amplifying the alternating current. Block  708  is converting the amplified alternating current to a direct current bio-impedance signal. Block  710  is converting the direct current bio-impedance signal to a digital bio-impedance signal. 
     Boundaries between the above-described operations are provided as examples. Multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments 
     Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operations may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be implemented in an intermittent and/or alternating manner. 
     It should also be noted that at least some of the operations for the methods described herein may be implemented using software instructions stored on a computer useable storage medium for execution by a computer. As an example, an embodiment of a computer program product includes a computer useable storage medium to store a computer readable program. 
     Alternatively, embodiments of the invention may be implemented entirely in hardware or in an implementation containing both hardware and software elements. In embodiments which use software, the software may include but is not limited to firmware, resident software, microcode, etc. 
     The connections as discussed herein may be any type of connection suitable to transfer signals or power from or to the respective nodes, units, or devices, including via intermediate devices. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, a plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. The term “coupled” or similar language may include a direct physical connection or a connection through other intermediate components even when those intermediate components change the form of coupling from source to destination. 
     The described examples may be implemented on a single integrated circuit, for example in software in a digital signal processor (DSP) as part of a radio frequency integrated circuit (RFIC). The described examples may also be implemented in hardware in a field programmable gate array (FPGA) or application specific integrated circuit (ASIC), or in other electronic devices. The described examples may be implemented in analog circuitry, digital circuitry, or a combination of analog and digital circuitry. Alternatively, the circuit and/or component examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner. These examples may alternatively be implemented as software or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language or any other appropriate form. 
     It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures may be arranged and designed in a wide variety of different configurations. Thus, the more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.