Patent Publication Number: US-11388524-B2

Title: Differential ultrasonic transducer element for ultrasound devices

Description:
RELATED APPLICATIONS 
     This application is a continuation claiming the benefit under 35 U.S.C. § 120 of U.S. application Ser. No. 16/016,359, titled “DIFFERENTIAL ULTRASONIC TRANSDUCER ELEMENT FOR ULTRASOUND DEVICES” filed on Jun. 22, 2018, which is hereby incorporated herein by reference in its entirety. 
     U.S. application Ser. No. 16/016,359 claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 62/524,285, titled “DIFFERENTIAL ULTRASONIC TRANSDUCER ELEMENT FOR ULTRASOUND DEVICES” filed on Jun. 23, 2017, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD 
     Generally, the aspects of the technology described herein relate to ultrasonic transducers. Some aspects relate to differential ultrasonic transducer elements. 
     BACKGROUND 
     Capacitive micromachined ultrasonic transducers (CMUTs) are known devices that include a membrane above a micromachined cavity. The membrane may be used to transduce an acoustic signal into an electric signal, or vice versa. Thus, CMUTs can operate as ultrasonic transducers. 
     SUMMARY 
     According to at least one aspect, an ultrasound circuit is provided. The ultrasound circuit comprises a differential micromachined ultrasonic transducer (MUT) element and an integrated circuit coupled to the differential MUT element and configured to operate the differential MUT element in a differential receive mode and/or a differential transmit mode. 
     In some embodiments, the integrated circuit is configured to operate the differential MUT element in the differential receive mode and the differential transmit mode. In some embodiments, the differential MUT element is integrated into an ultrasonic transducer array and wherein the integrated circuit and the ultrasonic transducer array are formed on a single semiconductor die. In some embodiments, the differential MUT element is a differential capacitive micromachined ultrasonic transducer (CMUT) element or a differential piezoelectric micromachined ultrasonic transducer (PMUT) element. 
     According to at least one aspect, an ultrasound circuit is provided. The ultrasound circuit comprises a differential micromachined ultrasonic transducer (MUT) element comprising a first MUT that is configured to be biased with a first bias voltage and a second MUT that is configured to be biased with a second bias voltage and an integrated circuit coupled to the differential MUT element and configured to operate the differential MUT element. 
     In some embodiments, the first bias voltage is different from the second bias voltage. In some embodiments, the integrated circuit comprises transmit circuit that is configured to operate the differential MUT element to transmit acoustic signals. In some embodiments, the transmit circuit comprises a differential pulser that is configured to generate a first pulse signal to drive the first MUT and a second pulse signal that has an opposite polarity of the first pulse signal that is configured to drive the second MUT. 
     In some embodiments, the integrated circuit comprises receive circuit that is configured to operate the differential MUT element to receive acoustic signals. In some embodiments, the receive circuit comprises a differential transimpedance amplifier (TIA) having a first input coupled to the first MUT, a second input coupled to the second CMUT, a first output coupled to the first input by a first impedance, and a second output coupled to the second input by a second impedance. In some embodiments, the receive circuit comprises a differential analog-to-digital converter having a first input coupled to the first output of the differential TIA and a second input coupled to the second output of the differential TIA. In some embodiments, the receive circuit comprises a first switch coupled between the first input of the differential TIA and the first MUT and a second switch coupled between the second input of the differential TIA and the second MUT. 
     In some embodiments, the integrated circuit is configured to operate the differential MUT element in a plurality of modes comprising at least one mode selected from the group consisting of: a single-ended receive mode, a differential receive mode, a single-ended transmit mode, and a differential transmit mode. In some embodiments, the ultrasound circuit further comprises a third MUT that is biased with the first bias voltage and a fourth MUT that is biased with the second bias voltage. In some embodiments, the first MUT and the third MUT are arranged in a first row of a 2 by 2 array and wherein the second MUT and the fourth MUT are arranged in a second row of the 2 by 2 array. In some embodiments, the first MUT and the second MUT are arranged in a first row of a 2 by 2 array and wherein the third MUT and the fourth MUT are arranged in a second row of the 2 by 2 array. 
     According to at least one aspect, a method of operating an ultrasound circuit comprising a differential micromachined transducer (MUT) element is provided. The method comprises biasing the differential MUT element at least in part by biasing a first MUT of the differential MUT element with a first bias voltage and biasing a second MUT of the differential MUT element with a second bias voltage and operating the differential MUT element after biasing the differential MUT element. 
     In some embodiments, operating the differential MUT element comprises operating the differential MUT element to transmit acoustic signals at least in part by driving the first MUT with a first pulse signal and driving the second MUT with a second pulse signal that has an opposite polarity of the first pulse signal. In some embodiments, operating the differential MUT element comprises operating the differential MUT element to receive acoustic signals at least in part by controlling a state of at least one switch to couple the first MUT to a first input of a differential transimpedance amplifier (TIA) and couple the second MUT to a second input of the differential TIA. In some embodiments, operating the differential MUT to receive acoustic signals comprises digitizing an output of an analog processing circuit that comprises the differential TIA using a differential analog-to-digital converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and embodiments will be described with reference to the following exemplary and non-limiting figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear. 
         FIGS. 1A and 1B  show exemplary ultrasound circuits including a differential micromachined ultrasound transducer (MUT) element, in accordance with some embodiments of the technology described herein; 
         FIGS. 2A and 2B  show exemplary differential MUT elements, in accordance with some embodiments of the technology described herein. 
         FIG. 3  shows an exemplary ultrasound circuit including a differential MUT element, in accordance with some embodiments of the technology described herein; 
         FIG. 4A  shows the exemplary ultrasound circuit in  FIG. 3  operating in a differential transmit mode, in accordance with some embodiments of the technology described herein; 
         FIG. 4B  shows the exemplary ultrasound circuit in  FIG. 3  operating in a single-ended transmit mode, in accordance with some embodiments of the technology described herein; 
         FIG. 4C  shows the exemplary ultrasound circuit in  FIG. 3  operating in a differential receive mode, in accordance with some embodiments of the technology described herein; 
         FIG. 4D  shows the exemplary ultrasound circuit in  FIG. 3  operating in a single-ended receive mode, in accordance with some embodiments of the technology described herein; 
         FIGS. 5A and 5B  each show an exemplary ultrasound circuit including a differential MUT element; 
         FIG. 6  shows an exemplary method of operating an ultrasound circuit comprising a differential MUT element, in accordance with some embodiments of the technology described herein; 
         FIG. 7  shows an exemplary ultrasound device comprising the ultrasound circuit of  FIG. 1A , in accordance with some embodiments of the technology described herein; 
         FIGS. 8A-8H  show a pill comprising an ultrasound device, in accordance with some embodiments of the technology described herein; 
         FIGS. 9A and 9B  show a handheld device comprising an ultrasound device and a display, in accordance with some embodiments of the technology described herein; 
         FIGS. 9C-9E  show a wearable patch comprising an ultrasound device, in accordance with some embodiments of the technology described herein; 
         FIG. 10  shows a handheld ultrasound device in accordance with some embodiments of the technology described herein; and 
         FIG. 11  shows a detailed diagram of the exemplary ultrasound circuit in  FIG. 3  in accordance with some embodiments of the technology described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Some ultrasound devices comprise a plurality of capacitive micromachined ultrasonic transducers (CMUTs) configured to transmit and/or receive acoustic signals. These CMUTs are typically controlled using only single-ended signaling techniques. For example, the plurality of CMUTs may be driven in unison by the same pulse signal during transmission of an acoustic signal. Similarly, the electrical signals generated by each of the CMUTs during receipt of an acoustic signal may be separately received and processed by a respective receiver in a set of receivers. The inventors have appreciated that, as a result of their single-ended nature, such ultrasound devices are susceptible to numerous noise sources that may undesirably degrade electric signals from (or going to) the CMUTs. For example, the electric signals from the CMUTs may be corrupted by noise from supply voltage lines, bias voltage lines, and/or ground lines. The signal degradation caused by these various sources may reduce the quality of ultrasound images formed using such ultrasound devices. 
     Accordingly, some embodiments of the present application provide an ultrasound circuit that utilizes differential micromachined ultrasonic transducer (MUT) technology. In particular, in accordance with an aspect of the present application, a differential MUT element is described herein that may be employed in combination with differential signaling techniques (e.g., pseudo differential signaling techniques and/or fully differential signaling techniques). The differential MUT elements described herein may be implemented using any of a variety of MUTs such as piezoelectric micromachined ultrasonic transducers (PMUTs) or CMUTs. Such a differential configuration and operating scheme may reduce or otherwise eliminate the degradation caused by various noise sources and decrease signal processing distortion. Thus, ultrasound devices including such differential MUT technology may be more robust and may produce higher fidelity images. 
     The differential MUT element may comprise multiple MUTs, such as PMUTs and/or CMUTs, that are biased with bias voltages. These bias voltages may be the same or different for MUTs within the differential MUT element. For example, the differential MUT element may comprise a first MUT that is biased with a positive voltage and a second MUT that is adjacent the first MUT and biased with a negative voltage, such that the electric signals generated by the first MUT during receipt of an acoustic signal may have an opposite polarity of those generated by the second MUT. Such biasing of the differential MUT element may facilitate the use of differential signaling techniques in some implementations. For example, a receive circuit coupled to the differential MUT element may process electric signals from the differential MUT element by identifying a difference between the electric signals from the first and second MUTs in the differential MUT element. As a result, noise that similarly impacts the electric signals from both MUTs (such as noise from nearby voltage supply lines) may be canceled out because such noise does not impact the difference between the two electric signals. In another example, a differential pulser driving a differential MUT element may nearly eliminate the current injected into the ground reference node, which reduces undesirable ground bounce that may interfere with circuit operation. Thus, the differential pulser can apply much larger transmit waveforms to the differential MUT before deleterious effects occur allowing for larger transmit pressures that enlarge the receive echoes. As a result, the quality of ultrasound data and/or images produced using such a differential MUT element may be improved. 
     The aspects and embodiments described above, as well as additional aspects and embodiments, are described further below. These aspects and/or embodiments may be used individually, all together, or in any combination of two or more, as the application is not limited in this respect. 
       FIG. 1A  shows an example ultrasound circuit  100 A comprising a differential MUT element  102 . The differential MUT element  102  comprises a MUT  104 A that is biased with a positive bias voltage  106 A and MUT  104 B that is biased with a negative bias voltage  106 B. The differential MUT element  102  is operated by (and coupled to) an integrated circuit  108 . The integrated circuit  108  comprises transmit (TX) circuits  110 , receive (RX) circuits  112 , and a signal conditioning/processing circuit  114 . 
     The differential MUT element  102  comprises MUTs  104 A and  104 B that may each include two electrodes (e.g., plates). In a CMUT, the two electrodes may be separated by a cavity. A first electrode (e.g., a top electrode) in the CMUT may be allowed to move with respect to the second electrode (e.g., a bottom electrode), and the electrical properties of the CMUT may change as the top electrode moves with respect to the bottom electrode. The top electrode may be implemented as, for example, a metalized membrane and the bottom electrode may be implemented as, for example, a doped silicon substrate. A CMUT may further comprise an insulating layer between the top and bottom electrodes to prevent the CMUT from electrically shorting in the event the top electrode comes in contact with the bottom electrode, as can happen during collapse mode operation, as an example. In a PMUT, the two electrodes may be separated by a piezoelectric material that generates an electric signal when deformed and, conversely, deforms when an electric signal is applied. 
     The MUTs  104 A and  104 B may be biased by, for example, coupling one of the two electrodes (e.g., the top electrode) to a bias voltage (e.g., positive bias voltage  106 A and/or negative bias voltage  106 B). In some embodiments, the MUTs  104 A and  104 B are biased with different voltages. For example, the MUT  104 A may be biased with a first voltage (e.g., the positive bias voltage  106 A) and the MUT  104 B may be biased with a second voltage that has an opposite polarity of the first voltage (e.g., the negative bias voltage  106 B). In examples where additional MUTs are employed in the differential MUT element  102 , a first portion (e.g., a first half) of the MUTs may be biased with the first voltage (e.g., the positive bias voltage  106 A) and a second portion of the MUTs (e.g., a second half) may be biased with the second voltage (e.g., the negative bias voltage  106 B). 
     The transmit circuit  110  may be configured to operate the differential MUT element  102  to generate acoustic signals. For example, the transmit circuit  110  may be configured to apply an alternating current (AC) signal (e.g., a pulse signal) to one of the electrodes (e.g., the bottom electrode) of one or more MUTs in the differential MUT element  102  (e.g., MUTs  104 A and/or  104 B) to generate an acoustic signal. In some embodiments, the transmit circuit  110  employs a pulser  116  to generate the pulse signal. The pulser  116  may be, for example, configured to generate unipolar pulses and/or bipolar pulses to drive the MUTs  104 A and/or  104 B. In these embodiments, the pulser  116  may receive a waveform from a waveform generator  118  and generate the pulse signal based on this received waveform. It should be appreciated that the pulses provided by the pulser  116  to the MUTs  104 A and  104 B need not be completely in-phase (e.g., have a 0 degree phase difference) or completely out of phase (e.g., have a 180 degree phase difference). For example, the pulses provided to the MUT  104 A may be delayed by a quarter pulse period (e.g., have a 90 degree phase difference) relative to the pulses provided to the MUT  104 B. 
     The receive circuit  112  may be configured to receive and process electronic signals generated by the differential MUT element  102  when acoustic signals impinge upon the element. In some embodiments, the receive circuit  112  comprises a switch  120  (sometimes referred to as a “receive switch”) that selectively couples one or more components of the receive circuit  112  to one or more MUTs in the differential MUT element  102  (e.g., the MUTs  104 A and/or  104 B) based on an operating mode of the ultrasound circuit  100 A (e.g., transmit mode or receive mode). For example, the switch  120  may be open when the ultrasound circuit  100 A is operating in a transmit mode and closed when the ultrasound circuit  100 A is operating in a receive mode. The receive circuit  112  may comprise one or more components to detect and/or process electronic signals generated by the differential MUT element  102 . For example, the receive circuit  112  may comprise analog processing circuit  122  that processes a signal (e.g., a voltage signal or a current signal) indicative of a displacement of a top electrode relative to the bottom electrode. The analog processing circuit  122  may comprise any of a variety of components such as: a transimpedance amplifier (TIA), a variable-gain amplifier, a delay line, a time-gain-compensation amplifiers, a buffer, and/or a mixer. An output signal of the analog processing circuit  122  may be digitized by an analog-to-digital converter (ADC)  124 . The ADC  124  may comprise a differential ADC and/or a single-ended ADC. Example ADCs include 8-bit, 10-bit, or 12-bit, 20 Msps, 25 Msps, 40 Msps, 50 Msps, or 80 Msps ADCs. Additional example ADCs include oversampled ADCs such as continuous-time or discrete-time, and/or low-pass or band-pass oversampled ADCs. The digital signal from the ADC  124  may be processed (e.g., filtered or otherwise manipulated) by a digital processing circuit  126 . The digital processing circuit  126  may comprise memory such as dynamic random-access memory (DRAM) and/or static random-access memory (SRAM). The memory may store, for example, information regarding a received ultrasound signal for processing (e.g., by a digital signal processor). 
     In some embodiments, the digital processing circuit  126  may filter the received ultrasound data from the ADC  124  (e.g., to reduce the data rate) and store the ultrasound data in memory. In turn, the ultrasound data stored in memory may be offloaded from the ultrasound circuit  100 A to another device. It should be appreciated that the rate at which the ultrasound data is captured may be different from the rate at which ultrasound data stored in memory is offloaded from the ultrasound circuit  100 A. For example, the rate at which the ultrasound data is captured may be faster than the rate at which the ultrasound data is transmitted to an external device. 
     The integrated circuit  108  may comprise a plurality of transmit circuits  110  and/or receive circuits  112  as shown in  FIG. 1A . For example, the differential MUT element  102  may be part of a transducer array that comprises a plurality of differential MUT elements  102 . In this non-limiting example, each of the differential MUT elements  102  may be coupled to a separate transmit circuit  110  and/or a separate receive circuit  112  in the integrated circuit  108 . However, other configurations are possible, such as two or more differential MUT elements  102  sharing a transmit circuit  110  and/or a receive circuit  112 . In some embodiments, all differential MUT elements  102  are coupled to share the same transmit circuit  110  and/or receive circuit  112 . 
     In embodiments where the ultrasound circuit  100 A comprises multiple receive circuits  112 , the outputs of all of the receive circuits  112  on the integrated circuit  108  may be fed to a multiplexer (MUX)  128  in the signal conditioning/processing circuit  114 . The MUX  128  multiplexes the digital data from each of the receive circuits  112 , and the output of the MUX  128  is fed to a multiplexed digital processing circuit  130  in the signal conditioning/processing circuit  114 , for final processing before the data is output from the integrated circuit  108  using, for example, one or more high-speed serial output ports and/or one or more lower speed, parallel output ports. 
     It should be appreciated that various alterations may be made to the integrated circuit  108  without departing from the scope of the present disclosure. In some embodiments, one or more components of the integrated circuit  108  may be removed or added. For example, the MUX  128  may be removed in embodiments where parallel signal processing is performed and/or the switches  120  may be removed in embodiments where the MUTs  104 A and/or  104 B are hardwired to the TX circuit  110  and/or the RX circuit  120 . Additionally (or alternatively), the switch  120  in the RX circuits  112  may be replaced with a switch matrix  121  in some embodiments. In these embodiments, the switch matrix  121  may selectively couple MUTs  104 A and/or  104 B within the differential MUT element  102  to particular transmit circuits  110 , receive circuits  112 , particular components within the transmit circuits  110 , and/or particular components with the receive circuits  112 . Thereby, the connections between the bottom electrodes of the MUTs  104 A and  104 B may be dynamically connected to components within the integrated circuit  108 . Such a feature may be employed to generate and/or receive acoustic signals using a selected portion of the MUTs  104 A and/or  104 B in a transducer array. The selected portion of the MUTs  104 A and/or  104 B may be selected consistent with, for example, a coding scheme such as a Hadamard coding scheme. 
     In some embodiments, the MUTs (e.g., MUTs  104 A and  104 B) in the differential MUT element  102  may be biased such that one or more MUTs, and in some situations each MUT, is adjacent at least one other MUT that is biased using a voltage with an opposite polarity. As shown in  FIGs. 1A and 1B  for example, the differential MUT element  102  may comprise MUT  104 A that is biased with the positive bias voltage  106 A and CMUT  104 B that is adjacent MUT  104 A and is biased with the negative bias voltage  106 B. In other examples, the differential MUT element  102  may comprise four MUTs arranged in a 2 by 2 array (e.g., an array with two rows and two columns).  FIGS. 2A and 2B  illustrate examples of such differential MUT elements. 
     As shown in  FIG. 2A , the differential MUT element  202 A comprises four MUTs arranged in a 2 by 2 array. The MUTs  104 A in the top left and bottom right corners are biased with the positive bias voltage  106 A and the MUTs  104 B in the top right and bottom left corners are biased with the negative bias voltage  106 B. Thus, in this non-limiting example, each of the MUTs is adjacent at least two other MUTs that are biased using a voltage with an opposite polarity. In some embodiments, one or more MUTs of a differential MUT element are adjacent at least two other MUTs biased using a voltage with an opposite polarity. The configuration shown in  FIG. 2A  may be a common-centroid configuration where the centroid of the MUTs  104 A is the same as the centroid of the MUTs  104 B. Such a common centroid configuration may advantageously reject noise caused by, for example, a linear gradient in one or more parameters of the MUTs  104 A and  104 B. 
     As shown in  FIG. 2B , the differential MUT element  202 B comprises four MUTs arranged in a 2 by 2 array. The MUTs  104 A in the top row are biased with the positive bias voltage  106 A and the MUTs  104 B in the bottom row are biased with the negative bias voltage  106 B. Thus, in this non-limiting example, each of the MUTs is adjacent at least one other MUT that is biased using a voltage with an opposite polarity, although other configurations are possible. For example, one or more MUTs of a differential MUT element may be adjacent at least one other MUT biased using a voltage with an opposite polarity. 
     It should be appreciated that the depictions of differential MUT elements  102 ,  202 A and  202 B in  FIGS. 1, 2A and 2B , respectively, with two or four MUTs with a circular shape is only for illustration. The differential MUT elements  102 ,  202 A, and/or  202 B may include additional (or fewer) MUTs. For example, the differential MUT elements  102 ,  202 A, and/or  202 B may include 3, 5, 6, 7, 8, or 9 MUTs. In some embodiments, the differential MUT elements  102 ,  202 A, and/or  202 B may have an even number of MUTs (e.g., 2, 4, 6, 8, 10, or 12 MUTs). Further, one or more of the MUTs in the differential MUT elements  102 ,  202 A, and  202 B may have a non-circular shape such as: a hexagonal shape or an octagonal shape. 
       FIG. 3  shows an exemplary ultrasound circuit  300  comprising a differential MUT element formed by MUTs  304 A and  304 B coupled to bias voltages sources  302 A and  302 B, respectively. The ultrasound circuit  300  further comprises transmit circuits  110 A and  110 B and receive circuit  112  coupled to the MUTs  304 A and  304 B. Each of the MUTs  304 A and  304 B comprises a first electrode  306 A and  306 B, respectively, and a second electrode  308 A and  308 B, respectively. In embodiments where the MUTs  304 A and  304 B are CMUTs, the first electrode  306 A and  306 B, respectively, may be allowed to move with respect to a second electrode  308 A and  308 B, respectively. The movement of the first electrodes  306 A and  306 B relative to the second electrodes  308 A and  308 B, respectively, may be analyzed by the receive circuit  112  to process received acoustic signals. The transmit circuits  110 A and  110 B may use pulse signals to cause the first electrodes  306 A and  306 B to move relative to the second electrodes  308 A and  308 B, respectively, to generate acoustic signals. In embodiments where the MUTs  304 A and  304 B are PMUTs, the potential across the first electrodes  306 A and  306 B and the second electrodes,  308 A and  308 B, respectively, may be measured by the receive circuit  112  to identify a deformation of a piezoelectric between the electrodes and, thereby, analyze received acoustic signals. Conversely, the transmit circuits  110 A and  110 B may use pulse signals to cause the piezoelectric material between the electrodes to deform and, thereby, generate acoustic signals. 
     The first electrodes  306 A and  306 B may be coupled to bias voltage sources  302 A and  302 B, respectively. The bias voltage sources  302 A and  302 B may generate bias voltages for the MUTs  304 A and  304 B, respectively. The bias voltage sources  302 A and/or  302 B may be located on the same chip as the MUTs  304 A and  304 B or another chip that is external to the MUTs  304 A and  304 B. The bias voltage sources  302 A and  302 B may be fixed voltage sources or variable voltage sources. For example, the bias voltage sources  302 A and  302 B may be variable voltage sources that receive voltage control signals  310 A and  310 B, respectively, and generate a voltage based on the respective control signal. Thereby, the bias voltage generated by the viable voltage sources may be adjusted differently for different modes of operation (e.g., a transmit mode of operation and a receive mode of operation). In some embodiments, the bias voltages generated by the bias voltage source  302 A and  302 B may have an opposite polarity. For example, the bias voltage source  302 A may generate a positive voltage and the bias voltage source  302 B may generate a negative voltage. 
     The second electrodes  308 A and  308 B may be coupled to transmit circuits  110 A and  110 B, respectively. The transmit circuits  110 A and  110 B may be configured to drive the MUTs  304 A and  304 B, respectively, in unison using one or more pulse signals. For example, the first electrode  306 A may be attracted to the second electrode  308 A when the first electrode  306 B is also attracted to the second electrode  308 B. The waveforms generated by the waveform generators  118 A and  118 B (and thereby the pulse signals from the pulsers  116 A and  116 B) may be adjusted using waveform control signals  314 A and  314 B, respectively, based on the bias voltages applied to the MUTs  304 A and  304 B. For example, the MUTs  304 A and  304 B may be biased with voltages that have an opposite polarity. In this example, the pulse signal generated by the pulser  116 A may have an opposite polarity of the pulse signal generated by the pulse  116 B such that the MUTs  304 A and  304 B are driven in unison. In another example, the bias voltage applied to both MUTs  304 A and  304 B may be the same. In this example, the pulse signal generated by the pulses  116 A and  116 B may be the same. 
     In some embodiments, the connections of the electrodes  306 A and  308 A of the MUT  304 A may be swapped relative to the connections of the electrodes  306 B and  308 B of the MUT  304 B. For example, the second electrode  308 B may be coupled to the bias voltage source  302 B while the second electrode  308 A is coupled to the transmit circuit  110 A and the receive circuit  112 . Further, the first electrode  306 B may be coupled to the transmit circuit  110 B and the receive circuit  112  while the first electrode  306 A may be coupled to the bias voltage source  302 A. Such a configuration of the ultrasound circuit  300  may be employed in, for example, embodiments where the MUTs  304 A and  304 B in a differential MUT element are implemented as PMUTs. 
     It should be appreciated that the transmit circuits  110 A and  110 B need not be two separate circuits with two separate pulsers  116 A and  116 B as shown in  FIG. 3 . For example, the transmit circuits  110 A and  110 B may be implemented in a single circuit with a single pulser (in place of the pulsers  116 A and  116 B) and a single waveform generator (in place of waveform generators  118 A and  118 B). The single pulser may be constructed using, for example, one or more differential or single-ended pulsers. The single pulser may be, for example, configured to generate two sets of pulse signals. For example, the single pulser may generate a first pulse signal for the MUT  304 A and a second pulse signal for the MUT  304 B. The first pulse signal may be phase shifted relative to the second pulse signal. For example, the first pulse signal may be phase shifted by 180 degrees (e.g., have an opposite polarity) relative to the second pulse signal. In another example, the first pulse signal may be phase shifted by less than 180 degrees relative to the second pulse signal (e.g., phase shifted by 120 degrees, 90 degrees, or 30 degrees). 
     The second electrodes  308 A and  308 B may also be coupled (e.g., switchably coupled) to the receive circuit  112 . The receive circuit  112  may comprise switches  120 A and  120 B that selectively couple one or more components of the receive circuit  112  (such as the analog processing circuit  122 , the ADC  124  and/or digital processing circuit  126 ) to the second electrodes  308 A and  308 B, respectively. The state of the switches  120 A and  120 B may be controlled by switch control signals  312 A and  312 B respectively. These control signals may be generated based on, for example, an operating mode of the ultrasound circuit  300 . For example, the ultrasound circuit may be operating in a transmit mode and the switches  120 A and  120 B may be open to avoid receiving the pulse signal from the pulsers  116 A and  116 B. Conversely, the switches  120 A and  120 B may be closed when the ultrasound circuit is operating in a receive mode to allow the receive circuit to detect signals from the MUTs  304 A and  304 B. 
     It should be appreciated that the receive circuit  112  may comprise more (or less) than two switches that selectively couple the second electrodes  308 A and  308 B to the receive circuit  112 . For example, the switches  120 A and  120 B may be omitted in some embodiments. In these embodiments, a portion of the MUTs in a given differential MUT element may be hardwired to the receive circuit  112 , the transmit circuit  110 A, and/or the transmit circuit  110 B. Such a configuration may reduce the transmit power and/or receive responsivity and advantageously eliminate any parasitic elements of the switches  120 A and  120 B. In other embodiments, the receive circuit  112  may comprise more than two switches (e.g., four switches) and/or a switch matrix that is configured to selectively couple each of the second electrodes  308 A and  308 B to two or more points in the analog processing circuit  122 . For example, the second electrode  308 A may be selectively coupled (e.g., using a switch matrix) to a first input terminal or a second input terminal of a TIA in the analog processing circuit  122 . 
       FIG. 11  shows an ultrasound circuit  1100  that is a more detailed diagram of the ultrasound circuit  300 . As shown, the ultrasound circuit  1100  comprises MUTs  304 A and  304 B that have a first electrode coupled to a positive bias voltage (VBIAS+) and a negative bias voltage (VBIAS−), respectively, and a second electrode coupled to pulsers  116 A and  116 B, respectively. As shown, the second electrode of the MUTs  304 A and  304 B may be switchably coupled to the analog processing circuit  122  by a set of transistors including, for example, those transistors in the switches  120 A and  120 B. 
     The pulsers  116 A and  116 B comprise two transistors coupled in series that are coupled between a positive supply voltage V+ and a negative supply voltage V−. The transistors in the pulsers  116 A and  116 B may be, for example, high-voltage transistors. The state of these transistors may be changed by control signals HI 1 , LO 1 , HI 2 , and LO 2  (e.g., generated by a waveform generator) in, for example, a fully differential or pseudo differential fashion. These control signals may, for example, control the transistors to selectively couple the second electrode of the MUTs  304 A and/or  304 B to the positive supply voltage V+ or the negative supply voltage V− to drive the MUTs  304 A and  304 B. The pulsers  116 A and  116 B may be controlled independently to, for example, enable a differential transmit mode where the second electrodes of the MUTs  304 A and  304 B are coupled to the positive supply voltage V+ at different times. The design of the ultrasound circuit  1100  advantageously implements the pulsers  116 A and  116 B with fewer transistors than simply putting two single-ended pulsers together. Thereby, the ultrasound circuit  1100  may consume less power than conventional approaches during operation (e.g., during transmit operation). 
     The switches  120 A and  120 B comprise two transistors coupled in series and a diode coupled there-between. The transistors in the switches  120 A and  120 B may be, for example, high-voltage transistors. The state of these transistors may be changed by control signals TR_G 1 , TR_S 1 , TR_G 2 , and TR_S 2  in, for example, a common-mode fashion (e.g., change states in unison). As shown, the switches  120 A and  120 B may be selectively coupled to each other by two transistors controlled by the control signal TR. These transistors between the switches  120 A and  120 B may be, for example, low voltage transistors. 
     The analog processing circuit  122  may comprise a low noise amplifier (LNA) with a first input that is coupled to the switch  120 A and a second input that is coupled to the switch  120 B. The LNA may comprise a first output coupled to the first input by a first impedance and a second output that is coupled to the second input by a second impedance. The LNA in combination with the first and second impedences may form a TIA. The outputs of the LNA may be provided to, for example, other components of the analog processing circuit  122  (not shown) and/or to an ADC (not shown). 
     Ultrasound circuits including differential MUT elements, such as the differential MUT elements described herein, may be operated in various modes. Example modes are described in connection with ultrasound circuit  300  and include: a differential receive mode, a single-ended receive mode, a differential transmit mode, and a single-ended transmit mode. Various combination of these modes may also be used, and the ultrasound circuit  300  may be configurable/controllable to allow for selection of a desired mode, or combination of modes, to suit a particular application. Example configurations of the ultrasound circuit  300  in each of these modes is shown in  FIGS. 4A-4D . Table 1 below shows the particular mode of operation depicted in each of  FIGS. 4A-4D . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Example Modes of Operation of a Differential 
               
               
                 CMUT Ultrasound Device 
               
            
           
           
               
               
               
            
               
                   
                 Mode of Operation 
                 FIG. Number 
               
               
                   
                   
               
               
                   
                 Differential transmit mode 
                 FIG. 4A 
               
               
                   
                 Single-ended transmit mode 
                 FIG. 4B 
               
               
                   
                 Differential receive mode 
                 FIG. 4C 
               
               
                   
                 Single-ended receive mode 
                 FIG. 4D 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 4A  shows the ultrasound circuit  300  operating in a differential transmit mode. The differential transmit mode may be achieved, for example, by: (1) biasing MUTs  304 A and  304 B with bias voltages having an opposite polarity, (2) opening the switches  120 A and  120 B to disconnect the receive circuit  112  from the MUTs  304 A and  304 B, and (3) driving the MUTs  304 A and  304 B with pulse signals having an opposite polarity. In the differential transmit mode, the biasing of the MUTs  304 A and  304 B in combination with the pulse signals causes the MUTs  304 A and  304 B to be driven in unison (e.g., the first electrodes  306 A and  306 B may move in the same direction at the same time) while a direction of the current  401 A in the top branch of the ultrasound circuit  300  is opposite a direction of the current  401 B in a bottom branch of the ultrasound circuit  300 . The opposite direction of current in the top and bottom branches of the circuit may advantageously reduce (or eliminate) ground bounce in the ultrasound circuit  300  that may impact operation of other components in the ultrasound circuit  300 . For example, the currents in the top and bottom branches of the ultrasound circuit  300  may destructively interfere because these branch currents may have an approximately equal (and/or exactly equal) magnitude and opposite polarity. As a result, little or no current leaves from (or enters) the ground node during differential transmit operation, which advantageously reduces (or eliminates) ground bounce. 
       FIG. 4B  shows the ultrasound circuit  300  operating in a single-ended transmit mode. The single-ended transmit mode may be achieved, for example, by: (1) biasing MUTs  304 A and  304 B with bias voltages having the same polarity, (2) opening the switches  120 A and  120 B to disconnect the receive circuit  112  from the MUTs  304 A and  304 B, and (3) driving the MUTs  304 A and  304 B with pulse signals that have the same polarity. In the single-ended transmit mode, the biasing of the MUTs  304 A and  304 B in combination with the pulse signals causes the MUTs  304 A and  304 B to be driven in unison (e.g., the first electrodes  306 A and  306 B may move in the same direction at the same time) while a direction of the current  403 A in the top branch of the ultrasound circuit  300  is the same as a direction of the current  403 B in a bottom branch of the ultrasound circuit  300 . 
       FIG. 4C  shows the ultrasound circuit  300  operating in a differential receive mode. The differential receive mode may be achieved, for example, by: (1) biasing MUTs  304 A and  304 B with bias voltages having an opposite polarity and (2) closing the switches  120 A and  120 B to connect the receive circuit  112  to the MUTs  304 A and  304 B. In the differential transmit mode, the biasing of the MUTs  304 A and  304 B causes a direction of the current  405 A in the top branch of the ultrasound circuit  300  to be opposite a direction of the current  405 B in a bottom branch of the ultrasound circuit  300 . Thus, the receive circuit  112  may measure the difference between the signals from the MUTs  304 A and  304 B to identify characteristics of the acoustic signal incident on the MUTs  304 A and  304 B. Employing the difference between signals from the MUTs  304 A and  304 B may advantageously cancel out noise from noise sources that similarly impact the electrical signals from both MUTs  304 A and  304 B. The receive circuit  112  may measure the difference between the signals using a differential TIA  402  in the analog processing circuitry  122 . The differential TIA  402  may have a first input coupled to the second electrode  308 A, a second input coupled to the second electrode  308 B, a first output coupled to the first input by an impedance  404 , and a second output coupled to the second input by an impedance  406 . The two outputs of the differential TIA  402  may be provided to additional circuitry within the analog processing circuit  122  (such as a variable-gain amplifier, a delay line, a time-gain-compensation amplifiers, a buffer, and/or a mixer) and then to the ADC  124  or provided directly to the ADC  124  (as shown in  FIG. 4C ). The ADC  124  may be implemented as, for example, a differential ADC that is configured to provide a digital value that is indicative of a difference between the voltages received at the two inputs. 
       FIG. 4D  shows the ultrasound circuit  300  operating in a single-ended receive mode. The single-ended receive mode may be achieved, for example, by: (1) biasing the first electrodes  306 A and  306 B with bias voltages having the same polarity and (2) closing the switches  120 A and  120 B to connect the receive circuit  112  to the MUTs  304 A and  304 B. In the single-ended transmit mode, the biasing of the MUTs  304 A and  304 B causes a direction of the current  407 A in the top branch of the ultrasound circuit  300  to be the same as a direction of the current  407 B in a bottom branch of the ultrasound circuit  300 . The receive circuit  112  may measure the signals from the MUTs  304 A and  304 B individually (e.g., without combining them). For example the receive circuit  112  may separately process and digitize the signals from the MUTs  304 A and  304 B. 
     In some embodiments, single-ended transmit and/or receive modes may allow fewer MUTs to be employed to obtain the same spatial resolution as differential transmit and/or receive modes without adversely impacting image quality in certain operating conditions where the signal-to-noise ratio is high (e.g., in shallow ultrasound imaging). In these embodiments, the ultrasound circuit may operate in single-ended transmit and/or single-ended receive modes to consume less power when operating in these conditions without noticeably degrading the resulting ultrasound image. 
     In some embodiments, the ultrasound circuit  300  may be configurable between a plurality of modes, such as two or more of the modes shown in Table 1. For example, the ultrasound circuit  300  may be configurable between: (1) a differential transmit mode and a differential receive mode; (2) a differential transmit mode and a single-ended receive mode; (3) a differential transmit mode, a single-ended receive mode, and a differential receive mode; (4) a single-ended transmit mode and a differential receive mode; (5) a single-ended transmit mode and a single-ended receive mode; (6) a single-ended transmit mode, a single-ended receive mode, and a differential receive mode; (7) a differential transmit mode, a single-ended transmit mode, and a differential receive mode; (8) a differential transmit mode, a single-ended transmit mode, and a single-ended receive mode; or (9) a differential transmit mode, a single-ended transmit mode, a single-ended receive mode, and a differential receive mode. The mode of operation of the ultrasound circuit  300  may be configurable using one or more control signals. The control signals may: (1) adjust a bias voltage applied by one or more of the bias voltage sources  302 A and  302 B such as voltage control signals  310 A and  310 B; (2) change a state of one or more of the switches  120 A and  120 B such as switch control signals  312 A and  312 B; and/or (3) change a waveform generated by one or more of the waveform generates  118 A and  118 B such as waveform control signals  314 A and  314 B. The control signals may be generated by control circuits (such as timing and control circuit  708  described below with reference to  FIG. 7 ) that may be located on the same chip as the ultrasound circuit  300  or on a different chip. 
     It should be appreciated that the ultrasound circuit  300  may be coupled to the MUTs  304 A and  304 B in a different way than illustrated in  FIG. 3 . The particular way in which the ultrasound circuit  300  is coupled to the MUTs  304 A and  304 B may, for example, depend on the construction of the MUTs  304 . In some embodiments, the MUTs  304 A and  304 B may be implemented as PMUTs where the polarity of the signal applied to the PMUTs may impact the performance of the PMUT. In these embodiments, the connections to the MUT  304 B may be reversed relative to the connections to MUT  304 A such that the current direction in the top and bottom branches of the ultrasound circuit  300  match during operation in a differential transmit mode and/or differential receive mode. An example of such an ultrasound circuit is shown in  FIG. 5A  by ultrasound circuit  500 A. As shown, the connections to the first electrode  306 B are swapped with the connections to the second electrode  308 B relative to the configuration shown in ultrasound circuit  300 . In particular, the TX circuits  110 A and  110 B and the RX circuit  120  are coupled to the second electrode  308 A of the MUT  304 A and coupled to the first electrode  306 B of the MUT  304 B. Further, the bias voltage source  302 A is coupled to the first electrode  306 A of the MUT  304 A and the bias voltage source  302 B is coupled to the second electrode  308 B of the MUT  304 B. 
     One or more switches may be integrated into the ultrasound circuits  300  and/or  500 A to enable the connections to the electrodes of the MUTs  304 A and/or  304 B to be swapped based on, for example, a current mode of operation of the ultrasound circuit. In some embodiments, the switches may be controlled such that the current direction in the top and bottom branches of the ultrasound circuit  300  match during one or more of (or all of) the operation modes. Controlling the switches in such a fashion may, for example, advantageously improve the performance of ultrasound circuits implemented using PMUTs where the polarity of the signal applied to the PMUTs impacts the performance of the PMUT. In these embodiments, the switches may be controlled such that the bias voltage sources  302 A and  302 B are coupled to first electrodes  306 A and  306 B, respectively, during operation in single-ended transmit mode and/or single-ended receive mode and the bias voltage sources  302 A and  302 B are coupled to first electrode  306 A and second electrode  308 B, respectively, during operation in differential receive mode and/or differential transmit mode. An example of such an ultrasound circuit is shown in  FIG. 5B  by ultrasound circuit  500 B. As shown, ultrasound circuit  500 B adds switches  502 A and  502 B that are controlled using switch control signals  504 A and  504 B, respectively, relative to the ultrasound circuits  500 A and  300  described above. 
     The switches  502 A and  502 B may each be constructed as, for example, a set of one or more switches that selectively couple any one of the inputs to any one of the outputs. For example, the switch  502 A may be constructed to selectively couple the bias voltage source  302 A to the first electrode  306 A or the second electrode  308 A and selectively couple the TX and RX circuits  110 A,  110 B, and  112  to the first electrode  306 A or the second electrode  308 A based on a received switch control signal  504 A. The switch  502 B may be constructed to selectively couple the bias voltage source  302 B to the first electrode  306 B or the second electrode  308 B and selectively couple the TX and RX circuits  110 A,  110 B, and  112  to the first electrode  306 B or the second electrode  308 B based on a received switch control signal  504 B. In a differential receive mode and/or a differential transmit mode, the switches  502 A and/or  502 B may be controlled such that the bias voltage sources  302 A and  302 B are coupled to first electrode  306 A and second electrode  308 B, respectively. Further, the bias voltage sources  302 A and  302 B may be controlled so as to generate output voltages with opposite polarities. In a single-ended receive mode and/or a single-ended transmit mode, the switches  502 A and/or  502 B may be controlled such that the bias voltage sources  302 A and  302 B are coupled to first electrodes  306 A and  306 B, respectively. Further, the bias voltage sources  302 A and  302 B may be controlled so as to generate output voltages with the same polarity (e.g., the same output voltage). Thus, the switches  502 A and  502 B may enable the ultrasound circuit  500 B to change the direction in which current is applied to the MUTs  304 A and/or  304 B such that, for example, the direction of current applied to the MUT  304 A matches the direction of current applied to the MUT  304 B. 
     It should be appreciated that various alterations may be made to the ultrasound circuit  500 B without departing from the scope of the present disclosure. In some embodiments, the ultrasound circuit  500 B may omit one of the switches  502 A and  502 B. Thus, the direction in which current is applied to one of the MUTs may be fixed for a given mode of operation. In these embodiments, the remaining switch (e.g., either switch  502 A or switch  502 B) may be controlled such that the direction of current applied to the second MUT matches the direction of current applied to the first MUT in the given mode of operation. Thus, the same effect of matching the current direction in each of the top and bottom branches in the ultrasound circuit  500 B may be achieved using a smaller number of switches. 
       FIG. 6  shows an example method  600  of operating an ultrasound circuit comprising a differential MUT element. As shown, the method  600  comprises an act  602  of biasing the differential MUT element and an act  603  of operating the differential MUT element. The act  603  of operating the differential MUT element may comprise, for example, an act  604  of driving the differential MUT element with a pulse signal, an act  606  of controlling a state of a switch, and an act  608  of receiving a signal from the differential MUT element. 
     In act  602 , the differential MUT element may be biased. The differential MUT element may be biased by, for example, applying a bias voltage to one electrode of the MUT(s) in the differential MUT element. The bias voltages may be generated by, for example, bias voltage sources. These bias voltage sources may be variable voltage sources that are capable of providing a plurality of different voltages. In some embodiments, the variable voltage sources may be controlled using one or more control signals (e.g., generated by one or more control circuits) based on a particular mode of operation of the ultrasound circuit. For example, the ultrasound circuit may be operating in a single-ended receive or transmit mode and the variable voltage sources may be controlled such that all of the MUTs in the differential MUT element are biased with the same voltage. In another example, the ultrasound circuit may be operating in a differential receive or differential transmit mode and the variable voltage sources may be controlled such that a first portion of the MUTs in the differential MUT element are biased with a first voltage and a second portion of the MUTs in the differential element are biased with a second voltage that has an opposite polarity of the first voltage. 
     In act  603 , the differential MUT element may be operated to transmit and/or receive acoustic signals based on a current mode of operation of the ultrasound circuit. For example, the differential MUT element may be operated to transmit acoustic signals when the ultrasound circuit is operating in a differential transmit or a single-ended transmit mode and operated to receive acoustic signals when the ultrasound circuit is operating in a differential receive or a single-ended receive mode. 
     The differential MUT element may be operated to transmit acoustic signals by, for example, performing act  604  of driving the differential MUT element with a pulse signal. The characteristics of the pulse signal that is applied to the differential MUT element may depend on whether the ultrasound circuit is operating in a differential transmit or a single-ended transmit mode. When the ultrasound circuit is operating in the single-ended transmit mode, the pulse signal provided to all of the MUTs in the differential MUT element may have the same polarity (and/or be the same signal). When the ultrasound circuit is operating in the differential transmit mode, the pulse signal provided to a first portion of the MUTs (e.g., a first half) in the MUT element may have a first polarity and the pulse signal provided to a second portion of the MUTs (e.g., a second half) in the differential MUT element have a second, opposite polarity. 
     The differential MUT element may be operated to receive acoustic signals by, for example, performing act  606  of controlling a state of a switch (e.g., switch  120 ) to couple receive circuit (e.g., receive circuit  112 ) to the differential MUT element and act  608  of processing a signal from the differential MUT element. The particular techniques employed to process the signal from the differential MUT element in act  608  may depend on whether the ultrasound circuit is operating in a differential receive or a single-ended receive mode. In the differential receive mode, the processing may comprise generating a digital signal representative of a difference between signals from two MUTs that are biased with voltages of an opposite polarity. In the single-ended receive mode, the processing may comprise generating a digital signal for each of the MUTs representative of the signal from the MUTs. 
     Various aspects of the technology described herein may be embodied as one or more processes, of which examples have been provided. The acts performed as part of each process may be ordered in any suitable way. Thus, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     Example Ultrasound Device 
       FIG. 7  shows the architecture of an ultrasound device  700  that employs differential MUT technology, such as the ultrasound circuits  100 A and  100 B described above. As shown, the ultrasound device  700  may include one or more transducer arrangements (e.g., arrays)  702 , transmit (TX) circuit  110 , receive (RX) circuit  112 , a timing and control circuit  708 , a signal conditioning/processing circuit  114 , a power management circuit  718 , and/or a high-intensity focused ultrasound (HIFU) controller  720 . In the embodiment shown, all of the illustrated elements of  FIG. 7  are formed on a single semiconductor die  712 . Thus, the ultrasound device  700  may be a monolithic ultrasound device. It should be appreciated, however, that in alternative embodiments one or more of the illustrated elements may be instead located off-chip. In some embodiments, the illustrated components may be disposed on two or more chips. For example, the transducer array  702 , a portion of the transmit circuit  110 , and/or a portion of the receive circuit  112  may be on one die and the other components may be on one or more other dies. In addition, although the illustrated example shows both transmit circuit  110  and receive circuit  112 , in alternative embodiments only transmit circuit  110  or only receive circuit  112  may be employed. For example, such embodiments may be employed in a circumstance where one or more transmission-only ultrasound devices  700  are used to transmit acoustic signals and one or more reception-only ultrasound devices  700  are used to receive acoustic signals that have been transmitted through or reflected off of a subject being ultrasonically imaged. 
     It should be appreciated that communication between one or more of the illustrated components may be performed in any of numerous ways. In some embodiments, for example, one or more high-speed busses (not shown), such as that employed by a unified Northbridge, may be used to allow high-speed intra-chip communication or communication with one or more off-chip components. 
     The one or more transducer arrays  702  may take on any of numerous forms, and aspects of the present technology do not necessarily require the use of any particular type or arrangement of transducer cells or transducer elements. Indeed, although the term “array” is used in this description, it should be appreciated that in some embodiments the transducer elements may not be organized in an array and may instead be arranged in some non-array fashion. As shown in  FIG. 7 , the transducer array  702  may comprise one or more differential MUT elements  102 . It should be appreciated that other transducer elements may be employed in place of or in conjunction with the differential MUT elements  102 . For example, the array transducer  702  may comprise one or more CMOS ultrasonic transducers (CUTs) and/or one or more other suitable ultrasonic transducers. In some embodiments, the transducer elements (e.g., differential MUT elements  102 ) of the transducer array  702  may be formed on the same chip as the electronics of the transmit circuit  110  and/or receive circuit  112 . The transducer array  702 , transmit circuit  110 , and receive circuit  112  may, in some embodiments, be integrated in a single ultrasound device. In some embodiments, the single ultrasound device may be a handheld device. In other embodiments, the single ultrasound device may be embodied in a patch that may be coupled to a patient. The patch may be configured to transmit, wirelessly, data collected by the patch to one or more external devices for further processing. 
     A MUT may, for example, include a cavity formed in a metal oxide semiconductor (MOS) wafer (e.g., a complementary MOS (or “CMOS”) wafer), with a membrane overlying the cavity, and in some embodiments sealing the cavity. Electrodes may be provided to create a transducer cell from the covered cavity structure. The MUT may include a piezoelectric layer sandwiched between the electrodes (e.g., in a PMUT implementation). The CMOS wafer may include an integrated circuit (e.g., integrated circuit  108 ) to which the transducer cell may be connected. The transducer cell and CMOS wafer may be monolithically integrated, thus forming an integrated ultrasonic transducer cell and integrated circuit on a single substrate (the CMOS wafer). 
     The transmit circuit  110  (if included) may, for example, generate pulses that drive the individual elements of, or one or more groups of elements within, the transducer array(s)  702  so as to generate acoustic signals to be used for imaging. The receive circuit  112 , on the other hand, may receive and process electronic signals generated by the individual elements of the transducer array(s)  702  when acoustic signals impinge upon such elements. 
     In some embodiments, the timing and control circuit  708  may, for example, be responsible for generating all timing and control signals that are used to synchronize and coordinate the operation of the other elements in the device  700 . In the example shown, the timing and control circuit  708  is driven by a single clock signal CLK supplied to an input port  716 . The clock signal CLK may, for example, be a high-frequency clock used to drive one or more of the on-chip circuit components. In some embodiments, the clock signal CLK may, for example, be a 1.5625 GHz or 2.5 GHz clock used to drive a high-speed serial output device (not shown in  FIG. 7 ) in the signal conditioning/processing circuit  114 , or a 20 Mhz or 40 MHz clock used to drive other digital components on the semiconductor die  712 , and the timing and control circuit  708  may divide or multiply the clock CLK, as necessary, to drive other components on the semiconductor die  712 . In other embodiments, two or more clocks of different frequencies (such as those referenced above) may be separately supplied to the timing and control circuit  708  from an off-chip source. 
     The power management circuit  718  may, for example, be responsible for converting one or more input voltages VIN from an off-chip source into voltages needed to carry out operation of the chip, and for otherwise managing power consumption within the device  700 . In some embodiments, for example, a single voltage (e.g., 12V, 80V, 100V, 120V, etc.) may be supplied to the chip and the power management circuit  718  may step that voltage up or down, as necessary, using a charge pump circuit or via some other DC-to-DC voltage conversion mechanism. In other embodiments, multiple different voltages may be supplied separately to the power management circuit  718  for processing and/or distribution to the other on-chip components. 
     As shown in  FIG. 7 , in some embodiments, a high-intensity focused ultrasound (HIFU) controller  720  may be integrated on the semiconductor die  712  so as to enable the generation of HIFU signals via one or more elements of the transducer array(s)  702 . In other embodiments, a HIFU controller for driving the transducer array(s)  702  may be located off-chip, or even within a device separate from the device  700 . That is, aspects of the present disclosure relate to provision of ultrasound-on-a-chip HIFU systems, with and without ultrasound imaging capability. It should be appreciated, however, that some embodiments may not have any HIFU capabilities and thus may not include a HIFU controller  720 . 
     Moreover, it should be appreciated that the HIFU controller  720  may not represent distinct circuit in those embodiments providing HIFU functionality. For example, in some embodiments, the remaining circuit of  FIG. 7  (other than the HIFU controller  720 ) may be suitable to provide ultrasound imaging functionality and/or HIFU, i.e., in some embodiments the same shared circuit may be operated as an imaging system and/or for HIFU. Whether or not imaging or HIFU functionality is exhibited may depend on the power provided to the system. HIFU typically operates at higher powers than ultrasound imaging. Thus, providing the system a first power level (or voltage level) appropriate for imaging applications may cause the system to operate as an imaging system, whereas providing a higher power level (or voltage level) may cause the system to operate for HIFU. Such power management may be provided by off-chip control circuit in some embodiments. 
     In addition to using different power levels, imaging and HIFU applications may utilize different waveforms. Thus, waveform generation circuit may be used to provide suitable waveforms for operating the system as either an imaging system or a HIFU system. 
     In some embodiments, the system may operate as both an imaging system and a HIFU system (e.g., capable of providing image-guided HIFU). In some such embodiments, the same on-chip circuit may be utilized to provide both functions, with suitable timing sequences used to control the operation between the two modalities. 
     In the example shown, one or more output ports  714  may output a high-speed serial data stream generated by one or more components of the signal conditioning/processing circuit  114 . Such data streams may, for example, be generated by one or more USB 3.0 modules, and/or one or more 10 GB, 40 GB, or 100 GB Ethernet modules, integrated on the semiconductor die  712 . In some embodiments, the signal stream produced on output port  714  can be fed to a computer, tablet, or smartphone for the generation and/or display of 2-dimensional, 3-dimensional, and/or tomographic images. In embodiments in which image formation capabilities are incorporated in the signal conditioning/processing circuit  114 , even relatively low-power devices, such as smartphones or tablets which have only a limited amount of processing power and memory available for application execution, can display images using only a serial data stream from the output port  714 . As noted above, the use of on-chip analog-to-digital conversion and a high-speed serial data link to offload a digital data stream is one of the features that helps facilitate an “ultrasound on a chip” solution according to some embodiments of the technology described herein. 
     Devices  700  such as that shown in  FIG. 7  may be used in any of a number of imaging and/or treatment (e.g., HIFU) applications, and the particular examples discussed herein should not be viewed as limiting. In one illustrative implementation, for example, an imaging device including an N×M planar or substantially planar array of CMUT elements may itself be used to acquire an ultrasonic image of a subject, e.g., a person&#39;s abdomen, by energizing some or all of the elements in the array(s)  702  (either together or individually) during one or more transmit phases, and receiving and processing signals generated by some or all of the elements in the array(s)  702  during one or more receive phases, such that during each receive phase the CMUT elements sense acoustic signals reflected by the subject. In other implementations, some of the elements in the array(s)  702  may be used only to transmit acoustic signals and other elements in the same array(s)  702  may be simultaneously used only to receive acoustic signals. Moreover, in some implementations, a single imaging device may include a P×Q array of individual devices, or a P×Q array of individual N×M planar arrays of CMUT elements, which components can be operated in parallel, sequentially, or according to some other timing scheme so as to allow data to be accumulated from a larger number of CMUT elements than can be embodied in a single device  700  or on a single die  712 . 
     In yet other implementations, a pair of imaging devices can be positioned so as to straddle a subject, such that one or more CMUT elements (e.g., differential CMUT elements) in the device(s)  700  of the imaging device on one side of the subject can sense acoustic signals generated by one or more CMUT elements in the device(s)  700  of the imaging device on the other side of the subject, to the extent that such pulses were not substantially attenuated by the subject. Moreover, in some implementations, the same device  700  can be used to measure both the scattering of acoustic signals from one or more of its own CMUT elements as well as the transmission of acoustic signals from one or more of the CMUT elements disposed in an imaging device on the opposite side of the subject. 
     Example Forms of Ultrasound Devices 
     The ultrasound devices described herein may be implemented in any of a variety of physical configurations, or form factors, including as part of a handheld device (which may include a screen to display obtained images) or as part of a patch configured to be affixed to the subject. Several examples are now described. 
     An ultrasound device may be implemented in any of a variety of physical configurations including as part of a pill to be swallowed by a subject, as part of a handheld device including a screen to display obtained images, or as part of a patch configured to be affixed to the subject. 
     In some embodiments, a ultrasound device may be embodied in a pill to be swallowed by a subject. As the pill travels through the subject, the ultrasound device within the pill may image the subject and wirelessly transmit obtained data to one or more external devices for processing the data received from the pill and generating one or more images of the subject. For example, as shown in  FIG. 8A , pill  802  comprising an ultrasound device may be configured to communicate wirelessly (e.g., via wireless link  801 ) with external device  800 , which may be a desktop, a laptop, a handheld computing device, and/or any other device external to pill  802  and configured to process data received from pill  802 . A person may swallow pill  802  and, as pill  802  travels through the person&#39;s digestive system, pill  802  may image the person from within and transmit data obtained by the ultrasound device within the pill to external device  800  for further processing. 
     In some embodiments, a pill comprising an ultrasound device may be implemented by potting the ultrasound device within an outer case, as illustrated by an isometric view of pill  804  shown in  FIG. 8B .  FIG. 8C  is a section view of pill  804  shown in  FIG. 8B  exposing views of the electronic assembly and batteries. In some embodiments, a pill comprising an ultrasound device may be implemented by encasing the ultrasound device within an outer housing, as illustrated by an isometric view of pill  806  shown in  FIG. 8D .  FIG. 8E  is an exploded view of pill  806  shown in  FIG. 8D  showing outer housing portions  806 A and  806 B used to encase electronic assembly  806 C. 
     In some embodiments, the ultrasound device implemented as part of a pill may comprise one or multiple ultrasonic transducer (e.g., CMUT) arrays, one or multiple image reconstruction chips, an FPGA, communications circuit, and one or more batteries. For example, as shown in  FIG. 8F , pill  808 A may include multiple ultrasonic transducer arrays shown in sections  808 B and  808 C, multiple image reconstruction chips as shown in sections  808 C and  808 D, a WiFi chip as shown in section  808 D, and batteries as shown in sections  808 D and  808 E. 
       FIGS. 8G and 8H  further illustrate the physical configuration of electronics module  806 C shown in  FIG. 8E . As shown in  FIGS. 8G and 8H , electronics module  806 C includes four CMUT arrays  812  (though more or fewer CMUT arrays may be used in other embodiments), bond wire encapsulant  814 , four image reconstruction chips  816  (though more or fewer image reconstruction chips may be used in other embodiments), flex circuit  818 , WiFi chip  820 , FPGA  822 , and batteries  822 . Each of the batteries may be of size 13 PR48. Each of the batteries may be a 300 mAh 1.4V battery. Other batteries may be used, as aspects of the technology described herein are not limited in this respect. 
     In some embodiments, the ultrasonic transducers of an ultrasound device in a pill are physically arranged such that the field of view of the device within the pill is equal to or as close to 360 degrees as possible. For example, as shown in  FIGS. 8G and 8H , each of the four CMUT arrays may a field of view of approximately 60 degrees (30 degrees on each side of a vector normal to the surface of the CMUT array) or a field of view in a range of 40-80 degrees such that the pill consequently has a field of view of approximately 240 degrees or a field of view in a range of 160-320 degrees. 
     In some embodiments, a ultrasound device may be embodied in a handheld device  902  illustrated in  FIGS. 9A and 9B . Handheld device  902  may be held against (or near) a subject  900  and used to image the subject. Handheld device  902  may comprise an ultrasound device (e.g., a ultrasound device) and display  904 , which in some embodiments, may be a touchscreen. Display  904  may be configured to display images of the subject generated within handheld device  902  using ultrasound data gathered by the ultrasound device within device  902 . 
     In some embodiments, handheld device  902  may be used in a manner analogous to a stethoscope. A medical professional may place handheld device  902  at various positions along a patient&#39;s body. The ultrasound device within handheld device  902  may image the patient. The data obtained by the ultrasound device may be processed and used to generate image(s) of the patient, which image(s) may be displayed to the medical professional via display  904 . As such, a medical professional could carry hand-held device (e.g., around their neck or in their pocket) rather than carrying around multiple conventional devices, which is burdensome and impractical. 
     In some embodiments, an ultrasound device may be embodied in a patch that may be coupled to a patient. For example,  FIGS. 9C and 9D  illustrate a patch  910  coupled to patient  912 . The patch  910  may be configured to transmit, wirelessly, data collected by the patch  910  to one or more external devices for further processing. 
       FIG. 9E  shows an exploded view of patch  910 . As particularly illustrated in  FIG. 9E , patch  910  comprises upper housing  914 , lower housing  916 , and circuit board  918  disposed there between. The circuit board  918  may be configured to support various components, such as for example a heat sink  920 , a battery  922 , and communications circuitry  924 . In one embodiment, communication circuitry  924  includes one or more short- or long-range communication platforms. Exemplary short-range communication platforms include, Bluetooth, Bluetooth Low Energy (BLE), and Near-Field Communication (NFC). Long-range communication platforms include, WiFi and Cellular. As further depicted in  FIG. 9E , the patch  910  may also comprise dressing  928  that provides an adhesive surface for both the lower housing  916  as well as to the skin of a patient. One non-limiting example of such a dressing  928  is TEGADERM, a transparent medical dressing available from 3M Corporation. 
     In some embodiments, a ultrasound device may be embodied in hand-held device  1000  shown in  FIG. 10 , which may considered an ultrasound probe. Hand-held device  1000  comprises a handle  1002  coupled to a probe head  1004 . The probe head  1004  may comprise one or more ultrasound chips that may be configured to transmit and/or receive acoustic signals. In some embodiments, the hand-held device  1000  may be configured to transmit data collected by the device  1000  wirelessly to one or more external device for further processing. In other embodiments, hand-held device  1000  may be configured transmit data collected by the device  1000  to one or more external devices using one or more wired connections, as aspects of the technology described herein are not limited in this respect. 
     Various aspects of the present disclosure may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments. 
     Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed. 
     The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value. 
     Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be object of this disclosure. Accordingly, the foregoing description and drawings are by way of example only.