Abstract:
An improved analog switch for use in an ultrasound elastography probe is disclosed. The improved analog switch results in less heat dissipation compared to prior art analog switches.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority to Chinese Patent Application No. 201510644537.6, filed on Oct. 8, 2015, and titled “An Optimized CMOS Analog Switch” (in Mandarin), which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    An improved analog switch for use in an ultrasound elastography device is disclosed. The improved analog switch results in less heat dissipation compared to prior art analog switches. 
       BACKGROUND OF THE INVENTION 
       [0003]    Recent developments in ultrasound imaging systems include the use of shear wave elastography.  FIG. 1  depicts prior art ultrasound elastography device  100 , which comprises transducer  105 . Transducer  105  uses acoustic radiation force to induce a “push” pulse  150  into soft tissue  110  that results in shear waves  140 . Soft tissue  110  comprises stiff lesion  120  (which could be a tumor or other medical abnormality), which includes or is near region of interest  130 . The tissue&#39;s stiffness is computed based upon how fast the resulting shear wave travels through the tissue. When detection pulses  160  interact with a passing shear wave  140 , the passing shear wave  140  reveals the wave&#39;s location at a specific time, allowing calculation of the speed of the shear wave  140 . This numerical value is related to the stiffness of the tissue within the region of interest. By using many near-simultaneous push pulses  150 , and by using an advanced ultrafast imaging technique to track shear waves  140 , the system can generate a two-dimensional quantitative map of the tissue&#39;s stiffness (the Young&#39;s modulus) every second. 
         [0004]      FIG. 2  depicts certain electrical aspects of prior art ultrasound elastography device  100 . Ultrasound elastography device  100  comprises an exemplary high voltage transmit path comprising amplifier  210  and diode pair  211  and an exemplary low voltage receive path comprising amplifier  250  coupled to high voltage isolation circuit  254  comprising resistors  252  and  253  and diode bridge  251 . Ultrasound elastography device  100  further comprises probe selection relays  221 ,  222 ,  223 , and  224 , and probes  231 ,  232 ,  233 , and  234 . Probe  231  is shown connected to multiplexor  270 , which comprises analog switches  241 ,  243 ,  245 , and  247 , which connect to transducers  242 ,  244 ,  246 , and  248 , respectively. It is to be understood that the same configuration of structures (multiplexor  270  and transducers  242 ,  244 ,  246 , and  248 ) are used for probes  232 ,  233 , and  234  as well. Transducer  105  in  FIG. 1  is representative of transducers  242 ,  244 ,  246 , and  248 . 
         [0005]    With reference to  FIGS. 1 and 2 , the vibration frequency of the acoustic push pulse  150  is in the 50-500 Hz range. To measure the speed of shear wave  140 , each detection pulse  170  could last for 300 ms. To detect a shear wave  140 , the analog switches  241 ,  243 ,  245 , and  247  in the device  100  need to drive the high-voltage transducers  242 ,  244 ,  246 , and  248  (represented by transducer  105 ) for about 300 ms. 
         [0006]    Prior art analog switches  241 ,  243 ,  245 , and  247  each requires two high-voltage switches working in parallel to avoid the excessive heat dissipation that could damage the circuits. However, two switches connected in parallel also double the parasitic capacitance and affect the image quality. 
         [0007]    Virtually all prior art analog switches  241 ,  243 ,  245 , and  247  in multiplexor  270  utilize a T-switch  300 , shown in  FIG. 3 . In ultrasound imaging applications, the use of T-switch  300  limits the on-resistance to approximately 16 Ohms for less than 15 pF parasitic capacitance and higher than 60 dB off-isolation. 
         [0008]      FIG. 3  shows the schematic of the conventional T-switch  300  structure used for analog switches  241 ,  243 ,  245 , and  247  in high-voltage multiplexer  270 . The T-switch  300  comprises NMOS transistor  310  in series with NMOS transistor  320 , with shunting NMOS transistor  330  to achieve an off-isolation of 60 dB. NMOS transistors  310 ,  320 , and  330  each comprise a thick gate oxide layer that allows for both positive and negative high gate voltage swings at the expense of on-resistance that generates excessive heat. A thick gate oxide layer typically ranges between 5000-10000 angstrom. 
         [0009]    The high-voltage capability of the devices further worsens the on-resistance/parasitic capacitance trade-off. Fixed gate bias over the varying source voltage makes the on-resistance for positive signal much larger than that for negative signal and introduces second harmonic distortion. 
         [0010]      FIG. 4  shows exemplary NMOS transistor  400 , which is representative of NMOS transistors  310 ,  320 , and  330 . NMOS transistor  400  actually comprises NMOS transistor  410  and diode  420 , which is due to the junction between the drain and the body of NMOS transistor  410 . Both drain and gate are capable of operating up to 200V. 
         [0011]    The drain/body diode structure of NMOS transistor  400  makes the high-voltage device essentially a rectifier even though it is turned off. As a result, the body of NMOS transistor  410  needs to be pulled down to the most negative voltage, e.g., −100V (V NN ) at off state just as shown in  FIG. 4 . Shunting NMOS transistor  330  terminates to −100V. The second series device, such as NMOS transistor  320 , isolates the transducer from the −100V termination. This device is not necessary if the transducer can be terminated to −100V but that is usually not the case for the piezoelectric transducers. 
         [0012]    Piezoelectric devices exhibit nonlinear behavior when subjected to high electric field. This strong nonlinear material behavior is induced by localized polarization switching (i.e. change of the polarization direction) at the subgrain level. Once the piezoelectric material is operating in nonlinear mode, the material likely is already damaged. Thus, terminating a piezoelectric transducer with a high voltage is destructive to the piezoelectric material. This behavior is shown in graphs  500  and  510  in  FIG. 5 . Graphs  500  and  510  show the change in polarization of piezoelectric material in response to a change in electric field. 
         [0013]    An improved T-switch previously invented by the Applicant is shown in  FIG. 6  T-switch  600  comprises butterfly transistor pair  615  (comprising NMOS transistors  610  and  620 ) in series with butterfly transistor pair  635  (comprising NMOS transistors  630  and  640 ) and shunt butterfly transistor pair  655  (comprising NMOS transistors  650  and  660 ) as shown. NMOS transistors  610 ,  620 ,  630 ,  640 ,  650 , and  660  each comprise a thin gate oxide layer. A thin gate oxide layer typically ranges between 100-200 angstrom. T-switch  600  uses a lower gate voltage and has a higher transconductance than T-switch  300 . Butterfly transistor pairs  615  and  635  together are comparable to a single 200V drain, 200V gate transistor in on-resistance/capacitance ratio. However, T-switch  600  experiences substantial heat dissipation. 
         [0014]    What is needed is an improved analog switch that results in less heat dissipation than prior art T-switches  300  and  600 . 
       SUMMARY OF THE INVENTION 
       [0015]    The preferred embodiment is a high-voltage CMOS switch circuit topology that reduces the on-resistance to 4-8 Ohms, which is an improvement by a factor of 2 to 4 without increasing the parasitic capacitance compared to the prior art. The circuit topology is straightforward to implement and is suitable for constructing, for example, a 4:1 ultrasound multiplexer (such as multiplexor  270 ) that can handle analog signals of ±100 V. The power supplies to this circuit are ±6 V and the control inputs are voltage levels of 0 and +5 V, compatible with standard CMOS circuits. The circuit is particularly useful in driving high-voltage transducers for ultrasound elastography probes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  depicts a prior art ultrasound elastography probe. 
           [0017]      FIG. 2  depicts electrical aspects of the prior art ultrasound elastography probe. 
           [0018]      FIG. 3  depicts a prior art analog T-switch. 
           [0019]      FIG. 4  depicts the structure of transistors used in the prior art analog T-switch of  FIG. 3 . 
           [0020]      FIG. 5  depicts certain physical characteristics of prior art piezoelectric transducers. 
           [0021]      FIG. 6  depicts another prior art analog T-switch. 
           [0022]      FIG. 7  depicts an embodiment of an improved analog switch. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]      FIG. 7  depicts an embodiment of the invention. Analog switch  700  is a modified version of T-switch  600  where one of the series butterfly transistor pairs has been removed. Analog switch  700  comprises butterfly transistor pair  705  (comprising NMOS transistors  710  and  720 ) and shunt butterfly transistor pair  725  (comprising NMOS transistors  730  and  740 ), which performs a shunting function. NMOS transistors  710 ,  720 ,  730 , and  740  each comprise a thin gate oxide layer. A thin gate oxide layer typically ranges between 100-200 angstrom. In this configuration, butterfly transistor pair  705  can be considered to be a conducting means for connecting a high voltage source to a transceiver, and shunt butterfly transistor pair  725  can be considered to be a shunting means for shunting current from a terminal of the conducting means to ground. 
         [0024]    The on-resistance of analog switch  700  is approximately half of the on-resistance of prior art T-switch  600 , and the parasitic capacitance of analog switch  700  is also largely reduced compared to the parasitic capacitance of prior art T-switch  600 . 
         [0025]    The topology of analog switch  700  is immune to piezoelectric material nonlinearity issues of prior art T-switch  300 , as the butterfly transistor pair  725  allows termination to ground, not to −100V as in prior art T-switch  300 . 
         [0026]    The following improvements compared to prior art T-switches  300  and  600  are achieved with analog switch  700 :
       (1) Can transfer voltages greater than ±100V while using only a power supply of approximately ±6V.   (2) Inputs compatible with standard 5V CMOS circuits.   (3) Analog signal capability up to 200V peak-to-peak, with peak analog signal currents of &gt;3 A.   (4) Signal independent on-resistance lower than 8 Ohms.   (5) Parasitic capacitance reduced to 10 pF.   (6) No off-isolation concern as the non-selected transducer is terminated.       
 
         [0033]    In conclusion, a high-voltage analog switch  700  using 100V thin gate oxide NMOS transistors is proposed. It is modified from a prior art T-switch  600  by removing one series device and shunting the non-selected transducer directly to ground. Circuits using this topology have wide potential application in ultrasound imaging, shear wave elastography, and even high-intensity focused ultrasound where high power ultrasound transmission is needed. For example, analog switch  700  is suitable for use in a 4:1 ultrasound multiplexer, such as multiplexor  270  in ultrasound elastography device  100 . 
         [0034]    Although the present invention has been fully described in connection with embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present invention as defined by the appended claims.