Supertex inc. MD2131DB1 MD2131 Ultrasound Beamforming Transmitter Demoboard Introduction General Description The MD2131 is a high speed, arbitrary waveform, push-pull This MD2131DB1 datasheet describes how the demoboard source driver. It is designed for medical ultrasound imaging is to used to generate the ultrasound transmit beamforming and HIFU beamforming applications. It also can be used waveform with the Gaussian prole, and the adjustable fre- in NDT, sonar and other ultrasound phase-array focusing quency, amplitude and phase angle. It also provides informa- beamforming applications. tion about how to design a user application circuit and PCB using the Supertex MD2131 and DN2625 devices. The integrated circuit (IC) consists of the CMOS digital logic input circuits, an 8-bit current DAC for the waveform The MD2131DB1 circuit uses two depletion-mode MOSFETs amplitude control, and four PWM current sources. These in the push-pull mode to drive the center tapped, wide band, current sources are constructed with the high speed in-phase ultrasound output transformer. The two depletion-mode and quadrature current switch matrix and the built-in sine and MOSFETs are packaged in a single 5x5mm DFN package. cosine angle-to-vector look-up table. The angular resolution The sources of the MOSFETs are directly driven by the of the vector table is 7.5 degrees per step with a total range MD2131s two outputs, whose maximum peak sinking current of 48 steps. There are four logic input signals to control the in- is up to 3.0A. These current source outputs are controlled by phase and quadrature PWM push-pull current sources output the MD2131s internal angular vector switch matrix and the timing frequency cycle in the burst and waveform envelope. in-phase and quadrature PWM input signals. The MD2131s output stage is designed to drive two depletion All of the MD2131s logic control signals are generated by mode, high voltage, Supertex DN2625 MOSFETs as a source two small CPLD programmable logic circuits clocked by an driver. The MOSFET drains are connected to a center-tapped on-board 160MHz crystal oscillator. The CPLD circuits not ultrasound frequency pulse transformer. The secondary only generate accurate timing for the high speed PWM con- winding of the transformer can connect to the ultrasound piezo trol waveforms, but also the serial data and clock to set and or capacitive transducer via a cable with a good impendence change the waveform amplitude DAC and phase angle data match. The MD2131 has a high speed serial data interface registers. The external clock input can be used if the on-board that quickly updates the data registers per-scan-line for oscillator is disabled. The external trigger input can be used changing the beamforming phase angles and apodization to synchronize the burst waveforms launch timing. There are amplitudes. ve push buttons for enabling and selecting the output wave- Demoboard Block Diagram +3.3V +2.5V JTAG +2.5V +5.0V +5.0V +3.3V VCC VCCIO EN T1 VLL VDD PA IA 1:1:1 EXTRG DN2625 EXTRG QA JUMP IB +70 to 100V XDCX QB V PP EXCLK CPLD MD2131 SDI 1k SDO C7 220pF SCK CLKIN OSC CS LOAD LD DN2625 160MHz DIS PB GND RFB PHO Wave DAC Freq PWR Ampl EN +3.3V +5.0V Phase ENA Doc. DSDB-MD2131DB1 Supertex inc. A070114 www.supertex.comMD2131DB1 form frequency, phase angle and amplitude. Four color LEDs possible. The supply voltage bypass capacitors and the indicate the power, chip enable and wave-form parameter MOSFET gate decoupling capacitors should be as close to selection states. The MD2131DB1 output waveform can be the pins as possible. The capacitors ground pin pads should displayed by using an oscilloscope and the high impedance have low inductance feed-through connections that are con- probe at the TP13 test point. It also can use an SMA to BNC, nected directly to a solid ground plane. The VDD and VPP 50, coaxial cable to directly connect to an oscilloscope, with supplies can draw fast transient currents of up to 3.5A, so an attenuation of 5:1 if R10 is 200. A cable can also be used they should be provided with a low impedance bypass capaci- to directly drive the users transducer. Jumper J4 can be used tor at the chips pins. A ceramic capacitor of 0.1 to 1.0F may to select whether or not to connect the on-board equivalent be used. Minimize the trace length to the ground plane, and load, which is formed by a 220pF capacitor in parallel with a insert a ferrite bead in the power supply lead to the capacitor 1.0k resistor. to prevent resonance in the power supply lines. For applica- tions that are sensitive to jitter and noise and using multiple Circuit Design & PCB Layout MD2131 ICs, insert another ferrite bead between VDD, and The thermal pad at the bottom of the MD2131 package must decouple each chip supply separately. Pay particular attention be connected to the VSUB pin on the PCB. The VSUB is con- to minimizing trace lengths and using sufcient trace width to nected to the ICs substrate. It is important to make sure that reduce inductance, not only on the supply pins but also on the the VSUB is well grounded. A proper supply voltage power-up CA/B and KA/B compensation pins. Very closely placed sur- sequence is needed to test the circuit. To prevent any supply face mount components are highly recommended. Be aware voltage polarity reversing, the circuit also has the protection of of the parasitic coupling from the high voltage outputs to the Schottky diodes D7, D8 and D9. input signal terminals of MD2131. This feedback may cause oscillations or spurious waveform shapes on the edges of sig- Due to the high current and high current slew rate nature of nal transitions. Since the input operates with signals down to this common gate, source driven and push-pull circuit topolo- 2.5V, even small coupling voltages may cause problems. The gy, the two cascading N-channel MOSFETs need to have very use of a solid ground plane and good power and signal layout low lead inductance. The DN2625DK6 MOSFET is designed practices will prevent this problem. Also ensure that the cir- for this application and works seamlessly with the MD2131K7. culating ground return current from a capacitive load cannot In particular, a good PCB layout design needs to shorten the react with common inductance to create noise voltages in the traces between the MD2131K7 output pins and DN2625DK6 input logic circuitry. source pins. It is also necessary to connect all three pairs of pins between them for the high current carrying capacity. Fur- This MD2131DB1 beamforming demoboard should be pow- thermore, because of the high di/dt in the output current of the ered up with multiple DC power supplies with current limiting MD2131, it is also necessary to have the Schottky diodes D5 functions. The power supply voltages and current limits used and D6 from the driver output pins to the +5.0V power supply in the testing are listed on page 11. There are examples of line as the clamping diodes. Note that the diodes must have the MD2131DB1 demoboard input and output waveform and enough speed and peak current capability. The RC snubber measurements shown in Figures 1 to 9. circuits of R8-C5 and R15-C28 at the output pins can dump the current pulse edge ringing effectively. Output Transformer Design A center tapped wide band ultrasound transformer is needed PCB designers need to pay attention to some of the connect- for the push pull output circuit. The transformer serves at least ing traces as high voltage and high speed traces. In particular, three functions: the balanced differential to single end RF out- low capacitance to the ground plane and more trace spacing put transformer the isolation barrier to the ultrasound probe need to be applied in this situation. and the impedance matching or low pass net work combined with the cable and transducer element. The MD2131 PWM High speed PCB trace design practices that are compatible clock may operate at a 40 to 160MHz frequency range, how- with operating speed of about 100 to 200 MHz are used for the ever the wide band transformer only needs to work in the fre- demo board PCB layout. The internal circuitry of the MD2131 quency band of the ultrasound being transmitted. Beside the can operate at quite a high frequency, with the primary speed bandwidth consideration, the transformer also needs enough limitation being load capacitance. Because of this high speed, peak current capacity and RF power coupling efciency to and the high transient currents that result when driving even make sure the ferrite magnetic core will not be saturated, very small inductive loads, ringing and even oscillations are have little leakage inductance and will be a small size. Doc. DSDB-MD2131DB1 Supertex inc. A070114 2 www.supertex.com