Design Con 2015

The unheralded side of medical ultrasound imaging: The high-voltage transmit path

-May 03, 2013

It seems that much of the focus in electronic solutions for medical ultrasound are the high resolution, high accuracy, multiple integrated components for the receive channels. Maxim Integrated has given the high-voltage transmit path arena an innovative analog switch that improves upon existing technology, especially by integrating the high voltage VPP.

The MAX4968/MAX4968A are 16-channel, high-linearity, high-voltage, bidirectional SPST analog switches with 18Ω (typ) on-resistance. The devices are ideal for use in applications requiring high-voltage switching controlled by a low-voltage control signal, such as ultrasound imaging and printers. The MAX4968A provides integrated 40kΩ (typ) bleed resistors on each switch terminal to discharge capacitive loads. Using HVCMOS technology, these switches combine high-voltage bilateral MOS switches and low-power CMOS logic to provide efficient control of high-voltage analog signals.

Having multiple sources is really important to designers and procurement. The MAX4968 is pin-to-pin compatible with the MAX14802 and Supertex HV2601. The MAX4968A is pin-to-pin compatible with the MAX14803 and Supertex HV2701.

The only difference, which is a benefit that saves board space and cost, as well as simplifying the board design/layout, is the VPP positive supply voltage level. The MAX4968/MAX4968A require a low +10V (typ) voltage (VPP), whereas the MAX14802/MAX14803 and HV2601/HV2701 require a high +100V supply voltage. The high voltage VPP is done internally in this IC!

In a typical ultrasound application, since these devices do not require a dedicated high-voltage supply, that implies a significant simplification of system requirement. The negative voltage supply can be shared with the transmitter, and the positive voltage supply is typically +10V.

The MAX4968 functionality1

Figure 1 shows the essential functional diagram of the MAX4968, which features 16 independent HV analog switches. Each switch's internal status can be programmed with an SPI™ interface. In most ultrasound applications, the HV analog switches are used to implement HV multiplexers.

 

Figure 1: The essential functional diagram of the MAX4968, features 16 independent HV analog switches

  • SW1A, SW1B can swing from VNN to VNN + 200V.
  • Although most industrial ultrasound applications are unipolar, the HV analog switches can operate both in bipolar and unipolar applications. The input/output voltage range can be one of the following cases:
    • (SW_) range = [+100V, -100V] bipolar
    • (SW_) range = [0, -200V] unipolar negative
    • (SW_) range = [+200V, 0] unipolar positive
  • VNN can vary from 0V to -200V, depending on the amplitude of the input signal and its polarity. VNN can be shared with the pulser (transmitter) negative supply.
  • VPP is a low-voltage supply (10V only)
  • Equivalent RON is flat in the entire input range (approximately 20Ω) and on-capacitance is 16pF only.

The ultrasound system overview focusing on the transmit path

A typical block diagram of an ultrasound system shows the high-voltage transmit path in the lower middle section of the image (Courtesy of Maxim Integrated)


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