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State-of-the-Art IC: Transmitter in Ultrasound Devices

Wed, 10/14/2009 - 9:18am

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Lower voltage IC technologies now enable ultrasound receiver chips with unprecedented high gain and low noise numbers.
By Ismail Oguzman and Arash Loloee, Texas Instruments

State-of-the-Art IC: Transmitter in Ultrasound Devices
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Figure 1. Typical block-diagram of a complete medical ultrasound system.
The medical imaging field is benefiting greatly from research and development in applied physics and electronics, especially in areas such as instrumentation, image acquisition and modeling. Resulting from its fully non-invasive nature, ultrasound holds a special position among the imaging modes, providing a reliable method for the study of internal organs. The ultrasound technique has been used for medical purposes for over half a century. However, the necessary equipment has been quite bulky and expensive, and until recently made exclusively out of discrete components.

By virtue of advances made in semiconductor process technologies, this trend is changing. It is now possible to manufacture an ultrasound transceiver entirely out of semiconductor ICs. Lower voltage IC technologies now enable ultrasound receiver chips with significantly high-gain and low-noise performance. Similarly, on the higher voltage side, there is growing interest in making transmitter ICs that drive the ultrasound transducer. This article surveys advancements as well as challenges when designing ultrasound transmitter chips.
Ultrasound System Overview: Transmit and Receive Functions
In simple terms, an ultrasound system's operation principle consists of generating acoustic waves to be used on a patient, followed by receiving and processing the reflected signals to create an image of the body. The original acoustic waves being sent into the body are generated by a transducer, which typically is excited by electrical pulses generated by a transmitter. Similarly, the reflected acoustic waves are received by the transducer and converted back into electrical form. The resulting signal is finally processed to determine the internal structure of the body parts of interest.

State-of-the-Art IC: Transmitter in Ultrasound Devices
click to enlarge

Figure 2. Typical ultrasound transmitter output at ±100 V power supplies and the corresponding instantaneous current into a parallel load of 100 and 300 pF.
Typical construction of a complete medical ultrasound system is shown in Figure 1. The transmit path can be realized in several different ways. This path may consist of a beamformer, level translators, gate drivers and high-voltage switches whose output is sent to the ultrasound transducer. The transducer, usually made of a piezoelectric material, converts the high-voltage electrical signal into an acoustic wave, which is the final output of the system.

In some systems, the signal's digital nature is preserved from beginning to end across the transmit path where the output stage is driven with digital logic. However, you can also create and transmit the signal to the transducer in an analog fashion. This involves a digital-to-analog converter (DAC) to convert the output of the beamformer into analog format. Then, analog amplification is used on the resulting signal before sending it to the transducer.

With an ultrasound system's receive path, an analogous approach is used. Because the received signal is far lower in amplitude than the transmitted signal, the front-end consists of a low-noise amplifier followed by some sort of gain control block. After filtering out the higher frequency components not of interest, the resulting signal is converted into digital format through an analog-to-digital converter (ADC) whose output is processed by a beamformer.

Other important pieces of the ultrasound transceiver system include a multiplexer to interleave the activities of multiple channels, and a transmit/receive switch to control the signal traffic between the transducer and the transceiver electronics. A critical function of the transmit/receive switch is to protect the receiver during transmit events, which involve much higher voltages across the transmission line than what the receiver block can withstand.
Transmit Path Challenges Voltage Range and Frequency of Operation
State-of-the-Art IC: Transmitter in Ultrasound Devices
click to enlarge

Figure 3. A fast return-to-zero (damping) function improves the linearity of th e output signal in an ultrasound transmitter system. This is especially important in the case of a few pulses.
The ultrasound system described thus far is expected to create various signal patterns to meet the requirements of different imaging modes. At one extreme of the range, you can find high-voltage (60 to 100 V), low-duty cycle (0.5 to 2.0%) signals required by B-mode and harmonic imaging applications. At the other extreme, there are low-voltage (3 to 10 V), 100% duty cycle signals required by continuous-wave (CW) Doppler-type imaging modes. This means that the ultrasound system's transmitter circuit is required to create output voltages ranging from ±3 V to ±100 V under commensurate duty cycle conditions, at a fundamental frequency ranging from 1 to 20 MHz.

It is quite obvious that the ±100 V requirement on the transmitter output requires high-voltage switches. When the transmitter consists of an IC, this requirement translates into high-voltage transistors, which are optimized to withstand large electric fields. As such, they underperform at low voltages (<10 V) typically used for the CW operation. Designing a transmitter to meet the product specifications at two quite distant ends of the voltage range remains a formidable challenge.

The wide range of output voltage is not the only difficulty involved in making an ultrasound transmitter device. There are further challenges.

Slew Rate
Based on the ranges mentioned earlier for the voltage swing and operation frequency, a transmitter may have to generate slew rates as high as 8 V/ns. Combined with a typical parallel load of 100 and 300 pF to represent the transducer, you can see that the transmitter is expected to supply close to 3 A of transient current in the most stringent case (see Figure 2).
Harmonic Distortion
The ideal output from an ultrasound transmitter is a sinusoidal signal that satisfies the highest voltage amplitude and operation frequency requirements. Rather than create this difficult-to-make analog signal, you can generate a square pulse. After being subject to the low-pass filtering characteristics of the transducer, this pulse is reduced to only the first few of its harmonics. Out of the remaining even harmonics, the second harmonic is typically the worst offender. Therefore, the amount of suppression in the second harmonic becomes a major figure-of-merit for an ultrasound transmitter.
Pulse Symmetry and Return-to-Zero
The symmetry requirement imposed on the ultrasound transmitter's output can be intuitively understood. However, what needs to be further understood here is that the output signal does not have to be a long pulse train. It may well consist of a single pair of positive and negative pulses preceded and followed by 0 V. As such, the quality of the signal's return to 0 V becomes critical. This is sometimes called the "damping" function (see Figure 3) and has a strong impact on ultrasound modes such as harmonic imaging where the nonlinearity of the human body is the primary source of information.

Consequently, the symmetry between the return to 0 V from a positive versus a negative pulse, as well as how quickly this occurs, become factors in determining the quality of the output signal's linearity.
On-Resistance
The resistance of an output transistor in the on-state is critical to the operation of the ultrasound transmitter. First, the on-resistance determines, along with the load, the rise and fall times of the output signal, setting the achievable output frequency. Second, it directly affects the power dissipation. Based on the aforementioned voltage and current ranges, large amounts of power dissipation are expected during an ultrasound transmit event. The extent of this dissipation is determined by the interplay between the high voltage and low duty-cycle in cases like B-mode or harmonic imaging, versus the low voltage and continuous operation of CW Doppler-type imaging modes.

Other key performance parameters of an ultrasound transmitter system include the output signal's jitter and phase noise, as well as the delay matching between channels.
Semiconductors on the Scene
The semiconductor technologies have been the backbone of the progress in the communications and computer industries over the past few decades. Now they hold the promise of a similar breakthrough in medical technologies, particularly in imaging applications. Ultrasound is no exception, already witnessing a move that started happening from the conventionally used discrete systems into fully integrated semiconductor chip-based solutions. Thanks to their inherent advantages of higher speed, lower power consumption and smaller area, semiconductor ICs can help medical imaging manufacturers reduce their time-to-market, enable portability of end devices, and improve product reliability and performance, while keeping costs in line.

It is now possible to realize the transmit and receive, as well as the transmit/receive switching functions with monolithic IC solutions. Currently available IC transmitters are capable of producing +/–100 V output voltages with slew rates up to 8 V/ns, and second harmonic distortion lower than –40 dBc. Pulse symmetry and fast return-to-zero are possible with active damping architectures. For instance, the TX734 from Texas Instruments is a ±90 V, ±2 A, 3-level, 4-channel, integrated transmitter with active damping capability. This integrated ultrasound pulser, along with the AFE5851, a 16-channel analog-front-end chip, and TX810, an eight-channel transmit/receive switch, are examples of IC solutions for ultrasound systems.
Conclusion
The medical imaging field has made significant strides in the past few decades. Ultrasound technology plays a particular role in these advancements, proving to be a versatile diagnostics tool in applications ranging from obstetrics to intravascular imaging, to the guiding of needles in some procedures, and even to the treatment of certain benign and malignant tumors. The semiconductor IC technologies are supporting this progress at an increasing pace. With the advent of different ICs fulfilling each of the primary functions of an ultrasound system, significant improvements such as portability, better image resolution and higher product reliability are beginning to be within reach of clinicians and other users.WDD

Ismail Oguzman is Senior IC Design Engineer and Member of Group Technical Staff. Ismail can be reached at ti_ismailoguzman@list.ti.com. Arash Loloee is Senior IC Design Engineer and Member of Group Technical Staff. He can be reached at ti_arashloloee@list.ti.com.

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