This article shows you how to use Silent Switcher to improve ultrasound noise and image quality!

The ultrasound system consists of a transducer, a transmitting circuit, a receiving circuit, a back-end digital processing circuit, a control circuit, and a display module. The digital processing module usually contains a field programmable gate array (FPGA), which generates a transmit beamformer and a corresponding waveform pattern according to the system configuration and control parameters. Then, the driver and high-voltage circuits in the transmitting circuit generate a high-voltage signal to excite the ultrasound transducer. The ultrasound transducer is usually made of PZT ceramic. The transducer converts the voltage signal into ultrasound waves that enter the human body and receives the echoes generated by the human tissue. The echoes are converted into small voltage signals and transmitted to the transmit/receive (T/R) switch. The main purpose of the T/R switch is to prevent the high-voltage transmit signal from damaging the low-voltage receive analog front end. After signal conditioning, amplification, and filtering, the analog voltage signal is transmitted to the integrated ADC of the AFE and then converted into digital data. The digital data is transmitted to the FPGA through the JESD204B or LVDS interface for receive beamforming, and then transmitted to the back-end digital part for further processing to create an ultrasound image.

Figure 1. Ultrasound system block diagram.

From the above ultrasound architecture, the system noise will be affected by many factors, such as transmit signal chain, receive signal chain, TGC gain control, clock and power supply. In this article, we will discuss how the power supply affects the noise.

Ultrasound systems offer different types of imaging modes, each with different requirements for dynamic range. This also means that the SNR or noise requirements depend on the different imaging modes. Black-and-white mode requires 70 dB dynamic range, pulsed wave Doppler (PWD) mode requires 130 dB, and continuous wave Doppler (CWD) mode requires 160 dB. For black-and-white mode, the noise floor is very important, as it affects the maximum depth of the smallest ultrasound echo that can be seen in the far field, that is, penetration, which is one of the key characteristics of black-and-white mode. For PWD and CWD modes, 1/f noise is particularly important. Both PWD and CWD images include low frequency spectra below 1 kHz, and phase noise affects the Doppler spectrum above 1 kHz. Since the ultrasound transducer frequency is usually 1 MHz to 15 MHz, any switching frequency noise in this range will affect it. If there are intermodulation frequencies in the PWD and CWD spectra (from 100 Hz to 200 kHz), there will be a noticeable noise spectrum in the Doppler image, which is unacceptable in ultrasound systems.

On the other hand, by considering the same factors, a good power supply can improve ultrasound images. Designers should be aware of several factors when designing a power supply for ultrasound applications.

As mentioned previously, it is important to avoid introducing unintended harmonic frequencies into the sampling band (200 Hz to 100 kHz). This type of noise can be easily found in power supply systems.

Most switching regulators use resistors to set the switching frequency. The error of this resistor will introduce different switching nominal frequencies and harmonics on the PCB. For example, in a 400 kHz DC/DC regulator, a 1% precision resistor provides ±1% error and a 4 kHz harmonic frequency. A better solution is to choose a power conversion switch with synchronization. An external clock will send a signal to all regulators through the SYNC pin, causing all regulators to switch to the same frequency and the same phase.

Additionally, for EMI considerations or higher transient response, some regulators have a 20% variable switching frequency, which can result in 0 kHz to 80 kHz harmonic frequencies in a 400 kHz power supply. Constant frequency switching regulators can help solve this problem. ADI's Silent Switcher family of power regulators and power modules feature constant frequency switching while maintaining excellent EMI performance without spread spectrum, as well as excellent transient response.

There are also many sources of white noise in ultrasound systems, which can cause background noise in ultrasound imaging. This noise mainly comes from the signal chain, clocks, and power supplies.

It is now common practice to add LDO regulators to the analog power pins of analog processing components. ADI’s next generation LDO regulators feature ultra-low noise of approximately 1 μV rms and cover currents from 200 mA to 3 A. The circuit and specification parameters are shown in Figure 2 and Figure 3.

Figure 2. Next-generation low-noise LDO regulator.

Figure 3. Low noise spectral density of the next-generation LT3073.

When designing a data acquisition board in an ultrasound system, there is often a trade-off between the high current power supply section and the highly sensitive signal chain section. The noise generated by the switching power supply can easily couple into the signal path traces and is difficult to remove through data processing. Switching noise is typically generated by the switch input capacitance (Figure 4) and the hot loops generated by the upper or lower side switches. Adding a snubber circuit can help manage electromagnetic radiation; however, it also reduces efficiency. The Silent Switcher architecture helps improve EMI performance and maintain high efficiency even at high switching frequencies.

In addition to the heating caused by ultrasound absorption, the temperature of the transducer itself has a significant influence on the temperature of the tissue near the transducer. Ultrasound pulses are generated by applying an electrical signal to the transducer. Some of the electrical energy is dissipated in the components, lens, and substrate material, resulting in heating of the transducer. In addition, electrical heating may be generated by electronic processing of the received signal in the transducer head. The removal of heat from the transducer surface can increase the temperature of the surface tissue by several degrees Celsius. The maximum permissible transducer surface temperature (TSURF) is specified in IEC standard 60601-2-37 (2007 edition). 1 The maximum permissible transducer surface temperature is 50°C when the transducer signal is transmitted into air and 43°C when transmitted into a suitable prosthesis. The latter limit means that the skin temperature (usually 33°C) can be increased by up to 10°C. In complex transducers, transducer heating is an important design consideration and in some cases these temperature limitations may effectively constrain the achievable acoustic output.

Silent Switcher module technology is a wise choice for ultrasonic power rail design. This mode was introduced to help improve EMI and switching frequency noise. Traditionally, we should focus on the circuit and layout design on the hot loop of each switching regulator. For the buck circuit, as shown in Figure 4, the hot loop contains the input capacitor, top MOSFET, bottom MOSFET, and parasitic inductance caused by traces, routing, boundaries, etc.

The Silent Switcher module mainly provides two design methods:

First, by creating opposing hot loops, most of the EMI will be reduced due to bidirectional radiation, as shown in Figures 4 and 5. By this approach, an optimization of nearly 20 dB will be achieved.

Figure 4. Schematic diagram of the split heat loop.

Figure 5. Comparing silent switching and non-silent switching EMI performance.

Second, as shown in Figure 6, instead of soldering directly around the chip, the Silent Switcher module uses a copper pillar flip-chip package, which helps reduce parasitic inductance and optimize spike and dead time.

Figure 6. Copper pillar flip chip package and its performance (LT8614) compared to traditional bonding technology (LT8610).

Additionally, as shown in Figure 7, Silent Switcher technology provides a high power density design and enables high current capability in a small package, keeping θ JA low for high efficiency (for example, the LTM4638 is capable of 15 A in a 6.25 mm × 6.25 mm × 5.02 mm package).

Figure 7. Inside view of the Silent Switcher power module package.