“ADI’s Silent Switcher power modules and LDO products provide a complete solution for ultrasonic power rail designs that minimize system noise levels and switching noise. This helps improve image quality, but also helps limit temperature rise and simplifies PCB layout design complexity.
The ultrasound market has grown rapidly since digital ultrasound technology was first introduced in 2000 (GE). Ultrasound technology has moved from being based on static to dynamic, and from black and white to color Doppler. As ultrasound applications grow, so do the demands on components, such as those related to probes, AFEs, and power systems.
In the field of medical diagnosis, more and more applications require higher image quality from ultrasound imaging systems. One of the key technologies to improve image quality is to improve the signal-to-noise ratio (SNR) of the system. The different factors that affect noise, especially the power supply, will be discussed below.
How does ultrasound work?
Ultrasound system consists of transducer, transmitting circuit, receiving circuit, back-end digital processing circuit, control circuit and Display module. The digital processing module usually contains a Field Programmable Gate Array (FPGA), which generates the transmit beamformer and the corresponding waveform pattern according to the configuration and control parameters of the system. The drive and high voltage circuits in the transmit circuit then generate high voltage signals to excite the ultrasound transducer. Ultrasonic transducers are usually made of PZT ceramics. The transducer converts the voltage signal into ultrasonic waves into the human body, and at the same time 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 passed to the AFE’s integrated ADC, where it is converted to digital data. The digital data is transmitted through the JESD204B or LVDS interface to the FPGA for receive beamforming, and then to the back-end digital section for further processing to create an ultrasound image.
Figure 1. Block diagram of an ultrasound system.
How does the power supply affect the ultrasound system?
From the ultrasound architecture described above, system noise can 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 power supplies affect noise.
Ultrasound systems offer different types of imaging modalities, each with different requirements for dynamic range. This also means that SNR or noise requirements depend on different imaging modalities. Black and white mode requires 70 dB of 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, 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. 1/f noise is especially important for PWD and CWD modes. Both PWD and CWD images include low frequency spectrum below 1 kHz, and phase noise affects the Doppler spectrum above 1 kHz. Since ultrasound transducer frequencies are typically 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 spectrum (from 100 Hz to 200 kHz), there will be a noticeable noise spectrum in the Doppler image, which is unacceptable in an ultrasound system.
On the other hand, a good power supply can improve ultrasound images by considering the same factors. Designers should be aware of several factors when designing power supplies for ultrasound applications.
As mentioned before, the introduction of unexpected harmonic frequencies into the sampling band (200 Hz to 100 kHz) must be avoided. In power systems, this type of noise is easy to find.
Most switching regulators use resistors to set the switching frequency. Errors in this resistor 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 transfer switch with synchronization capabilities. An external clock will signal all regulators through the SYNC pin to switch all regulators to operate at the same frequency and the same phase.
Additionally, some regulators have a 20% variable switching frequency due to EMI concerns or higher transient response, which results in 0 kHz to 80 kHz harmonic frequencies in the 400 kHz supply. Constant frequency switching regulators help to solve this problem. ADI’s family of Silent Switcher power regulators and power modules feature constant frequency switching while maintaining excellent EMI performance and excellent transient response without turning on spread spectrum.
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, clock and power supply.
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 ultralow noise around 1 μV rms and cover currents from 200 mA to 3 A. The circuit and specifications are shown in Figure 2 and Figure 3.
Figure 2. Next-generation low-noise LDO regulators.
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 high current power supply sections and highly sensitive signal chain sections. Noise generated by switching power supplies is easily coupled into the signal path traces and is difficult to remove with data processing. Switching noise is typically caused by switching input capacitance (Figure 4) and hot loops generated by the upper or lower side switches. Adding a snubber circuit can help manage electromagnetic radiation; it also reduces efficiency. The Silent Switcher architecture helps improve EMI performance and maintain high efficiency even at high switching frequencies.
Handheld Digital Probe
In addition to the heating caused by the absorption of ultrasound, the temperature of the transducer itself greatly affects the temperature of the tissue near the transducer. Ultrasonic pulses can be generated by applying an electrical signal to the transducer. Some of the electrical energy is dissipated in the element, lens, and substrate material, causing the transducer to heat up. Additionally, Electronic processing of the signal received in the transducer head may also generate electrical heat. Removing heat from the transducer surface increases the temperature of the surface tissue by several degrees Celsius. The maximum allowable transducer surface temperature (TSURF) is specified in IEC Standard 60601-2-37 (2007 edition). 1 The maximum allowable transducer surface temperature is 50°C when the transducer signal is emitted into air and 43°C when emitted into a suitable prosthesis. The latter limit means that the skin temperature (usually 33°C) can rise 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.
The safety standard IEC 60601-2-37 (2007 edition)1 limits the temperature of the transducer surface to below 50°C when the transducer is operating in air, and when the transducer is operated at 33°C (for external applications transducer) or 37°C (for internal transducers) when in contact with the prosthesis, the standard limits its surface temperature to below 43°C. Often these temperature limitations (rather than limitations on the maximum intensity in the beam) constrain the acoustic output of the transducer. Silent Switcher devices convert power (with wide switching bandwidths up to 3 MHz) to the different voltage domains of digital probes with the highest efficiency. This means that power losses during power conversion are low. This helps a lot with the cooling system because not much extra power is lost as heat.
Silent Switcher mode helps a lot
Silent Switcher module technology is a smart choice for ultrasonic power rail designs. 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 a buck circuit, as shown in Figure 4, the hot loop contains the input capacitance, top MOSFET, bottom MOSFET, and parasitic inductance due to traces, routing, bounding, etc.
The Silent Switcher module mainly provides two design methods:
First, as shown in Figures 4 and 5, by creating opposing thermal loops, most EMI will be reduced due to bidirectional radiation. With this method, nearly 20 dB will be optimized.
Figure 4. Schematic of split thermal loop.
Figure 5. Comparing silent and non-silent switching EMI performance.
Second, as shown in Figure 6, instead of soldering directly around the chip, the Silent Switcher module is packaged in 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 offers high power density designs and enables high current capability in a small package, keeping θ JA low and achieving high efficiency (for example, the LTM4638 can operate at 6.25 mm × 6.25 mm × 5.02 15 A in mm package).
Figure 7. Inside view of the Silent Switcher power module package.
Table 1. Overview of Silent Switcher modules
Table 2. Popular Silent Switcher Products
In addition, many Silent Switcher modules also feature fixed frequency, wide frequency range and peak current architectures for low jitter and fast transient response. See Table 2 for popular products in this product line.
ADI’s Silent Switcher power modules and LDO products provide a complete solution for ultrasonic power rail designs that minimize system noise levels and switching noise. This helps improve image quality, but also helps limit temperature rise and simplifies PCB layout design complexity.