Thermal imaging & vision systems
and white to colour doppler. A growing number of ultrasound applications has led to increased component requirements such as those related to the probe, AFE, and power system. In the field of medical diagnostics, there are ever increasing demands for higher image quality in ultrasound imaging systems. One of the key techniques for improving image quality is to enhance the signal-to-noise ratio (SNR) of the system. The different factors that affect noise will be discussed below, especially the power supplies.
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HOW DOES ULTRASOUND WORK? An ultrasound system is composed of transducers, transmitting circuits, receiving circuits, back-end digital processing circuits, control circuits, a display module, etc. The digital processing module usually comprises a field programmable gate array (FPGA), which generates the transmit beamformers and corresponding waveform patterns according to the configuration and control parameters of the system. The transmit circuits’ driver and the high voltage circuit then generate a high voltage signal to excite the ultrasound transducers. The ultrasound transducer is usually made of PZT ceramic. It converts a voltage signal into ultrasound waves that enter into the human body while receiving the echoes produced by the tissues. The echoes are converted into a small voltage signal and passed to a transmitting/receiving (T/R) switch. The primary objective of the T/R switch is to prevent the high voltage transmit signal from damaging the low voltage receive analogue front end. The analogue voltage signal after signal conditioning, gaining, and filtering is passed to the integrated ADC of the AFE and then converted into digital data. The digital data is transmitted through a JESD204B or LVDS interface to the FPGA for receive beamforming and then to the back-end digital parts for further processing to create the ultrasound image.
HOW DOES A POWER SUPPLY INFLUENCE ULTRASOUND SYSTEMS? From the ultrasound architecture described above, system noise can be affected by many factors such as the transmit signal chain, the receive signal chain, TGC gain control, clocking, and power supplies. In this article, we will discuss how the power supply can affect noise.
There are different kinds of image modes in an ultrasound system, and each image mode has different requirements for the dynamic range. This also means that the SNR or noise requirements depend on the varying image modes. 70dB dynamic range is required for black and white mode, 130dB is required for pulse wave doppler (PWD) mode, and 160dB is required for continuous wave doppler (CWD) mode. The noise floor is important for the black and white mode, and it impacts the maximum depth the smallest ultrasound echo can be seen in the far field, which is called penetration, one of the key features of black and white mode. The 1/f noise is particularly
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he ultrasound market has grown rapidly following the introduction of the first digital ultrasound (by GE) in 2000. Ultrasound technology has shifted from static-based to dynamic and from black
LOW NOISE SILENT SWITCHER μMODULE AND LDO REGULATORS IMPROVE ULTRASOUND NOISE AND IMAGE QUALITY
Here, Yu Lu, field applications engineer, and Hugh Yu, healthcare systems applications manager at Analog Devices, present a brief introduction to ultrasound imaging systems and provide a detailed analysis of some of the challenges and solutions in ultrasound power management design.
Figure 1. Ultrasound system block diagram.
important for the PWD and CWD modes. Both PWD and CWD images include the low frequency spectrum below 1kHz, and the phase noise impacts the doppler frequency spectrum higher than 1kHz. As the ultrasound transducer frequency is typically from 1MHz to 15MHz, it will be affected by any switching frequency noise within this range. If there are intermodulated frequencies within the PWD and CWD spectrums (from 100Hz to 200kHz), the obvious noise spectrums will appear in the doppler images, which is unacceptable in the ultrasound system. On the other hand, a good power supply can improve ultrasound images by taking into account the same considerations. There are several factors a designer should understand when designing a power supply for an ultrasound application.
Switching Frequency
As mentioned, it is necessary to avoid introducing unexpected harmonic frequency into the sampling band (200Hz to 100kHz). It is easy to find this kind of noise in a power system.
The majority of switching regulators use a resistor to set the switching frequency. The error of this resistor introduces different switching nominal frequencies and harmonics on the PCB. For
Figure 2. Next-generation low noise LDO regulator.
April 2023 Instrumentation Monthly
example, one per cent accuracy resistors provide ± one per cent error and 4kHz harmonic frequency in a 400kHz DC-to-DC regulator. A better solution is to select power switchers with a sync function. The external clock will send a signal to all regulators via the SYNC pin so that all regulators switch at the same frequency and same phase. Also, some regulators feature a variant switching frequency of 20 per cent for EMI consideration or higher transient response, which leads to 0kHz to 80kHz harmonic frequency in a 400kHz power supply. Switching regulators with constant frequency help avoid this issue. ADI’s family of Silent Switcher voltage regulators and
µModule regulators features constant frequency switching, but at the same time keep excellent EMI
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