MEDICAL ANALOG DEVICES
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. white mode, and it impacts the maximum depth the smallest ultrasound echo can be seen in the far features of black and white mode. The 1/f noise is particularly 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
Figure 2: Next-generation low noise LDO regulator
There are also many white noise sources in an ultrasound system, which leads to the background noise in ultrasound imaging. This noise mainly comes from the signal chain, clock, and power.
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 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 example, 1% accuracy resistors provide
±1% 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% 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 performance without spread spectrum on and keep excellent transient response.
WHITE NOISE There are also many white noise sources in an ultrasound system, which leads to the background noise in ultrasound imaging. This noise mainly comes from the signal chain, clock, and power. Adding an LDO regulator at the analogue
Figure 3: The low noise spectrum density in the next-generation LDO regulator: LT3045 14 December/January 2025 Irish Manufacturing power pin of the analogue processing component
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