How to improve ultrasound system image quality using ultra-low-noise supplies
noise versus potential efficiency, the need for ultra-low noise in the ultrasound application should prevail. After all, a few more milliwatts of dissipation should not be that much of a burden at the “big-picture” system level. Further, why not increase the energy pulsed by the transducer to increase the pulse signal strength and thus the reflected SNR?
in the transducer head, this will result in heating and a temperature rise at the probe. There is a limit on the maximum allowable transducer surface temperature. IEC standard 60601- 2-37 (Rev 2007) restricts this temperature to 50°C when the transducer is transmitting into air, and 43°C when transmitting into a suitable phantom (a standard body simulator); the latter limit implies that skin (typically at 33°C) can be heated by 10°C at most. Thus, transducer heating is a significant design consideration in complex transducers. These temperature limits may effectively restrict the acoustic output that can be employed, independent of available DC power.
Conclusion
Switching Noise Harmonics
High Thermal Performance
Low Frequency Noise
Ultrasound imaging is a widely used, invaluable, non-invasive, and risk-free medical imaging tool. Although the basic principle is conceptually simple, designing an effective imaging system requires a significant amount of complex circuitry, along with multiple DC regulators to power its various subcircuits. These regulators and associated power must be efficient, but also be very low noise due to the extreme SNR and dynamic range mandates on the reflected acoustic signal energy. As shown, LDOs and Silent Switcher ICs from Analog Devices meet these requirements without compromising space, EMI, or other key attributes.
Ultra-low noise reference in Silent Switcher 3 device
Silent Switcher technology plus Cu pillar package
Silent Switcher technology plus heatsink in package
Architecture
Same performance as an LDO regulator in terms of low f noise Removes the need for post-LDO regulator while keeping the same image quality
Low EMI, low switching noise
High power density Lower thermal resistance
Feature
But this tradeoff has another complication: self-heating in the handheld digital probe that contains the transducer,
High frequency with high efficiency
Minimize degrading for the same current level
Benefit in Application
piezoelectric element driver, AFE, and other electronic circuitry. Some of the probe’s electrical energy is dissipated in the piezoelectric element, lens, and backing material, thus causing transducer heating. Along with wasted acoustic energy
Figure 7: Shown are the key attributes of the Silent Switcher family of regulators relative to important design perspectives. Image source: Analog Devices
component can resolve this
capacitor, top-side MOSFET, bottom-side MOSFET, and parasitic inductances due to wiring, routing, and bonding The standard solution is to add a snubber circuit to reduce electromagnetic emission, but this decreases efficiency. The Silent Switcher architecture improves performance and maintains high
efficiency even at a high switching frequency by creating an opposite hot loop (called “splitting”) using bidirectional emissions, reducing EMI by about 20 dB (Figure 9). Efficiency versus noise It may seem that if there is a tradeoff between power supply
ADI’s next-generation LDO regulators, such as the LT3045 , feature an ultra-low noise level of around 1 microvolt (μV) rms (10Hz to 100kHz), and provide a current output of up to 500 mA at a typical dropout voltage of 260 mV (Figure 8). Operating quiescent current is nominally 2.3 mA and drops to much lower than 1 μA in shutdown mode. Other low-noise LDOs are available to cover current from 200 mA to 3 A. ■ Board layout : in most PC board layouts, there is a conflict between high-current signal traces from the switching power supplies and the adjacent low- level signal traces, as noise from the former can couple into the latter. This switching noise is usually generated by the “hot loop” created by the input
Figure 9: By establishing an opposing “hot loop” that splits the current-flow path, the Silent Switcher significantly cuts EMI by about 20 dB. Image source: Analog Devices
Figure 8: The LT3045 LDO regulators are noted for their ultra-low noise of around 1 μV rms over a current range from 200 mA to 3 A. Image source: Analog Devices
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