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  Ultrasound Imaging Syst...  
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Anti-Alias Filter (AAF) and ADC
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アンチエイリアスフィルタ(AAF)とADC
  Ultrasound Imaging Syst...  
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The ultrasound image-path receivers are used to detect 2D as well as pulsed-wave Doppler (PWD) signals necessary for color-flow imaging and spectral PWD. The receivers include a Tx/Rx switch; a low-noise amplifier (LNA); a variable-gain amplifier (VGA); an anti-alias filter (AAF); and an analog-to-digital converter (ADC).
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超音波画像パスレシーバは、カラーフロー画像処理やスペクトルパルス波ドップラー(PWD)に必要な2D信号とPWD信号を検出するために使用します。このレシーバには、Tx/Rxスイッチ、低ノイズアンプ(LNA)、可変利得アンプ(VGA)、アンチエイリアスフィルタ(AAF)、およびアナログ-デジタルコンバータ(ADC)が内蔵されています。
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Tx/Rx开关可以保护低噪声放大器免受高压发射脉冲的影响，同时在接收间歇期间隔离低噪声放大器输入和发射器。该开关一般采用一组正确偏置的二极管阵列实现，当有高压发射脉冲出现时，它们会自动闭合或断开。Tx/Rx开关必须具备很快的恢复时间，以保证接收器在发射一个脉冲后能够立刻开启。这些快速恢复时间对于浅埋成像和提供低导通电阻确保接收灵敏度至关重要。
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Figure 2 shows that the receiver noise floor in the MAX2082 transceiver at low gains is about 50nV/√Rt. This noise is the output noise contribution of the 12-bit ADC, VGA, and AAF. In a well-designed receiver the ADC should dominate this noise.
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In this example it should be noted that we assumed an LNA gain of 18.5dB, as this is typically the most useful LNA gain setting. This gain setting provides sufficient LNA input range and very good noise figure. In most cases, higher LNA gain settings tend to reduce the LNA input range and restrict near-field imaging with marginal improvements in noise figure. For example, a 6dB increase in LNA gain typically lowers the input range by a factor of 2. Lower LNA gains, however, increase the allowable input range but sacrifice noise performance to an unacceptable level.
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A typical high-performance ultrasound receiver lineup is shown in Figure 1. It consists of a low-noise amplifier (LNA), VGA, anti-alias filter (AAF), and ADC. The LNA buffers the incoming signal and provides sufficient gain to overcome the noise of the subsequent stages.
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The minimum gain of the receiver was selected to ensure that the largest allowable LNA input signal would not saturate the ADC in the near field. With the MAX2082 receiver, the maximum input signal with the LNA gain at 18.5dB is 330mVP-P. The maximum input range of the 12-bit ADC is 1.5VP-P. As a result, the minimum receiver gain needs to be no greater than 20 × log (1.5/0.33) or about 13.2dB. For the MAX2082, the minimum gain is actually 5.9dB, which provides an additional 7.3dB of margin.
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A typical high-performance ultrasound receiver lineup is shown in Figure 1. It consists of a low-noise amplifier (LNA), VGA, anti-alias filter (AAF), and ADC. The LNA buffers the incoming signal and provides sufficient gain to overcome the noise of the subsequent stages.
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The minimum gain of the receiver was selected to ensure that the largest allowable LNA input signal would not saturate the ADC in the near field. With the MAX2082 receiver, the maximum input signal with the LNA gain at 18.5dB is 330mVP-P. The maximum input range of the 12-bit ADC is 1.5VP-P. As a result, the minimum receiver gain needs to be no greater than 20 × log (1.5/0.33) or about 13.2dB. For the MAX2082, the minimum gain is actually 5.9dB, which provides an additional 7.3dB of margin.
  Compact Medical Ultraso...  
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Analog IC manufacturers have responded to this challenge with more highly integrated solutions. Thus, it is now common to find octal receivers that include the LNA, VGA, AAF, and ADC in packages as small as 10mm x 10mm.
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Power is also a major concern in these highly integrated designs. Many of these ultrasound systems are portable and must run from a battery for an hour or more between charges. Heat management is also problematic as the component density is very high and the PCBs can be very close together, leaving little room for airflow. The ultrasound transceivers represent a significant portion of the overall system power budget and, therefore, warrant significant design attention.