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Linear Array CWDP Doppler Estimator BJT  LNA CMOS  LNA
Predictive ADC


The main advantage of pulsed Doppler ultrasound is its capability to provide spatial information associated to velocity values.  However, in the Doppler modes, both  the weak flow signals and the large-magnitude echoes from the surrounded tissue (commonly referred as a clutter) are simultaneously converted into digital. Thus, to avoid ADC saturation, the analog gain should be set in accordance with the clutter swing. 
It is widely accepted that signals from the blood scatterers can be 40-100 dB weaker than echoes from clutter. Consequently, to map the flow signals into sufficient amount of bits, a high-resolution ADC should be employed. Otherwise, the receive channel may provide an unacceptably low flow resolution due to the quantization noise. For instance, having a -60 dB flow-to-clutter ratio and a 12-Bit ADC, the flow signal resolution is only 2 Bit. 
High-resolution (> 12 Bit) ADCs are more expensive, operate at lower sampling frequency, and consume more power.  To avoid its implementation, a number of commercial ultrasound scanners incorporates a separate analog beamformer for Doppler processing that increases the system complexity and cost.  Therefore, it would be beneficial to develop a technology allowing acquisition of Doppler data by a regular ADC.
In the early phase of Doppler imaging,  it has been proposed to move the ADC within a feedback loop. This technique seeks to arrange an ADC so that the large-amplitude echoes caused by stationary and near-stationary targets are filtered out prior to A-to-D conversion.  Operating as a stationary canceller (or wall filter), such converter incorporates an extrapolative function by which a predicted input signal value is compared with the actual one to generate an error signal and the error signal is converted to digital.  This architecture is known as a Predictive ADC. The same technique is often referred as Predictive Coding, Differential Pulse Code Modulation (DPCM), or Delta Modulation.

We, at EchoeScan, have further developed the concept of tracking/predictive  ADC. The proposed  converter exhibits a feedback loop comprising a conventional ADC, a DAC, a predictive filter, and  a fixed gain amplifier. 
Predictive A/D Converter
The above arrangement  operates as a closed-loop high-pass filter  whose frequency response has a steep transition around zero Hz and substantial flatness.  According to the original concept, the amplitude resolution of  tracking ADC is dictated  by the dynamic range of the DAC. However, implementing a high-resolution DAC per channel would cause the same kind of problems as those associated with high-resolution ADCs.  On the contrary, the proposed architecture utilizes ADC and DAC of the same resolution.  A novel prediction error filter  contributes not only to cancellation of  stationary echoes but simultaneously boosts the weak flow signals.  Thus, these signals are moved into higher bits of the ADC, thereby improving the SNR for  Doppler modalities.  The input-referred frequency response of the predictive ADC is shown below.
A rigorous analysis and thorough simulation of the proposed technique reveal a 4-5 bits reduction in required word-length for a tracking ADC comparing with a conventional counterpart.

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