Front-end IC decides the overall performance of the ultrasound system

Ultrasound system is widely used in the most sophisticated signal processing machines.

As with any complex instrument, physical performance, and cost requirements, implementation to make many trade-offs. Have system-level of knowledge required for a full understanding of front-end IC functionality and performance level is necessary, particularly low-noise amplifier (LNA), time gain compensation amplifiers (TGC, a variable gain amplifier) and analog-to-digital converter (ADC). These analog signal processing IC is to determine the overall performance of the system. Front-end IC characteristics provides system performance, once the introduction of noise and distortion, which is virtually impossible to get rid of them.

In all of the ultrasonic system, contains 48 256 SMIC coaxial cable for relatively long cable (approx. 2m) ends with a multi sensor array.

In some array using high speed (HV) multiplexer or multi-channel distributor to decrease the transmit and receive hardware complexity, but to use the flexibility to price. Using the most flexible systems are phased array columns digital beamformer system-they are also often the most expensive system, because of the need to implement all channel full electronic control. However, like the AD8332 dual variable gain amplifier (VGA) and AD8335 four 12-bit VGA and AD9229 four ADCs that front-end IC are led per channel continues to reduce the cost and power consumption, thus even making low-cost systems are likely to achieve all channels of full electronic control.

In the send (Tx), Tx beamformer first decided to set the desired delay sending the focus mode, and then use the driver sends a high voltage sensor amplifier beamformer output.

In the receive (Rx) end, a transceiver (T/R) switch, it is usually a segregation of high voltage Tx pulsed diode rectifier bridge, they followed a LNA and one or more VGA.

Figure 1: DBF system basic diagram.

Zoom in after completing the simulation Beamforming (ABF) or digital Beamforming (DBF).

In addition to continuous wave (CW) Doppler processing, its dynamic range too large and cannot be used with the imaging channel, the current system mostly DBF. Finally, handle the Rx-beam to display grayscale images, 2D colored image and (or) color Doppler output.

The purpose of the ultrasound system is the body's internal organs of the exact image and the second is through the Doppler signal processing to determine the status of blood flow in the body movement.

The following analysis of ultrasonic system in achieving these aims, the signal attenuation, power consumption, as well as the dynamic range of technology challenges and front-end IC selection considerations.

Signal attenuation

Ultrasound system has three major gets modes: B mode (grayscale imaging, 2D), F (Colorflow imaging or Doppler imaging, blood testing) and D (spectral Doppler).

Medical ultrasonic frequency range 1MHz-40MHz, external Imaging usually use 1MHz? 15MHz frequency range, but the frequency of the venous system use is as high as 40MHz.

For a given penetration distance attenuation, organizations will decay signal frequency. Signal experiences about 1dB/cm/MHz attenuator, i.e. for a signal and 10MHz 5cm penetration depth, round-trip signal will decay 5 × 2 × 10 = 100dB.

The large dynamic range of the received signal presented a serious challenge: one issue is receiving circuits must also have very low noise and large signal processing power, another important issue is called quick overload recovery capabilities.

Even if the T/R switch should prevent the receiver receives big pulse, the pulse is still a small part from switch leakage and enough to make the receiver overload. Poor overload recovery will make the receiver is in "blind" status until it is resumed, it will be on the resulting image from skin surface distance direct impact.

ABF and DBF system

In the simulation of the ABF and DBF ultrasound system, first of all to all channel delay or storage along the beam reflection from a specific focus, and then receive pulse chronological, and its coherence sum-this provides spatial processing gain because the channel is not relevant, between noise and signal is related; it produces 10 * log (N) processing gain of theory, where n is the number of channels.

The image can be formed in two ways: one way is to use analog delay line delay analog sequence values they sum and the sum is converted into digital values (ABF); another option is to pass on as closely as possible sensor array unit of digitizing analog values, put them into a sample storage (FIFO), and then on their digital sum (DBF).

As the figure gives a DBF system basic diagram, ABF system with variable delay line instead of ADC and FIFO.

Both systems require a very good match between channels. It should be noted that these two systems implementations need VGA, ABF system only requires a very high resolution and low speed of ADC (after the sum on the signal down-conversion), DBF system requires a lot of high speed, high resolution ADC because it to radio frequency (RF) bandpass sampling.

Dynamic range

The base of the LNA noise decided to receive weak signals.

But at the same time LNA also must be able to handle very large signal, in particular continuous wave (CW) Doppler signal processing. Therefore, the requirements for LNA have the maximum dynamic range is very important (in General, because of the noise and signal distortion limitations in any filtering before the LNA is not possible).

CW Doppler signal with ultrasound system all signal maximum dynamic range.

In CW signal processing period, half sensor arrayContinuous send sine wave, while the other half sensor array to receive the signal. Tx signal may leak to Rx-end. Because of the Doppler signal very weak, it is not easy in the demodulation on before leakage signal filtering, so any CW signal processing IC needs to have very large dynamic range.

According to the current level of technology development, cannot handle from DBF system main B and F mode channel of CW Doppler signal.

For this reason, in order to address as shown by the CW Doppler signal needs an ABF. Of course, DBF ultrasound system of the "Holy Grail" (Holy Grail) can link through to DBF realistic costs and power consumption for all modes.

Power considerations

Because the ultrasonic system requires a lot of channels, so all IC and low power consumption is critical.

In order to ultimately bring all ultrasound modes are integrated into a beamformer, people always seek to increase the dynamic range front end; meet the ultrasound system reduces the volume and the corresponding demand led portable and power requirements. Digital circuit power consumption is reduced as the power supply voltage decreases, but for analog or mixed signal circuit, you may not. Decrease the analog circuit "voltage allowance" will reduce the dynamic range, so one expects the dynamic range, the power supply voltage to achieve much lower is limited.

This article concludes

This article attempts to give a decision ultrasound system front-end IC required key specification technical background.

The most important are: the large dynamic range, high overload capacity, low intermodulation distortion (LNA) and flat group delay (excellent large signal bandwidth), and perhaps most important is the very low power consumption. Practical application of the results showed that the ultrasonic system on IC's request is very close to the communications or radar system on high-performance receiver applications.