Transmit/receive chipset for portable ultrasound systems offers eight channels, superior imaging

TechOnline India - September 08, 2010

Targeting low-power and portable multichannel ultrasound systems, a four-chip array from National Semiconductor provides medical instrumentation system designers with what the vendor says is the industry's first eight-channel transmit/receive chipset with an extensive array of features and functions. Using this eight-channel chipset, a high-resolution, 128-channel battery-operated unit yielding enhanced image quality is now practical.

September 7, 2010

Santa Clara, CA—Targeting low-power and portable multichannel ultrasound systems, a four-chip array from National Semiconductor provides medical instrumentation system designers with what the vendor says is the industry's first eight-channel transmit/receive chipset with an extensive array of features and functions. Using this eight-channel chipset, a high-resolution, 128-channel battery-operated unit yielding enhanced image quality is now practical.

The major functions of the chipset include a receive-side analog front end (AFE) which interfaces directly to the ultrasound transducer assembly, a transmit/receive switch to block out high-power transit pulses from the receiver front end to avoid saturation and recovery time lag, a high-voltage transmit pulser, and a configurable transmit beamformer (note: National did not include the receive beamformer; they told me that medical ultrasound OEMs have various proprietary "secret sauce" algorithms for this function which they prefer to implement themselves). The chipset, which implements the analog, "real world" functions and I/O, is designed to interface to an FPGA.

The chipset ICs, all tagged with the PowerWise designation, are:

1) LM96511 Ultrasound Receive Analog Front End (AFE) provides eight channels low-noise amplification, followed by digitally controlled variable gain amplifier (DVGA) and a 40-50 MHz ADC with LVDS outputs and eight demodulators for continuous wave (CW) Doppler beamforming. Power consumption in B-mode is 110 mW/channel, and channel-to-channel gain matching within 0.06 dB (typical), which National says is four times better than competitive units. The dynamic range in CW Doppler mode is 161 dB, which allows measurement of low-velocity blood flow in organs such as the liver, while phase noise is -144 dBc/Hz at 5 kHz offset, and phase match is 0.35 degree.

2) LM96530 Ultrasound Transmit/Receive Switch, which has eight T/R switches and includes clamping diodes to allow individual channels to be shut off. Input-referred noise is 0.5 nV/rt-Hz, and on-resistance is just 16 ohms (these parameters are related to receiver sensitivity and image resolution), each of which the vendor maintains is at least twice as good as units on the market. The daisy-chained SPI interface used for control dramatically reduces the number of FPGA pins needed to program this function.

3) LM96550 Ultrasound Transmit Pulser, also an octal device, and which generates the bipolar 50V pulses needed by the transducer. It supplies peak currents up to 2A at rates up to 20 MHz (B mode); CW mode pulsing is 10V at 0.6A. The rise/fall delay match is 2.5 nsec, and it monitors on-chip temperature to provide overtemperature protection.

4) LM96570 Ultrasound Configurable Transmit Beamformer simplifies board layout, since it can be placed next to the pulser, instead of performing the beamforming in the FPGA and then having to route I/O signal lines from the FPGA to the pulser. Maximum pulse rate is 80 MHz, and delay resolution is 1/1280 microseconds, with a delay range of 100 microseconds and maximum pattern length of 64 pulses. Its 25 ps pk-pk jitter performance exceeds FPGA-based beamforming by an order of magnitude, yielding much-higher resolution images in B-mode, and low blood-velocity measurements in CW Doppler mode. Typical power dissipation is 63 mW.

By providing all these functions in a functionally matched chipset, the vendor claims that OEMs will get far better performance at lower cost, quicker time to market, and all with lower power. For example, the DVGA offers better dynamic range, spectral performance, and channel matching than an analog VGA. The continuous-time sigma-delta (CTSD) ADC provides a "brick-wall" anti-aliasing filter functions, but at lower power than the low-order anti-aliasing filters of conventional pipeline ADCs. It also has faster recovery from overloads, such as occur when the ultrasound signal reflects from a bone rather than softer tissue.

Development support: Full reference designs, evaluation boards, and tool sets are offered along with this chipset. The development package includes WaveVision 5 acquisition analysis hardware and software, and a user-friendly GUI. A signal-source board provides three different types of signals for the AFE; another board demonstrates AFE performance in different operating modes, and a third board captures AFE output for post processing; in addition, an evaluation board with 8 independent inputs and SMA connectors is available.—Bill Schweber

To see a video demonstration of the LM96511 AFE operating in B-mode and CW Doppler-mode, with input signals that emulate a real ultrasound environment, visit http://bit.ly/LM965xxDemoVideo"> Demo Video

Packaging, Pricing, Power and Availability: Available now, the LM96511 AFE is supplied in an 11 mm x 17 mm, 376-pin BGA package and is priced at $55 each in 1,000-unit quantities (1.2, 3.3, and 5V). The LM96530 transmit/receive switch is packaged in a 9 mm x 9 mm, 60-pin LLP (+5, -5, 3.3, and -65V), the LM96550 pulser in a 12 mm x 12 mm, 80-pin LLP (+50, -50, +10, -10, and 3.3V), and the LM96570 beamformer in a 5 mm x 5 mm, 32-pin LLP (1.8 and 3.3V). The LM96530, LM96550 and LM96570 are priced at $8, $20 and $6, respectively, each in 1,000-unit quantities. All three latter devices are sampling now, with production volumes available by November 2010.

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