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VGAs integrate on-board CWD beamformers

Posted: 12 Aug 2009 ?? ?Print Version ?Bookmark and Share

Keywords:gain amplifier? VGA? beamformer?

MAX2036/MAX2038 VGA

From Maxim Integrated Products comes what it claims to be the first fully-integrated SiGe variable-gain amplifiers (VGAs) with on-board continuous-wave Doppler (CWD) beamformers designed specifically for high-performance ultrasound-imaging applications.

The MAX2036 and MAX2038 deliver the highest levels of integration, sensitivity, and dynamic range, along with the lowest output-referred noise of any competing integrated solution. The fully integrated CWD beamformer eliminates the need for external phase rotator ICs, delay lines, analog switches, and mixers.

The MAX2036 and MAX2038 combine the linearity and noise performance of the previously introduced MAX2035/MAX2037 VGAs with the added functionality of an on-chip, octal quadrature mixer array and programmable LO phase generators. During normal operation, the MAX2036/MAX2038 are configured so that either the VGA path is enabled while the mixer array is powered down (VGA mode), or the quadrature mixer array is enabled while the VGA path is powered down (CW mode). When in CW mode, the LO phase dividers can be programmed to allow four, eight or 16 quadrature phases for optimal beamforming resolution.

Detecting Doppler shifts
The MAX2036 and MAX2038 possess the necessary dynamic range and noise performance to detect Doppler shifts ranging from 100kHz to less than 1kHz. The mixers and LO generators are designed to have exceptionally low thermal and jitter noise performance of -155dBc/Hz at 1kHz offset from a 900mVp-p, 1.25MHz carrier signal. This exceptional noise performance is critical for detecting weak, low-velocity, Doppler-shift blood-flow signals. In ultrasound systems, these low-velocity Doppler-shift signals are the most difficult to detect due to the existence of exceptionally strong blocking signals commonly referred to as clutter. Since Doppler measurements must be made in the presence of a continuous-wave transmit signal, the amount of clutter signal generated from stationary tissue and bone reflections, as well as transmit-to-receive crosstalk in the transducer, can be as high as 200mVp-p at the receiver's input. The presence of these clutter signals degrades the receiver's ability to accurately detect weak, low-velocity Doppler signals (i.e., those which are closest in frequency to the CW carrier signal). Because the MAX2036/MAX2038 have high close-in noise performance, they excel in making these difficult-to-detect, velocity blood-flow measurements.

When combined with the MAX2034 quad LNA, the MAX2036/MAX2038 VGA mode yields an incredibly low, cascaded output-referred noise of only 20nV/rt-Hz2x less noise than the closest competing chipset solution. Another key feature of these VGAs is that they have been tailored to support either 10- or 12bit ADCs. The MAX2036 has been optimized to support 10bit ADCs, thus meeting the need for greater variable-gain range performance. The MAX2038 is optimized for 12bit ADCs, for applications where superior dynamic range performance is needed to capitalize on the ADC's inherent resolution.

The MAX2036/MAX2038 are available in Pb-free, 100-pin TQFP packages. Prices start at $39.98 (1,000-up, FOB USA).

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