Fully Integrated Octal VGAs with On-Board CW Doppler Beamformers for High-Performance Ultrasound-Imaging Equipment

The MAX2036/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 such 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 4, 8, or 16 quadrature phases for optimal beamforming resolution.

The MAX2036/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 such excellent 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-Hz--2x less noise than the closest competing chipset solution. Another key feature of these VGAs is that they have been tailored to support either 10-bit or 12-bit ADCs. The MAX2036 has been optimized to support 10-bit ADCs, thus meeting the need for greater variable-gain range performance. The MAX2038 is optimized for 12-bit ADCs, for applications where superior dynamic range performance is needed to capitalize on the ADC's inherent resolution.