TX810

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SLLS996A – SEPTEMBER 2009 – REVISED APRIL 2010

APPLICATION INFORMATION

Similar to discrete T/R switch solutions, external components can be used to optimize system performance.

Inductor L and resistor RLOAD before the low voltage receiver amplifier (LVRx) can improve overload recovery

time and reduce reflection. The L acts as a high pass filter thus overshoot or recovery response spikes can be

suppressed to minimal. The L and RLOAD terminate the entire signal path and can reduce reflection; therefore

axial resolution in ultrasound image might be improved. However, the combined impedance of L and RLOAD may

affect the system sensitivity. The insertion loss of T/R switch is determined by the input impedance of receiver

amplifier and RON of the TX810. L also creates a DC path for any offset caused by mismatching.The inductor can

be as low as 10s µH to suppress low frequency signals from transmitter, transducer, multiplexer, and TX810. The

optimization of L and RLOAD is always an important topic for system designers. AC coupling are typically used

between transmitter and T/R switch or T/R switch and amplifier. Thus amplifiers with DC biased inputs will not

interference with T/R switch.

One challenge for integrating multiple channel circuits on a small package is how to reduce cross talk. In

ultrasound systems, acoustic cross talk from adjacent transducer elements is a dominant source. The cross talk

from transducer elements is in a range of -30 to -35dBc for array transducers. Circuit cross talk is usually at least

20dB better than the transducer cross talk. The special considerations were implemented in both TX810 design

and layout. The cross talk among TX810 channels is reduced to below -60 dBc as show in the specification

table.

In ultrasound Doppler applications, modulation effect in system can influence image quality and sensitivity.

Ultrasound system is a complex mixed-signal system with all kinds of digital and analog circuits. Digital signals

and clock signals can contaminate analog signals on system level or on chip level. Nonlinear components, such

as transistors and diodes, can modulate noise and contaminate signals. In Doppler applications, the Doppler

signal frequency could range from 20Hz to >50KHz. Meanwhile, multiple system clocks are also in this range,

such as frame clock, image line clock, and etc. These noise signals could enter chip through ground and power

supply pins. It is important to study the power supply modulation ratio (PSMR) at chip level. Noise signal with

certain frequency and amplitude can be applied on supply pins. Side band signals could be found if modulation

effect exists. The PSMR is expressed as an amplitude ratio between carrier and side band signals. Beside

PSMR, 3rd order intermodulation ratio (IMD3) is a standard specification for mixed-signal ICs. Users can use

IMD3 to estimate the potential artifact Doppler mirror signals. Both specs can be found in the specification table.

The schematic of the basic connection for TX810 is shown in Figure 15. Optional inductors and resistors can be

used at TX810 outputs depending on transducer characteristics as discussed above. Standard decoupling

capacitors 0.1µF should be placed close to power supply pins. The pin out of TX810 is optimized for PCB layout.

All signals are going from left to right straightly.

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