1 Introduction

Modern ultrasound systems employ as many as 256 transducer elements in a probe. Increasingly, there is growing demand to add more transducer elements to a traditional corded ultrasound probes while keeping their size the same. Additionally, there is growing demand develop portable ultrasound smart probes where more analog circuitry is added into the probe (so it can be portable) while keeping the probe form factor the same size (or making the probe even smaller). These portable ultrasound smart probes have smaller effective space for designing in all needed circuitry and need to consume less power to enable battery powered portability (cordless ultrasound smart probes). One way to design a high channel count system is to drive each transducer element with one transmitter channel. However, ultrasound transmitters consume more power than multiplexers which impedes portability; a device which consumes more power will naturally require a larger battery for the same run time. Furthermore, the number of transmitters needed in a high channel count system take up more PCB area than using a combination of transmitters and multiplexers.

Taking a closer look at how high voltage multiplexers fit into an ultrasound system are shown in Figure 1-1. In this context, the multiplexer is responsible for connecting the transmit and receive circuitry to the transducers. The channels of the multiplexer are activated in turn to drive sequential sections of the probe elements. Each multiplexer channel passes the transmitter’s ±100V pulses to each piezoelectric element. When an element subsequently receives the return signal from the subject of imaging, the multiplexer also passes this signal back to the receiver circuitry for amplification, digital conversion, processing, and display.

The need for multiplexers is present in traditional ultrasound devices in addition to newer, portable ultrasound devices. Consider cart-based ultrasound systems, where power and board area are not as constrained as in smart hand-held ultrasound devices. For such systems, a limiting factor for designing a high channel count system is cable and probe design. Broadly speaking, each ultrasound probe element needs a corresponding driver signal source. For example, to maintain good signal integrity, a 256-channel system with only transmitters requires that each of probe element has a corresponding signal and ground wire in the connecting cable. The size of the cable is reduced in a probe design containing high voltage multiplexers. The multiplexers in the probe body route a reduced number of transmitter signal lines in the cable to the original quantity of transducer elements; the signal source for each element is shared between the wires of the cable, creating a lighter and more material efficient design. Figure 1-2 illustrates a 1:2 configuration, where one transmitter and receiver channel are connected to two transducer elements.

Depending on the design of the multiplexer IC, the IC is powered in one of two ways: either with high voltage supplies, or a low voltage supply. TI’s latest multiplexer, TMUX9832, is a 32 channel 1:1 device operating on a nominal 5V single ended supply. This eliminates the need for HV isolation within the probe, compared to probes with a dedicated HV supply. It also reduces heat and power dissipation because of the lower supply voltage involved.