Analog Circuitry Cures Medical Application Ills

Portable and lower cost medical equipment has created a need for new analog circuitry. According to market research firm Frost & Sullivan, the market for portable or hand-carried ultrasound systems will increase with a 19.2 percent compound annual growth rate and reach $330 million by 2010.

“What the medical community wants to do is go toward more of a handheld, battery-operated environment,” says Art Eck, senior product marketing manager, portable applications, Microchip Technology.

In fact, medical imaging such as ultrasound is an application target for several analog IC suppliers. “It used to be that these machines were large consoles, like pushcart types and they had 128 channels or 256 channels,” says Suresh Ram, key marketing segment leader for medical imaging, National Semiconductor. “Those things operated out of the wall outlet, so as long as you had about 15A, you were fine.”

Power is not the only problem analog circuitry has to address in portable, battery-power applications. For plugged-in applications, engineers could use more channels to provide better image capability and quality. Emerging point-of-care situations have dictated portability. One important portable application is a patient being rushed in an ambulance to the emergency room. While the machines may be laptop or even smaller units, users do not want to sacrifice image quality or diagnostic capability. This drives the need for technologies with lower power consumption with comparable performance. “That’s the unique challenge,” says Ram. “How do you provide performance at lower power?”

For some medical applications, the specifications for performance are dictated. For example, in patient monitoring, the monitors follow strict guidelines that are set out by AAMI (Assn. for the Advancement of Medical Instrumentation) and IEC requirements. Signal limits and bandwidths in electrocardiogram (ECG or EKG) are set by standards that must be met for end customers to sell the equipment. Ultrasound and imaging equipment also have safety standards, but performance enhancements are easier to achieve. In these applications, image quality can become a differentiating factor.

For Analog Devices, integration was the solution for differentiation. ADI introduced the AD9271 just over a year ago. While ADI offers separate low noise amplifiers (LNAs), variable gain amplifiers (VGAs), op amps that could be used for the anti-aliasing filter (AAF) and analog-to-digital converters (ADCs), the AD9271 combines all of these functions into a single chip. “Basically you have the transducer signal coming in and you have digital out and it’s a fully integrated solution,” says Scott Pavlik, strategic marketing manager, healthcare segment, Analog Devices.

Since portable products tend to have smaller form factors, some as small as the size of an iPod or PDA, small space and low power are major factors. The 8-channel integrated device has a serial peripheral interface (SPI) port for programming the gain on the LNA and the VGA, filter cut-off frequency and other post processing parts of the ADC.

Addressing the Portable Environment

Portable medical equipment can pose special challenges, especially for units used in home health care. In hospital usage, routine processes include alcohol swabbing for a good reduced-noise contact and using fairly expensive medical probes that have amplifiers in them. For personal use, measurements include pulse, blood pressure, EKG, brain waves and more. Since a person who is not a medical professional handles the equipment, the preparation suffers and noise increases. In addition, the electrodes can be abused a lot more, so they need to be low cost. As a result, the analog designer has to deal with poor contacts and is not allowed to put expensive silicon in the end of the probes. This requires more gain and much lower noise amplifiers and high common-mode rejection.

Eliminating the line cord for portable applications causes other issues for amplifiers. Instead of the dual supply typically used in line-powered patient monitors, a single supply is usually employed for simplified power management in batterypower applications. “The operating environment for patient monitors that are line connected are usually for the bedside or a surgical suite and they have far stricter RFI requirements, so you need a much wider common-mode range for RFI immunity and filtering and you tend to need the dual supplies,” says ADI’s Pavlik. Meeting the AAMI spec of ±200 mV consumes much of the common mode range with the offset voltage caused by leads that are connected to the patient, where the resistance varies over time due to the integrity of the contact to the body. But the dual supplies can be dc-coupled without major problems. “The single supply you can still dc-couple but it is a little more challenging because you have less head room,” says Pavlik.

One answer is Analog Devices’ AD8613/AD8617/AD8619, single, dual and quad micropower, rail-to-rail input and output amplifiers. These units have low supply current, low input voltage and low current noise. “If you operate on 5V or even 3.3V, you can operate to the rail fully, so you’re eating up say ±300 mV of offset and you still have plenty of headroom,” says Pavlik.

Power consumption in portable devices is important for many reasons. “You want higher efficiency because you want the battery to last, but most of the folks I am talking to want higher efficiency just because the device itself is so compact that there isn’t a real easy way to extrude the heat,” says Microchip’s Eck. He considers that efficiency is being more heavily driven in handheld products by the need to reduce heat.

To address the power consumption issue in ADCs, Microchip’s MCP3551 22-bit ?S converter automatically goes into shutdown mode and consumes less than 1 µA. But that is just one of the problems it addresses. “Most A to Ds have a lot of noise, so you will get a 24-bit ?S but you can only get 18 noise free bits out of it,” says Eck.“The 3551 will give you 21.9 noise free bits if you do your layout correctly.”

This noise level is so low that special boards have to be used with special design considerations. Microchip’s AN1007, "Designing with the MCP3551 Delta-Sigma ADC," provides considerable details for system engineers to avoid noise problems. “To address noise, you attack resolution but users frequently throw out bits because of their noise floor,” says Eck. The tradeoff is one of those choices the designer must make.

National Semiconductor’s Ram also points out the problem of transitioning from plugged-in to battery-power operation. He says it is not that difficult to provide front ends with very low noise amplifiers that use a little more power in the line-powered ADC to provide a better signal to noise ratio (SNR). However, he insists providing those capabilities in a laptop requires different technologies.

A pipeline ADC is a good example. “If you look at pipeline converters, the lowest power converter available from a competitor today is about 64 mW a channel,” says Ram. In National’s ADC12EU050, a 12-bit 40 to 50 Msps ADC, lower power of 44 mW/channel (typ) at 50 Msps or 40 mW/ch (typ) at 40 Msps was accomplished with little to no decrease in performance thanks to a unique architecture.

“Typically with pipeline converters, you have a sample and hold stage and depending on how fast the converter runs, what bandwidth the converter is a capable of handling, determines how exotic that sample and hold stage is,” says Ram. A converter with a 12-bit or a bandwidth of 100 or 200 MHz is not that difficult to drive or there is nothing special about the sample and hold circuitry. However, with National’s technology, a sample and hold is not required, just a resistive input, eliminating a stage that frequently consumes a bit of power. Even though the National part works at 1.2V supply, it can be driven at 2.1V peak-to-peak input. This provides increased dynamic range.

Texas Instruments tackled the space limitation, noise consideration and power consumption requirements of portable ultrasound systems with a fully integrated IC. Specifically designed for medical applications, Texas Instrument’s AFE5805 is an analog front end that boasts 50 percent smaller, 40 percent less noise and 20 percent less power than existing integrated solutions.

According to Xiaochen Xu, systems and applications engineer for TI’s Medical Business Unit, “A fully integrated analog front end allows you to design portable ultrasound systems with superior imaging quality and with the lowest power consumption.” The chip consumes 122 mW/channel at 40 Msps with a noise factor of 0.85 nV/vHz.