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A closer look at ADS1298 AFE for health monitoring

Posted: 21 Aug 2012 ?? ?Print Version ?Bookmark and Share

Keywords:health care devices? vital signs monitoring? electrocardiogram? analog front end?

We are in an exciting time for advances in health care devices. Improvements in health device design are placing vital signs monitoring in the hands of athletes and fitness enthusiasts but more importantly, a new family of patient monitoring devices promise to improve the comfort and outcomes of hospital stays.

Until recently, vital signs monitoring systems have had limited portability due to their bulk and high power consumption. As a result continuous monitoring in a clinical setting is only used for acute patients, leaving 60-70 percent of patients with only periodic monitoring. Patient safety, outcomes and comfort can be improved if affordable, low power, portable monitoring systems can be implemented. These attributes also extend this devices family to the monitoring of chronically ill patients in non-hospital environments.

Chips from Texas Instruments have been playing an instrumental role is this device evolution. Released in 2010 the ADS129x family of devices deliver fully integrated analog front end (AFE) capability for patient monitoring, portable and high-end electrocardiogram (ECG) and electroencephalogram (EEG) equipment. The engineering team developing the integrated circuit had to balance some challenging design targets: power consumption less than 750uW per channel, noise compliant with IEC standards for ECG (10uVp-p input), a size of 8mm x 8mm to enable portable and disposable ECG patch applications and low cost.

We took a look under the hood of one of the ADS129x family of devices. Figure 1 is a die photo of the ADS1298, an 8-channel, 24bit integrated AFE for ECG/EEG applications. The BGA package size is 8mm x 8mm and the die size is 6mm x 5.6mm. The engineers' attention to layout allowed them to integrate 43 discrete IC capabilities with a combined area of 1800 mm2.

Figure 1: Plan view of TI's ADS1298 8 channel, 24bit integrated AFE. The functionality of 43 separate ICs has been integrated into this 8mmx8mm die.

Much care is needed to manage input impedance since the input signals in these applications are weak. Differential input signals pass through an EMI filtering block and a low-noise chopper-stabilized PGA which eliminates 1/f noise. TI's team designed a new analog to digital converter (ADC) core based on a delta sigma (difference sum) converter for the ADS129x family. This ADC is extremely sensitive while consuming 3x lower power than competitors. Highly accurate reference voltage (Vref) signals are provided through the use of a 1.2V bandgap circuit. In addition to attention to layout we noted a design and process emphasis on electrical isolation. There is widespread use of decoupling capacitors to provide electrical isolation between circuit blocks and there are triple potential well structures for complete electrical isolation.

Looking deeper
Looking a little more deeply, Figure 2 shows a SEM cross-section of the ADS1298. A robust and high yielding 0.35um, 4-layer Aluminum Bi-CMOS process has been used. The Bi-CMOS process and low noise bipolar amplifiers contribute to the low power consumption and noise performance.

Figure 2: SEM cross-section of the TI ADS1298 chip showing use of the well-established, high yield and reliable 4-layer Al 0.35?m process.

The ADS129x series is featured in the Sotera Wireless' ViSi Mobile Patient Monitor that received FDA clearance in April 2012. This first generation system from Sotera Wireless is a hospital-based continuous vital signs monitoring system. The system is small, worn on the wrist and includes three sensors:
???Optical thumb sensor for measuring pulse rate and SpO2
???Chest sensor for measuring ECG/HR, respiration, and skin temperature
???Cuff module for single-inflation or automatic NIBP monitoring.

Figure 3: Sotera's ViSi Mobile System takes advantage of the ADS1298's small size and lower power consumption in its wrist-worn monitor. (Image Credit: Sotera Wireless)

The small size improves patient comfort and mobility. The system's low cost brings the benefits of continuous monitoring to a wider range of patients. Under development, and not yet cleared by the FDA, are versions that include additional functionality such as wireless connectivity. Sotera Wireless CTO, Jim Moon, explains the importance of the ADS129x family to their device development: "TI's ADS1298R analog front end for patient monitoring provided the means to make the ViSi Mobile Patient Monitor possible. The reduction in size and power consumption that it delivers is unparalleled."

The medical electronics design sector shares many engineering challenges with consumer electronics, with the added requirement of very high reliability. Devices such as TI's ADS129x family demonstrate that these challenges are being met and that this sector is very much worth watching for technical innovation.

About the author
Treena Grevatt is product marketing manager at UBM TechInsights, a division of UBM LLC, the publisher of EE Times.

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