Inside TI's ADS1298 analog front end for health monitoring
8/9/2012 10:53 AM EDT
It's 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, 24-bit 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, 24-bit 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.