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An overview of photometry applications, trends

13 May 2013  | Robert Burnham, Antoine Lourdes Praveen Aroul, Lijoy Philipose

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Hardware: Previously clinical pulse-oximeter and optical heart rate monitors have been discrete component solutions. Variations in component and manufacturing tolerances require calibration after the device is manufactured. Advances in electronic technology have enabled design of fully integrated pulse oximeter and optical heart rate monitor solutions.

For example, the Texas Instruments integrated analogue front-end for pulse oximeters (AFE4490) and integrated analogue front-end for heart rate monitors and low-cost pulse oximeters (AFE4400) devices are leading the way, by providing a fully integrated analogue front-end (AFE) for pulse-oximeter and photo-based heart rate monitor solutions. This family of devices will be enhanced as additional versions are introduced in the near future.

Figure 3: Block diagram representing the ambient cancellation loop (closed by the host processor) using the integrated AFE devices.

In addition to the low-noise receiver channel, LED transmit section with LED current control capability, and diagnostics for LED and photodiode fault detection, this AFE integrates a completely configurable timer module. This timer module offers programmability and flexibility in controlling the various timing edges for sampling and conversion of LEDs and ambient phases. The advantage of the timer module is that it completely off-loads the pulse sequencing and timing control usage of the timers from the host processor to synchronise the LED sampling and data conversions. Thus, loading on a microcontroller is reduced and operation in lower power modes is maximised. The timer module also provides high resolution of control on the LED turn on/off times, which allows for optimisation of power versus performance. The device's integrated fault detection provides warning of open circuit and short circuit of the system LEDs and photodiode.

Another unique feature offered by these AFE devices is the ambient light cancellation scheme. The receiver of the AFE4400 provides digital samples corresponding to ambient duration. The host processor can run an algorithm to process these ambient values and estimate the amount of ambient light leakage. Based on the algorithm, the host processor sets the value for the ambient cancellation digital-to-analogue converter (DAC). The ambient cancellation stage can subtract the ambient component and gain up the pleth components of the received signal. The gain settings of this stage helps to achieve lower input referred noise.

When compared to existing discrete solutions, integrated analogue front-end devices hold over eleven different circuits into a single IC. This integration is touted to provide significant improvements: 60 per cent less cost, reduced size by 90 per cent, and reduced power by at least 50 per cent. Consequently, the overall bill of materials is also 90 per cent less. These savings and reductions make it possible for more cost-effective pulse-oximeter products to be developed that can reach the growing worldwide markets.

About the authors
Robert John Burnham is HealthTech Strategic Marketing Manager at TI where he is responsible for new product definition, business case, marketing strategy, system engineering and customer support. Robert has more than three decades of engineering experience with at least 20 patents to his credit. Robert received his BSEE with Honours from Anglia University, UK.

Antoine Lourdes Praveen Aroul is an applications engineer at Texas Instruments where he is responsible for designing new semiconductor products. He also supports silicon evaluation reviews and customers through the complete design-in process. Antoine received his PhD in Computer Engineering degree from the University of Texas at Dallas, Richardson, Texas

Lijoy Philipose is an applications manager with more than 15 years of engineering experience. Currently, Lijoy is responsible for TI's HealthTech business unit where he develops and builds reference examples for end equipment, provides customer technical support, and trains analogue applications engineers and customers on using TI analogue and mixed-signal parts to build end products. Lijoy received his BSEE degree from the University of Illinois at Urbana Champaign, Illinois.

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