Both product approaches have been very successful in the marketplace. Below is a more detailed analysis of success drivers:
1. Texas Instruments’ AFE44xx family is suited to the pulse oximeter application, which inevitably results with an optimized and compact solution. Integrated additional features (like LED driver and fault diagnostics) make the AFE44xx an “all-in-one” solution for that photometric application. TI’s approach, however, also makes the AFE44xx family very application-specific. The drawback is that AFE44xx is not suitable or could be redundant for many other sensors and applications. On the other hand, Systemcom’s AFE family is very modular and therefore far more versatile and applicable to a wide range of current output sensors and applications.
2. Unlike Systemcom’s SC-I-AFE-180F110, TI’s AFE44xx features a fully-differential transimpendance amplifier (TIA) which eliminates the need for a single-ended to differential converter. This provides better overall simplicity in design but probably at the expense of linearity. The fully-differential TIA inherently biases the photodiode to the voltage of 0V which increases the photodiode’s capacitance. SC-I-AFE-180F110 is designed to bias the photodiode with higher reverse voltage, thus reducing its capacitance and consequently its response time. Such a sensor biasing solution makes SC-I-AFE-180F110 suitable for interfacing other current output sensors, e.g., electrochemical sensors.
3. The four-stage-PGA in SC-I-AFE-180F110 offers a gain of up to 1296 and a transresistance of 64kΩ by TIA. Such a combination of gain and transresistance allows significantly smaller resistance values in TIA and consequentially faster response time. The AFE44xx family provides 1MΩ transresistance by TIA, combined with a gain up to 4 in a single-stage-PGA. The PGA used in the SC-I-AFE-180F110 is based on the switched capacitor architecture, which in general exhibits lower noise performance when compared to solutions based on resistive amplifiers. Both products use the averaging technique to reduce noise and improve SNR.
4. AFE44xx provides limited ambient (light) cancellation (up to 10uA), which involves a procedure run by a microcontroller and DAC to perform this cancellation. Such a solution requires more time to complete and suffers from residual offset (because of 10 discrete steps of DAC). The relative measurement in SC-I-AFE-180F110 offers ambient light suppression performed by the analog circuitry itself regardless of the amount of ambient light. In other words, this allows high gain amplification of the desired signal relative to the ambient light. Furthermore, the ambient light suppression doesn’t rely on the procedure run by a microcontroller or digital post-processing, so it is therefore simpler and faster. The amount of ambient light to be suppressed depends only on the selected current range (up to 1mA) and is not performed by DAC; this reduces the residual offset.
5. The pulse oximeter application allows small low-gain amplifier and ambient light suppression to be done by a microcontroller instead of the analog circuitry; this ultimately simplifies the design and reduces power consumption. In the end, it all comes down to the requirement for specific application and the sensor used. Versatility means a wide spectrum of sensors and possible applications but it also requires an effort to implement it for a specific application. In contrast, a dedicated application means a more optimal, off-the-shelf solution.