Teardown: Inside the art of pulse oximetry

"These are very small physiological signals—very sensitive to noise and artifacts—so all the electronics, from the sensor all the way through to the signal processing, is all fundamentally new and proprietary," said Sampath.

The signals being captured are 10x to 100x smaller than those encountered at the input of an RF radio system, Sampath said, emphasizing the importance of board layout and good design practices in maintaining signal integrity by ensuring proper signal handling.

But there's more to it than that, he added, noting that many companies deal with signals in this range. For Masimo, it has more to do with having the technology necessary to accommodate patients with varying degrees of skin pigmentation, bone density, tissue scattering and so on. "Because of our signal integrity, we are able to work with the most people on the planet, literally, simply because of the amount of attention we've given to this topic," he said.

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At the heart of the MX-1 is the signal-processing IC: the AD90801 custom ROM product from Analog Devices Inc. Sampath acknowledged the device was based on a Sharc-family processor from ADI but would not specify which one. An online search of the SEC site and other data, however, points to a digital signal processor from the ADSP-2136x line.

Masimo's designers chose that DSP because for their application, "the most important function was performance, especially floating-point performance for the precision that we needed," said Sampath. At the time of the initial design, circa 2004-2005, the part in question was the best choice for the tasks at hand, he said.

The AD90801 is fed by an AKM5355VT low-voltage, 16-bit analog-to-digital converter and is supported by Silicon Storage Technology's SST34HF1621C Combo-Memory, which integrates 16 Mbits of flash with 2 Mbits of SRAM.

The AKM5355VT is a delta-sigma converter originally intended for audio applications, so it comes with an input gain amplifier, making it ideal for the MX-1's low-input signal levels. The analog signal input of the AKM5355 is single-ended, eliminating the need for external filters. That not only saves space and cost but also improves overall signal integrity by narrowing the opportunities for signal loss and interference. The part is housed in a space-saving 16-pin TSSOP.

Further down the signal chain, at the connector where the signals from the sensor first enter the board, is another critical IC: ADI's AD820, a precision, low-power FET input op amp (get quote) that amplifies the low-level signals from the sensor. The part can operate from a single supply of 5 V to 36 V, or from dual supplies of +/"2.5 V to +/"18 V. It features true single-supply capability, with an input voltage range extending below the negative rail. That range lets the device accommodate input signals below ground in the single-supply mode, another prerequisite for meeting the MX-1's needs.

On the digital-to-analog conversion side, the main IC is ADI's DAC8043AF, a high-accuracy 12-bit multiplying DAC featuring serial input, double buffering and 5-V operation at less than 10 mA with a 1-microsecond settling time (get quote). Control, housekeeping and other processing functions are performed by Texas Instruments' ultralow-power MSP430F169 16-bit microcontroller (get quote).

The back of the MX-1 is mainly devoted to driving the LEDs, in this case using a bank of eight AD5449YRU 12-bit, dual-channel current-output DACs from ADI (get quote). Also on the back are three SN74LVC573A octal transparent D-type latches from TI that feature three-state outputs designed specifically for driving highly capacitive or relatively low-impedance loads (get quote).

The output of the DSP is fed via a 10-pin connector on the back of the MX-1 to the main instrument board, which performs the core operational functions of a patient monitor, such as computation and display of measurement results, as well as user-interface and other control functions. Another Sharc shows up here: the ADSP-21062L, a high-performance signal processor originally intended for communications, graphics and imaging applications (get quote).

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Having a high-end DSP on a board that's essentially geared for low-end control and minor processing functions seems like overkill. Sampath acknowledged that the choice here didn't have to be a Sharc and could have been any embedded-type processor, but he added that since Masimo was already using the ADI tools, "using it [the 21062L] on the main board was more an artifact of convenience."

Newer Masimo systems do, in fact, use a generic IC with an ARM core, but Sampath would not identify the device or its maker.

The DSP is supported by 16 Mbits of single-die MirrorBit flash from Spansion (S29AL016D90TF102) and 4 Mbits of low-power SRAM from Alliance (AS6C4008). Next to those devices sits the SC28L92A1B, a 3.3/5.0-V dual universal asynchronous receiver/transmitter (DUART) from NXP (get quote).

Other logic on the top side of the board includes an Epson RTC-72423 4-bit real-time clock module with built-in crystal (get quote), an NXP 7555CD general-purpose CMOS timer (get quote), a Fairchild Semiconductor LCX245 low-voltage bidirectional transceiver (get quote) and a TI HC163 4-bit synchronous binary counter. The HC163 features an internal carry look-ahead for high-speed counting (get quote).

The back of the control board includes four TI HC595 8-bit shift registers with three-state output registers; two HC165 8-bit parallel-load shift registers; one HC138 three-line to eight-line decoder/multiplexer; one TI HC163 4-bit binary counter; and, from Fairchild, an HC32 quad two-input OR gate and an HC541 octal line driver. NXP contributes its HEF4013BT dual D-type flip-flop.

The third board on the Radical-7's handset is the main display board, driven by an Epson S1D13A04F00A1 LCD controller. (Epson note: Not recommended for a new design.)

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