December 2004, Issue 173

Light-to-Frequency Conversion (Part 1)
TSL230R-Based Pulse Oximeter

TAOS

In 1999, Texas Advanced Optoelectronic Solutions (TAOS) acquired licensing to produce and market the optoelectronic family of sensors from Texas Instruments. Its mission has been to develop and manufacture semiconductor devices combining photodetectors with precision mixed-signal functionality to give you a light-sensing solution with improved performance and design simplicity. TAOS products include light-to-voltage, light-to-frequency, linear-array, ambient, color, and color-reflective optosensors.

For this project, I chose the TSL230R for its sensitivity and wide spectral response. This programmable light-to-frequency converter has an array of photodiodes and a current-to-frequency converter in an eight-pin package. All the I/O is TTL-compatible, so analog isn’t involved. This removes any concerns associated with small analog signals.

Although a photodiode array isn’t mentioned in the TSL230R datasheet, you can see a 10 × 10 array on the device (see Figure 2). Two inputs to the TSL230R are used to select the device’s sensitivity. Selections include 1×, 10×, and 100×, which leads me to surmise that these selections choose the array size of the photodiodes. Using the 100× selection, device responsiveness is given as 770 Hz/µW/cm2. The spectral bandwidth covers the two areas of interest pertaining to this project: red (600– 700 nm) and infrared (800–940 nm). Although the frequency out is directly proportional to the light intensity, total darkness is not represented by zero frequency. The device always produces output. The minimum might be approximately 1 Hz. At 100 µW, the maximum frequency is approximately 100 kHz (using the 100× mode—the complete array). As expected, the 10× and 1× modes produce this frequency at 10× and 100× the light level of the 100× mode.

The eight-pin TSL230R consists of the photodiode array (with its two sensitivity inputs) with a current-to-frequency converter. Two other inputs select a divisor for the output frequency. This leaves connections for power, ground, frequency output, and an output enable input.