Datasheet Report - February 14 2007


PDF from Texas Advanced Optoelectronic Solutions (TAOS)
TAOS website
Available from Mouser for $7.20 each

Description
On an eight-pin dual inline package, this ROHS-compliant linear sensor array uses a 64x1 array of photodiodes for a variety of intended uses, including "mark detection and code reading, optical character recognition (OCR), contact imaging, edge detection, and positioning, as well as optical linear and rotary encoding." This sensor includes internal control logic which needs only a serial-input (SI) and a clock input.

Each of the 64 photodiodes measures 120m x 70m and are placed 55m apart, which translates to 200 dots-per-inch (DPI) sensor pitch(resolution?).

The overall dimensions (in inches) of the sensor are: 0.44W x 0.26H x 0.175 thick.

How it Works
The clock (5-5000kHz) controls the integration time for each of the 64 pixels. Each pixel has 64 clock pulses to 'read' the light hitting it before the outputting the sensor data and the integration time is reset. Each pixel is reset sequentially - meaning pixel 2 will dump its data and be reset one clock pulse after pixel 1 and so on.

How it Really Works
As light strikes each photodiode (pixel), photocurrent is generated and sent to (integrated with) the 'active integration circuitry' associated with that pixel. The integration circuitry charges a sampling capacitor directly proportionally to the light intensity and the integration time. The integration time is 64 clock pulses (one pulse for every pixel).

Each capacitor discharges sequentially through an analog switch to an output amplifier, which then generates a voltage output. So as pixel 1 is discharging, pixel 32 is halfway through its integration period. This sequential output is controlled by a 64-bit shift register. The shift register is controlled by the serial input pulse, which must happen on the 65th pulse of the clock. On that 65th pulse, the shift register is reset, which starts the sequence all over. The shift register may be reset as early as the 66th pulse (meaning the first clock pulse of a new output cycle).

The voltage output (VO) is an analog value generated from this equation:

Vout = Vdrk + (Re) (Ee) (tint)

Where:

Vout is the analog output voltage for white condition
Vdrk is the analog output voltage for dark condition
Re is the device responsivity for a given wavelength of light given in V/(W/cm2)
Ee is the incident irradiance in W/cm2
tint is integration time in seconds


Typical Output ranges: The output is nominally 0 V for no light input, 2 V for normal white-level, and 3.4 V for saturation light level. When the device is not in the output phase, the analog out pin (AO) is in a high impedance state.

Note: A 330-Ohm resistor must be placed between pins 6+7(Ground pins) and pin 3 (AO).


Operating Characteristics

  1. The recommended operating voltage of this device is between 4.5V - 5.5V, though +5.0V is the nominal value.
  2. The operating temperature range is -25C to +85C, though the suggested maximum value is only +70C.
  3. The clock frequency range is 5 - 5000 kHz.
  4. Input light source wavelength must be between 400 - 1000 nanometers. In terms of the color of that range, the photodiodes on the TSL-201 are responsive to just above the 'Near-UV' range (which is 200 - 400nm) through about half of the 'Near-Infrared range' (750 - 1400nm) and all of the visible light in between. The bell curve of the spectral responsivity peaks near 685-700nm (a range that humans would perceive as the color 'red').
  5. It is recommended that a 0.01F to 0.1F capacitor with short leads is used to decouple the power supply lines close to the device package for optimum performance.


Questions
Terminology

  1. What is "Sensor Pitch?" (page 1 of Datasheet)
  2. What is "Clamp Current?" (page 3)
  3. What is "Solder Reflow," "Load Resistance," and "Sensor Integration Time?" (table on page 3)
  4. What is the difference between "normal white level" and "saturation light level?"


How this works

  1. The datasheet mentions a "voltage follower" (page 2) to read in the Voltage Output. Does this mean that what ever is reading in that voltage also needs to be running at the same clock frequency as the TSL-201?
  2. I'm unsure of the nature of the Serial Input pulse. The table on page 4 states the minimum duration of that pulse must be 20nanoseconds. But no 'information' (as in an ASCII character) needs to be sent to the sensor - the pin on the sensor only needs to be set 'high', right?