TSL245R
INFRARED LIGHT-TO-FREQUENCY CONVERTER
TAOS060D − SEPTEMBER 2007
APPLICATION INFORMATION
Power-supply considerations
Power-supply lines must be decoupled by a 0.01-μF to 0.1-μF capacitor with short leads placed close to the
TSL245R (Figure 5). A low-noise power supply is required to minimize jitter on output pulse.
V
DD
MCU
2
0.1 μF
3
TSL245R
Timer/Port
1
Figure 5. Typical TSL245R Interface to a Microcontroller
Device Operational Details
The frequency at the output pin (OUT) is given by:
fO = fD + (Re) (Ee)
where:
fO
is the output frequency
fD
Re
Ee
is the output frequency for dark condition (Ee = 0)
is the device responsivity for a given wavelength of light given in kHz/(μW/cm2)
is the incident irradiance in μW/cm2
f is an output frequency resulting from leakage currents. As shown in the equation above, this frequency
D
represents a light-independent term in the total output frequency f . At very low light levels, this dark frequency
O
can be a significant portion of f . The dark frequency is temperature dependent. For optimum performance of
O
any given device over the full output range, the value of f should be measured (in the absence of light) and
D
later subtracted from subsequent light measurement (see Figure 1).
Output interface
The output of the device is designed to drive a standard TTL or CMOS logic input over short distances. If lines
greater than 12 inches are used on the output, a buffer or line driver is recommended.
Measuring the frequency
The choice of interface and measurement technique depends on the desired resolution and data-acquisition
rate. For maximum data-acquisition rate, period-measurement techniques are used.
Period measurement requires the use of a fast reference clock with available resolution directly related to
reference-clock rate. The technique is employed to measure rapidly varying light levels or to make a fast
measurement of a constant light source.
Maximum resolution and accuracy may be obtained using frequency-measurement, pulse-accumulation,
or integration techniques. Frequency measurements provide the added benefit of averaging out random- or
high-frequency variations (jitter) resulting from noise in the light signal. Resolution is limited mainly by available
counter registers and allowable measurement time. Frequency measurement is well suited for slowly varying
or constant light levels and for reading average light levels over short periods of time. Integration, the
accumulation of pulses over a very long period of time, can be used to measure exposure — the amount of light
present in an area over a given time period.
Copyright E 2007, TAOS Inc.
The LUMENOLOGY r Company
r
r
4
www.taosinc.com
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