TSL245

INFRARED LIGHT-TO-FREQUENCY CONVERTER

SOES018 – MAY 1995

5

POST OFFICE BOX 655303

APPLICATION INFORMATION

power-supply considerations

For optimum device performance, power-supply lines should be decoupled by a 0.01-

µF to 0.1-µF capacitor

with short leads (see Figure 7).

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 techniques depends on the desired resolution and data-acquisition

rate. For maximum data-acquisition rate, period-measurement techniques should be used.

Period measurement requires using a fast reference clock with available resolution directly related to reference

clock rate. The technique measures rapidly varying light levels or provides a fast measurement of a constant

light source.

Maximum resolution and accuracy can be obtained using frequency-measurement, pulse-accumulation, or

integration techniques. Frequency measurements provide the added benefit of averaging random- or

high-frequency variations (jitter) resulting from noise in the light signal. Resolution is limited primarily 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.

TSL245

Timer / Port

MCU

0.1

µF

VDD

2

3

1

Figure 7. Typical TSL245 Interface to a Microcontroller