TSL1401CL

128 × 1 LINEAR SENSOR ARRAY WITH HOLD

TAOS136 − JULY 2011

9

The LUMENOLOGY

r Company

r

r

Copyright

E 2011, TAOS Inc.

www.taosinc.com

APPLICATION INFORMATION

The minimum integration time can be calculated from the equation:

1

maximum clock frequency

(n * 18) pixels ) 20ms

where:

n

is

the number of pixels

In the case of the TSL1401CL with the maximum clock frequency of 8 MHz, the minimum integration time would

be:

(128 * 18) ) 20ms + 33.75ms

It is good practice on initial power up to run the clock (n+1) times after the first SI pulse to clock out indeterminate

data from power up. After that, the SI pulse is valid from the time following (n+1) clocks. The output will go into

a high-impedance state after the n+1 high clock edge. It is good practice to leave the clock in a low state when

inactive because the SI pulse required to start a new cycle is a low-to-high transition.

The integration time chosen is valid as long as it falls in the range between the minimum and maximum limits

for integration time. If the amount of light incident on the array during a given integration period produces a

saturated output (Max Voltage output), then the data is not accurate. If this occurs, the integration period should

be reduced until the analog output voltage for each pixel falls below the saturation level. The goal of reducing

the period of time the light sampling window is active is to lower the output voltage level to prevent saturation.

However, the integration time must still be greater than or equal to the minimum integration period.

If the light intensity produces an output below desired signal levels, the output voltage level can be increased

by increasing the integration period provided that the maximum integration time is not exceeded. The maximum

integration time is limited by the length of time the integrating capacitors on the pixels can hold their accumulated

charge. The maximum integration time should not exceed 100 ms for accurate measurements.

It should be noted that the data from the light sampled during one integration period is made available on the

analog output during the next integration period and is clocked out sequentially at a rate of one pixel per clock

period. In other words, at any given time, two groups of data are being handled by the linear array: the previous

measured light data is clocked out as the next light sample is being integrated.

Although the linear array is capable of running over a wide range of operating frequencies up to a maximum

of 8 MHz, the speed of the A/D converter used in the application is likely to be the limiter for the maximum clock

frequency. The voltage output is available for the whole period of the clock, so the setup and hold times required

for the analog-to-digital conversion must be less than the clock period.