ISL29125_14 Datasheet, PDF (14/17 Pages) Intersil Corporation – Digital Red, Green and Blue Color Light Sensor with IR Blocking Filter

ISL29125
adjustment is piecewise linear the proper setting can be
determined by extrapolating from a pair of measurements and
calculating the closest intersection of the sources of interest.
The Configuration register (Reg 0x02[7:0]) allows coarse tuning
B7 and fine tuning (B[5:0] of the residual infrared component
from the ALS output.
The recommended procedure for determining ALS IR
compensation is as follows:
• Illuminate the ISL29125 based design configuration with a no
IR F2 light source. Record the ALS measurement and the Lux
level.
• Illuminate the device with A and D65 with heavy IR and the F2
light sources. Take an ALS measurement and Lux level
measurement.
• It really depends on the system setup in order to adjust the
Configuration register (Reg 0x02, B7 and B[5:0]) to
compensate for the IR contribution.
• Repeat steps above until the IR light source contribution to the
ALS measurement is under 10 percent assuming no change in
Lux level due to IR light source.
Figure 14 is an example shows how to calculate the
compensation for varying level of infrared components such as
A, F2 and D65 (see Notes 11, 12 and 13). With compensation
adjustment from 0% to 100%. The crossing point is the IR
compensation value which makes tighter variation of varying
level of infrared components. This setup system is sensor without
IR tinted glass and illuminates with 3 different light sources.
Since it is not under IR tinted glass, then reg0x2 setups like
b7 = ‘0’ and B[5:0] is at about 25% compensation
adjust(%/range) which means about 40 codes.
12000
10000
8000
6000
A
D65
Noise Rejection
Electrical AC power worldwide is distributed at either 50Hz or
60Hz. Artificial light sources vary in intensity at the AC power
frequencies. The undesired interference frequencies are infused
on the electrical signals. This variation is one of the main sources
of noise for the light sensors. Integrating type ADC’s have
excellent noise-rejection characteristics for periodic noise
sources whose frequency is an integer multiple of the conversion
rate. By setting the sensor’s integration time to an integer
multiple of periodic noise signal, the performance of an ambient
light sensor can be improved greatly in the presence of noise. In
order to reject the AC noise, the integration time of the sensor
must to adjusted to match the AC noise cycle. For instance, a
60Hz AC unwanted signal’s sum from 0ms to k*16.66ms
(k = 1,2...k_i) is zero. Similarly, setting the device’s integration
time to be an integer multiple of the periodic noise signal greatly
improves the light sensor output signal in the presence of noise.
Digital Inputs and Termination
The ISL29125 digital inputs are guaranteed to CMOS levels. The
internal register is updated on the rising edge of the clock. To
minimize reflections, proper termination should be
implemented. If the lines driving the clock and the digital inputs
are 50Ω lines, then 50Ω termination resistors should be placed
as close to the sensor inputs as possible, connected to the digital
ground plane (if separate grounds are used).
Temperature Coefficient
The limits stated for temperature coefficient (Tempco) are
governed by the method of measurement. The overwhelming
standard for specifying the temperature drift of a reference is to
measure the reference voltage at two temperatures, take the
total variation, (V_HIGH - V_LOW) and divide by the temperature
extremes of measurement (T_HIGH - T_LOW). The result is divided by
the nominal reference voltage (at T = +25°C) and multiplied by
106 to yield ppm/°C. This is the "Box" method for specifying
temperature coefficient.
Layout and Board Mounting
Considerations
4000
F2
2000
0
0
20
40
60
80
100
COMPENSATION ADJUSTMENT (% RANGE)
FIGURE 14. IR COMPENSATION VALUE
Calculating Lux
Y-coordinate is Ev measured in lux. The data can be converted to
lux by using an equation. There are two different data sensing
ranges (375 lux and 10,000 lux) and also two different resolution
selections (16 bits and 12 bits) on this device. Equation 2 is
dependent on both these parameters.
Ev = Y = (CYRxRed + CYGxGreen + CYBxBlue)xRange
(EQ. 2)
Suggested PCB Footprint
It is important that users check TB477 “Surface Mount Assembly
Guidelines for Optical Dual Flat Pack No Lead (ODFN) Package”
before starting ODFN product board mounting.
Board Mounting
For applications requiring the light measurement, the board
mounting location should be reviewed. The device uses an
Optical Dual Flat Pack No Lead (ODFN) package, which subjects
the die to mild stresses when the printed circuit (PC) board is
heated and cooled, which slightly changes the shape. Because of
these die stresses, placing the device in areas subject to slight
twisting can cause degradation of reference voltage accuracy. It
is normally best to place the device near the edge of a board, or
on the shortest side, because the axis of bending is most limited
in that location.
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FN8424.2
January 24, 2014