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ISL29020
TABLE 10. INTEGRATION TIME OF n-BIT ADC
a window lens design. The bigger the diameter of the
R EXT
(k Ω)
250
500**
1000
1500
2000
n = 16-BIT
50ms
100ms
200ms
300ms
400ms
n = 12-BIT
3.2ms
6.25ms
12.5ms
18.8ms
25ms
n = 8-BIT
200μs
390μs
782μs
1.17ms
1.56ms
n = 4-BIT
12.5μs
24μs
49μs
73μs
98μs
window lens, the wider the viewing angle is of the ISL29020.
Table 11 shows the recommended dimensions of the optical
window to ensure both 35° and 45° viewing angle. These
dimensions are based on a window lens thickness of 1.0mm
and a refractive index of 1.59.
WINDOW LENS
t
**Recommended R EXT resistor value
?
D1
D TOTAL
INTEGRATION TIME IN EXTERNAL TIMING MODE
The External Timing Mode is recommended when the
integration time is needed to synchronize to an external
signal, such as a PWM to eliminate noise.
The synchronization can be implemented by using I 2 C sync
ISL29020
D LENS
command. The 1st I 2 C sync command starts the conversion.
The 2nd completes the conversion then starts over again to
E=
DATA
2 16
x 1000
? = VIEWING ANGLE
commence the next conversion. The integration time, t int , is
FIGURE 4. FLAT WINDOW LENS
t int = -----------------
the time interval between the two sync pulses:
Timer
f OSC
(EQ. 8)
TABLE 11. RECOMMENDED DIMENSIONS FOR A FLAT
WINDOW DESIGN
D LENS @ 35° D LENS @ 45°
where Timer is the number of internal clock cycles obtained
from data registers and f OSC is the internal oscillator frequency.
The internal oscillator, f OSC , operates identically in both the
internal and external timing modes. However, in External
Timing Mode, the number of clock cycles per integration is no
longer fixed at 2 n . The number of clock cycles varies with the
chosen integration time, and is limited to 2 16 = 65,536. In order
D TOTAL
1.5
2.0
2.5
3.0
3.5
D1
0.50
1.00
1.50
2.00
2.50
VIEWING ANGLE
2.25
3.00
3.75
4.30
5.00
VIEWING ANGLE
3.75
4.75
5.75
6.75
7.75
t int < ------------------
to avoid erroneous readings the integration time must be short
enough not to allow an overflow in the counter register.
65,535 (EQ. 9)
f OSC
t=1
D1
D LENS
D TOTAL
Thickness of lens
Distance between ISL29020 and inner edge of lens
Diameter of lens
Distance constraint between the ISL29020 and lens
outer edge
Noise Rejection
In general, integrating type ADCs have excellent
noise-rejection characteristics for periodic noise sources
whose frequency is an integer multiple of the conversion
rate. 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.
Optical Design
Flat Window Lens Design
A window lens will surely limit the viewing angle of the
ISL29020. The window lens should be placed directly on top
of the device. The thickness of the lens should be kept at
minimum to minimize loss of power due to reflection and
also to minimize loss due to absorption of energy in the
plastic material. A thickness of t = 1mm is recommended for
7
* All dimensions are in mm.
Window with Light Guide Design
If a smaller window is desired while maintaining a wide
effective viewing angle of the ISL29020, a cylindrical piece of
transparent plastic is needed to trap the light and then focus
and guide the light onto the device. Hence, the name light
guide or also known as light pipe. The pipe should be placed
directly on top of the device with a distance of D1 = 0.5mm
to achieve peak performance. The light pipe should have
minimum of 1.5mm in diameter to ensure that whole area of
the sensor will be exposed. See Figure 5.
FN6505.1
August 20, 2009
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