7
FN6505.1
August 20, 2009
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
command. The 1st I
2
C sync command starts the conversion.
The 2nd completes the conversion then starts over again to
commence the next conversion. The integration time, t
int
, is
the time interval between the two sync pulses:
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
to avoid erroneous readings the integration time must be short
enough not to allow an overflow in the counter register.
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
a window lens design. The bigger the diameter of the
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 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.
TABLE 10. INTEGRATION TIME OF n-BIT ADC
R
EXT
(kΩ) n = 16-BIT n = 12-BIT n = 8-BIT n = 4-BIT
250 50ms 3.2ms 200µs 12.5µs
500** 100ms 6.25ms 390µs 24µs
1000 200ms 12.5ms 782µs 49µs
1500 300ms 18.8ms 1.17ms 73µs
2000 400ms 25ms 1.56ms 98µs
**Recommended R
EXT
resistor value
t
int
Timer
f
OSC
-----------------
=
(EQ. 8)
t
int
65,535
f
OSC
------------------
<
(EQ. 9)
TABLE 11. RECOMMENDED DIMENSIONS FOR A FLAT
WINDOW DESIGN
D
TOTAL
D1
D
LENS
@ 35°
VIEWING ANGLE
D
LENS
@ 45°
VIEWING ANGLE
1.5 0.50 2.25 3.75
2.0 1.00 3.00 4.75
2.5 1.50 3.75 5.75
3.0 2.00 4.30 6.75
3.5 2.50 5.00 7.75
t = 1 Thickness of lens
D1 Distance between ISL29020 and inner edge of lens
D
LENS
Diameter of lens
D
TOTAL
Distance constraint between the ISL29020 and lens
outer edge
* All dimensions are in mm.
D
LENS
∅
t
D1
D
TOTAL
∅ = VIEWING ANGLE
WINDOW LENS
ISL29020
FIGURE 4. FLAT WINDOW LENS
E =
DATA
2
16
x 1000
ISL29020
