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8
FN7464.3
October 8, 2007
COUNTER is the number increments accrued for between
integration time for External Timing Mode.
Gain/Range, Range(k)
The Gain/Range can be programmed in the control register
to give Range (k) determining the FSR. Note that Range(k)
is not the FSR. See Equation 3. Range(k) provides four
constants depending on programmed k that will be scaled by
R
EXT
. See Table 9. Unlike R
EXT
, Range(k) dynamically
adjusts the FSR. This function is especially useful when light
conditions are varying drastically while maintaining excellent
resolution.
Number of Clock Cycles, n-bit ADC
The number of clock cycles determines “n” in the n-bit ADC; 2
n
clock cycles is a n-bit ADC. n is programmable in the command
register in the width function. Depending on the application, a
good balance of speed, and resolution has to be considered
when deciding for n. For fast and quick measurement, choose
the smallest n = 4. For maximum resolution without regard of
time, choose n = 16. Table 12 compares the trade-off between
integration time and resolution. See Equations 10 and 11 for the
relation between integration time and n. See Equation 3 for the
relation of n and resolution.
External Scaling Resistor R
EXT
and f
osc
The ISL29003 uses an external resistor R
EXT
to fix its
internal oscillator frequency, f
OSC
. Consequently, R
EXT
determines the f
OSC
, integration time and the FSR of the
device. f
OSC
, a dual speed mode oscillator, is inversely
proportional to R
EXT
. For user simplicity, the proportionality
constant is referenced to fixed constants 100k
Ω
and
655kHz:
Ω
EXT
f
OSC
1 is oscillator frequency when Range1 or Range2 are
set. This is nominally 327kHz when R
EXT
is 100k
Ω
.
f
OSC
2 is the oscillator frequency when Range3 or Range4
are set. This is nominally 655kHz when R
EXT
is 100k
Ω
.
When the Range/Gain bits are set to Range1 or Range2,
f
OSC
runs at half speed compared to when Range/Gain bits
are set to Range3 and Range4.
The automatic f
OSC
adjustment feature allows significant
improvement of signal-to-noise ratio when detecting very low
lux signals.
Integration Time or Conversion Time
Integration time is the period during which the device’s
analog-to-digital ADC converter samples the photodiode
current signal for a lux measurement. Integration time, in
other words, is the time to complete the conversion of analog
photodiode current into a digital signal (number of counts).
Integration time affects the measurement resolution. For
better resolution, use a longer integration time. For short and
fast conversions use a shorter integration time.
The ISL29003 offers user flexibility in the integration time to
balance resolution, speed and noise rejection. Integration time
can be set internally or externally and can be programmed in
the command register 00(hex) bit 5.
INTEGRATION TIME IN INTERNAL TIMING MODE
This timing mode is programmed in the command register
00(hex) bit 5. Most applications will be using this timing
mode. When using the Internal Timing Mode, f
OSC
and
n-bits resolution determine the integration time. t
int
is a
function of the number of clock cycles and f
OSC
.
2
n
osc
n = 4, 8, 12, and16. n is the number of bits of resolution.
2
n
therefore is the number of clock cycles. n can be
programmed at the command register 00(hex) bits 1 and 0.
Since f
OSC
is dual speed depending on the Gain/Range bit,
t
int
is dual time. The integration time as a function of R
EXT
and n is:
R
Ω
t
int1
is the integration time when the device is configured
for Internal Timing Mode and Gain/Range is set to Range1
or Range2.
t
int2
is the integration time when the device is configured
for Internal Timing Mode and Gain/Range is set to Range3
or Range4.
TABLE 12. RESOLUTION AND INTEGRATION TIME
SELECTION
n
RANGE1
f
OSC
= 327kHz
RESOLUTION
LUX/COUNT
RANGE4
f
OSC
= 655kHz
RESOLUTION
(LUX/COUNT)
t
INT
(ms)
200
t
INT
(ms)
100
16
0.01
1
12
12.8
0.24
6.4
16
8
0.8
3.90
0.4
250
4
0.05
62.5
0.025
4000
R
EXT
= 100k
Ω
(EQ. 6)
fosc1
1
2
--
-----------------
655
×
kHz
×
=
(EQ. 7)
fosc2
Ω
EXT
-----------------
655
×
kHz
=
(EQ. 8)
f
OSC
1
1
2
--
f
OSC
2
(
)
=
t
int
----------
×
=
(EQ. 9)
for Internal Timing Mode only
t
int
1
2
n
---------------------------------------------
×
=
(EQ. 10)
t
int
2
2
n
R
Ω
---------------------------------------------
×
=
(EQ. 11)
ISL29003