ISL29003
The transfer function effectively for each timing mode
becomes:
TABLE 12. RESOLUTION AND INTEGRATION TIME
SELECTION
Range ( k ) × ------------------
R EXT
E = ---------------------------------------------------- × DATA
2
INTERNAL TIMING MODE
100k Ω
n
(EQ. 4)
n
15
RANGE1
f OSC = 327kHz
t INT RESOLUTION
(ms) LUX/COUNT
200 0.06
RANGE4
f OSC = 655kHz
t INT RESOLUTION
(ms) (LUX/COUNT)
100 2
Range ( k ) × ------------------
R EXT
E = ---------------------------------------------------- × DATA
EXTERNAL TIMING MODE
100k Ω
COUNTER
(EQ. 5)
11
7
3
12.8
0.8
0.05
1.0
15.6
250
6.4
0.4
0.025
62.5
1,000
16,000
f OSC 1 = --- × ------------------ × 684 kHz
R EXT
f OSC 2 = ------------------ × 684 kHz
f OSC 1 = --- ( f OSC 2 )
n = 3, 7, 11, or 15. This is the number of clock cycles
programmed in the command register.
Range(k) is the user defined range in the Gain/Range bit
in the command register.
R EXT is an external scaling resistor hardwired to the R EXT
pin.
DATA is the output sensor reading in number of counts
available at the data register.
2 n represents the maximum number of counts possible in
Internal Timing Mode. For the External Timing Mode the
maximum number of counts is stored in the data register
named COUNTER.
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 = 3. For maximum resolution without regard of
time, choose n = 15. 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.
7
R EXT = 100k Ω
External Scaling Resistor R EXT and f OSC
The ISL29012 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
in Equations 6 and 7:
1 100k Ω (EQ. 6)
2
100k Ω (EQ. 7)
R EXT
f OSC 1 is oscillator frequency when Range1 or Range2 are
set. This is nominally 342kHz when R EXT is 100k Ω .
f OSC 2 is the oscillator frequency when Range3 or Range4
are set. This is nominally 684kHz 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 by using Equation 8:
1 (EQ. 8)
2
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 ISL29012 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.
FN6476.2
November 17, 2011
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