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ISL29018
TABLE 9. RANGE/FSR LUX
FSR (LUX) @ ALS
FSR @ IR
Here, Range(k) is defined in Table 9. Count max is the maximum
output counts from the ADC.
BITS 1:0 k
RANGE(k)
SENSING
SENSING
The transfer function used for n-bit ADC becomes Equation 3:
E cal = --------------------------- × DATA
2
00
01
1
2
Range1
Range2
1,000
4,000
Range ( k )
n
(EQ. 3)
10
11
3
4
Range3
Range4
16,000
64,000
Here, n = 4, 8, 12 or 16. This is the number of ADC bits
programmed in the command register. 2 n represents the
maximum number of counts possible from the ADC output. Data
t int = 2 × -------------- = 2 × ----------------------------------------------
f OSC
Data Registers (02 hex and 03 hex)
The device has two 8-bit read-only registers to hold the data from
LSB to MSB for ADC. The most significant bit (MSB) is accessed
at 03 hex, and the least significant bit (LSB) is accessed at 02
hex. For 16-bit resolution, the data is from D0 to D15; for 12-bit
resolution, the data is from D0 to D11; for 8-bit resolution, the
data is from D0 to D7. The registers are refreshed after every
conversion cycle.
TABLE 10. DATA REGISTERS
ADDRESS
is the ADC output stored in the data registers (02 hex and 03
hex).
Integration and Conversion Time
The ADC resolution and f OSC determines the integration time, t int
as shown in Equation 4.
n 1 n R EXT (EQ. 4)
725kHz × 499k Ω
where n is the number of bits of resolution and n = 4, 8, 12 or 16.
2 n , therefore, is the number of clock cycles. n can be programmed
at the command register 01(hex) bits 3 and 2.
(hex)
CONTENTS
TABLE 11. INTEGRATION TIME OF n-BIT ADC
02
03
D0 is LSB for 4, 8, 12 or 16-bit resolution, D3 is MSB for
4-bit resolution, D7 is MSB for 8-bit resolution
D15 is MSB for 16-bit resolution, D11 is MSB for 12-bit
resolution
R EXT
(k Ω )
499**
n = 16-BIT
(ms)
90
n = 12-BIT
(ms)
5.63
n = 8-BIT
(μs)
351
n = 4-BIT
(μs)
21.6μs
**Recommended R EXT resistor value
Range = ------------------ × Range ( k )
f OSC = ------------------ × 725 kHz
499k Ω
E cal = α × DATA
α = ----------------------------
Range ( k )
Interrupt Registers (04, 05, 06 and 07 hex)
Registers 04 and 05 hex set the low (LO) threshold for the
interrupt pin and the interrupt flag. 04 hex is the LSB and 05 hex
is the MSB. By default, the Interrupt threshold LO is 00 hex for
both LSB and MSB.
Registers 06 and 07 hex set the high (HI) threshold for the
interrupt pin and the interrupt flag. 06 hex is the LSB and 07 hex
is the MSB. By default, the Interrupt threshold HI is FF hex for
both LSB and MSB.
Test Register (08 hex)
Register 8 is a reserved register that holds 00h during normal
operation.
Calculating Lux
The ISL29018’s ADC output codes, DATA, are directly
proportional to lux in the ambient light sensing.
(EQ. 1)
Here, E cal is the calculated lux reading. The constant α is
determined by the Full Scale Range and the ADC’s maximum
output counts. The constant is independent on the light sources
(fluorescent, incandescent and sunlight) because of the light
sources’ IR component is removed during the light signal
process. The constant can also be viewed as the sensitivity: the
smallest lux measurement the device can measure as shown in
Equation 2.
(EQ. 2)
Count max
12
External Scaling Resistor R EXT for f OSC and
Range
The ISL29018 uses an external resistor R EXT to fix its internal
oscillator frequency, f OSC and the light sensing range, Range. f OSC
and Range are inversely proportional to R EXT . For user simplicity,
the proportionality constant is referenced to 499k Ω as shown in
Equations 5 and 6:
499k Ω (EQ. 5)
R EXT
(EQ. 6)
R EXT
Noise Rejection
In general, integrating type ADC’s 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.
ADC Output in IR Sensing
The ISL29018’s ADC output codes, DATA, are directly
proportional to the IR intensity received in the IR sensing.
DATA IR = β × E IR (EQ. 7)
Here, E IR is the received IR intensity. The constant β changes
with the spectrum of background IR noise like sunlight and
FN6619.4
October 8, 2012
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