ISL29001 Datasheet, PDF (8/11 Pages) Intersil Corporation – Light-to-Digital Sensor

ISL29001
The internal oscillator, F_OSC, operates identically in both the
internal and external timing modes, with the same
dependence on R_EXT. However, when using one of the three
external timing modes, the number of clock cycles per
integration is no longer fixed at 32,768, but varies with the
chosen integration time, and is limited to 65,536. In order to
avoid erroneous lux readings the integration must be short
enough not to allow an overflow in the counter register.
t_in
t
<
6_----5_---,_--5_---3_---6_--
f_OSC
(EQ. 6)
where:
t_int = user defined integration time
F_OSC = 327.6kHz*100kΩ/R_EXT. ISL29001’s internal
oscillator (not to be confused with the I²C’s frequency).
R_EXT = user defined external resistor to adjust F_OSC.
100kΩ recommended.
The number of clock cycles in the previous integration period
is provided in the third and fourth bytes of data read across
the I²C bus. This two-byte value is called the integration
counter value.
When using one of the three external timing modes, the
ISL29001’s resolution varies with the integration time. The
resolution is determined by the ratio of the max lux range to
the number of clock cycles per integration.
Equation 7 describes the light intensity as a function of
sensor reading, integration counter value, and integration
time:
E(Lux)
=
--------¹---⁰---^,---⁰----⁰---⁰----^l--^u---^x---------
(R_EXT ⁄ 100kΩ)
⋅
D_-----a_----t_--a_----1_--
Data2
(EQ. 7)
where E is the measured light intensity, Data1 is the sensor
reading, Data2 is the integration counter value and R_EXT is
external resistor value.
Noise Rejection and Integration Time
In general, integrating type ADC’s have an excellent
noise-rejection characteristics for periodic noise sources
whose frequency is an integer multiple of the integration
time. For instance, a 60Hz AC unwanted signal’s sum from
0ms to n*16.66ms (n = 1,2...n_i) is zero. Similarly, setting the
ISL29001’s integration time to an integer multiple of periodic
noise signal greatly improves the light sensor output signal
in the presence of noise. The integration time, t_int, of the
ISL29001 is set by an external resistor R_EXT. See
Equation 2.
DESIGN EXAMPLE 1
Using the ISL29001, determine a suitable integration time,
t_int, that will ignore the presence of both 60Hz and 50Hz
noise. Specify the R_EXT value accordingly, given that the I²C
clock is at f_I2C = 10kHz.
Solution 1 - Using Internal Timing
t_int = n(1/60Hz) = m(1/50Hz). In order to achieve both 60Hz
and 50Hz AC rejection, the integration time needs to be
adjusted to coincide with an integer multiple of the AC noise
cycle times.
n/m = 60Hz/50Hz = 6/5. The first instance of integer values
at which t_int rejects both 60Hz and 50Hz is when m = 5, and
n = 6.
t_int = 6(1/60Hz) = 5(1/50Hz) = 100ms
From Equation 2:
R_EXT = t_int * (100kΩ/100ms) = 100kΩ. By populating
R_EXT = 100kΩ, the ISL29001 defaults to 100ms integration
time and will reject the presence of both 60Hz and 50Hz
power line signals.
Solution 2 - Using External Timing
From Solution 1, the desired integration time is 100ms. Note
that the R_EXT resistor does not determine the integration
time when using external timing mode. Instead, the
integration and the 16-bit counter starts when an external
timing mode command is sent and end when another
external timing mode is sent. In other words, the time
between two external timing mode command is the
integration time. The programmer determines how many
clock cycles to wait between two external timing commands.
i_I2C = f_{I2C *} t_int, where i_I2C = number of I²C cycles
i_I2C = 10kHz _*100ms
i_I2C = 1,000 I²C clock cycles. An external timing command
1,000 cycles after another external timing command rejects
both 60Hz and 50Hz AC noise signals.
IR Rejection
Any filament type light source has a high presence of infrared
component invisible to the human eye. A white fluorescent
lamp, on the other hand has a low IR content. As a result,
output sensitivity may vary depending on the light source.
Maximum attenuation of IR can be achieved by properly
scaling the readings of Diode1 and Diode2. The user obtains
data reading from sensor diode 1, D₁, which is sensitive to
visible and IR, then reading from sensor diode 2, D₂ which is
mostly sensitive from IR. The graph in Figure 9 shows the
effective spectral response after applying Equation 8 of the
ISL29001 from 400nm to 1000nm. Equation 8 describes the
method of cancelling IR in internal timing mode.
D₃ = n(D₁ – kD₂)
(EQ. 8)
Where:
data = lux amount in number of counts less IR presence
D₁ = data reading of Diode 1
D₂ = data reading of Diode 2
n = 1.85. This is a fudge factor to scale back the sensitivity
up to ensure Equation 8 is valid.
8
FN6166.6
December 10, 2008