8

FN7465.2

December 1, 2006

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.

where:

recommended.

The number of clock cycles in the previous integration period

is provided in the third and fourth bytes of data read across

counter value.

When using one of the three external timing modes, the

ISL29002’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.

The following equations describe the light intensity as a

function of sensor reading, integration counter value, and

integration time:

where L is the measured light intensity, Data1 is the sensor

reading, Data2 is the integration counter value, T is the

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

ISL29002’s integration time to an integer multiple of periodic

noise signal greatly improves the light sensor output signal

See Equation 3.

Design Example 1

Using the ISL29002, determine a suitable integration time,

Solution 1 - Using Internal Timing

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

6.

From Equation 3:

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

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.

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 on Figure 9 shows the effective

spectral response after applying Equation 7 of the ISL29002

from 400nm to 1000nm. The equation below describes the

method of cancelling IR in internal timing mode.

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 2 is valid.

k = 7.5. This is a scaling factor for the IR sensitive Diode 2.

T

int

65,536

f

OSC

------------------

<

(EQ. 5)

Elux

()

50 000lux

,

R

⁄

()

-------------------------------------------

Data1

Data2

-----------------

⋅

=

(EQ. 6)

D3

n D1

kD2

–

()

=

(EQ. 7)

ISL29002