ISL76683

11

FN7697.1

January 31, 2011

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

function is especially useful when light conditions are varying

drastically while maintaining excellent resolution.

Number of Clock Cycles, n-bit ADC

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.

simplicity, the proportionality constant is referenced to fixed

constants 100k

Ω and 655kHz:

runs at half speed compared to when Range/Gain bits are set to

Range3 and Range4.

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 ISL76683 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.

n = 4, 8, 12, and 16. n is the number of bits of resolution.

at the command register 00(hex) bits 1 and 0.

Internal Timing Mode and Gain/Range is set to Range1 or

Range2.

Internal Timing Mode and Gain/Range is set to Range3 or

Range4.

TABLE 12. RESOLUTION AND INTEGRATION TIME SELECTION

n

RANGE1

RANGE4

RESOLUTION

LUX/COUNT

RESOLUTION

(LUX/COUNT)

16

200

0.01

100

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

(EQ. 6)

fosc1

1

2

---

100k

Ω

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

655

×

kHz

×

=

(EQ. 7)

fosc2

100k

Ω

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

655

×

kHz

=

(EQ. 8)

1

2

---

()

=

(k

Ω)

RANGE1

RANGE2

RANGE3

RANGE4

n = 16-BIT

n = 12-BIT

n = 12-BIT

n = 4

50

100

6.4

3.2

0.013

100**

200

13

6.5

0.025

200

400

26

13

0.050

500

1000

64

32

0.125

*Integration time in milliseconds

2

n

1

----------

×

=

(EQ. 9)

for Internal Timing Mode only

n

327kHz 100k

Ω

×

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

×

=

(EQ. 10)

n

655kHz 100k

Ω

×

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

×

=

(EQ. 11)