ISL76683_1103 Datasheet, PDF (11/17 Pages) Intersil Corporation – Light-to-Digital Output Sensor

ISL76683
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; 2ⁿ 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 = 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.
TABLE 12. RESOLUTION AND INTEGRATION TIME SELECTION
RANGE1
f_OSC = 327kHz
n t_INT (ms)
RESOLUTION
LUX/COUNT
16 200
0.01
RANGE4
f_OSC = 655kHz
t_INT (ms)
RESOLUTION
(LUX/COUNT)
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
R_EXT = 100kΩ
External Scaling Resistor R_EXT and f_osc
The ISL76683 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 100kΩ and 655kHz:
fosc1 = ¹-- × ¹----⁰----⁰----^k---^Ω--- × 655kHz
2 R_EXT
(EQ. 6)
fosc2
=
1_----0_----0_---k_----Ω_---
R_EXT
×
655 k
H
z
(EQ. 7)
f_OSC1 is oscillator frequency when Range1 or Range2 are set.
This is nominally 327kHz when R_EXT is 100kΩ.
f_OSC2 is the oscillator frequency when Range3 or Range4 are
set. This is nominally 655kHz when R_EXT is 100kΩ.
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.
f_OSC1
=
1_--
2
(
f_O
S
C
2)
(EQ. 8)
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 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.
When using the Internal Timing Mode, f_OSC and n-bits resolution
determine the integration time. t_int is a function of the number of
clock cycles and f_OSC.
t_int
=
2ⁿ × ----¹-----
f_osc
for Internal Timing Mode only
(EQ. 9)
n = 4, 8, 12, and 16. n is the number of bits of resolution.
Therefore, 2ⁿ is the number of clock cycles. n can be programmed
at the command register 00(hex) bits 1 and 0.
Since f_OSC is dual speed depending on the Gain/Range bit, t_int is
dual time. The integration time as a function of R_EXT and n is:
t_int1 = 2ⁿ × ₃----₂----₇----_k---_H--^R--_z---^E--_×-^X---^T₁----₀---₀----_k----_Ω---
(EQ. 10)
t_int1 is the integration time when the device is configured for
Internal Timing Mode and Gain/Range is set to Range1 or
Range2.
t_int2 = 2ⁿ × ₆----₅----₅----_k---_H--^R--_z---^E--_×-^X---^T₁----₀---₀----_k----_Ω---
(EQ. 11)
t_int2 is the integration time when the device is configured for
Internal Timing Mode and Gain/Range is set to Range3 or
Range4.
TABLE 13. INTEGRATION TIMES FOR TYPICAL R_EXT VALUES
R_EXT
(kΩ)
RANGE1
RANGE2
n = 16-BIT n = 12-BIT
RANGE3
RANGE4
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
**Recommended R_EXT resistor value
11
FN7697.2
March 17, 2011