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ISL29004
Since fosc 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:
The internal oscillator, f OSC , operates identically in both the
internal and external timing modes, with the same
dependence on R EXT . However, in External Timing Mode,
T int 1 = 2 × ----------------------------------------------
n R EXT
327kHz × 100k Ω
(EQ. 10)
the number of clock cycles per integration is no longer fixed
at 2 n . The number of clock cycles varies with the chosen
T int1 is the integration time when the the device is
configured for Internal Timing Mode and Gain/Range is set
to Range1 or Range2.
integration time, and is limited to 2 16 = 65,536. In order to
avoid erroneous Lux readings the integration time must be
short enough not to allow an overflow in the counter register.
T int 2 = 2 × ----------------------------------------------
T int < ------------------
n R EXT
655kHz × 100k Ω
(EQ. 11)
65,535
f OSC
(EQ. 13)
T int2 is the integration time when the 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
f osc = 327kHz*100k Ω /R EXT . When Range/Gain is set to
Range1 or Range2.
f osc = 655kHz*100k Ω /R EXT . When Range/Gain is set to
Range3 or Range4.
RANGE1
RANGE3
Noise Rejection
R EXT
(k Ω)
RANGE2
n = 16-BIT n = 12-BIT
RANGE4
n = 12-BIT n=4
In general, integrating type ADC’s have an excellent noise-
rejection characteristics for periodic noise sources whose
50
100
6.4
13
0.013
frequency is an integer multiple of the integration time. For
instance, a 60Hz AC unwanted signal’s sum from 0ms to
100**
200
500
200
400
1000
13
26
64
26
52
128
0.025
0.050
0.125
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.
*Integration time in milliseconds
T int = ----------
T × 327kHz × 100 k Ω
R EXT = -----------------------------------------------------------------
R = 50k Ω
**Recommended R EXT resistor value
INTEGRATION TIME IN EXTERNAL TIMING MODE
This timing mode is programmed in the command register
00(hex) bit 5. External Timing Mode is recommended when
integration time can be synchronized to an external signal
such as a PWM to eliminate noise.
For Mode0 or Mode1 operation, the integration starts when
the sync_iic command is sent over the I 2 C lines. The device
needs two sync_iic commands to complete a photodiode
conversion. The integration then stops when another
sync_iic command is received. Writing a logic 1 to the
sync_iic bit ends the current adc integration and starts
another one.
For Mode3, the operation is a sequential Mode0 and Mode1.
The device needs three sync_iic commands to complete two
photodiode measurments. The 1st sync_iic command starts
the conversion of the diode1. The 2nd sync_iic completes the
conversion of diode1 and starts the conversion of diode2. The
3rd sync_iic pules ends the conversion of diode2 and starts
over again to commence conversion of diode1.
The integration time, T int , is determined by the following
equation:
i I2C
(EQ. 12)
f I2C
i I2C is the number of I 2 C clock cycles to obtain the T int.
f I2C is the I 2 C operating frequency
11
Design Example 1
The ISl29004 will be designed in a portable system. The
ambient light conditions that the device will be exposed to is
at most 500Lux which is a good office lighting. The light
source has a 50/60Hz power line noise which is not visible
by the human eye. The I2C clock is 10kHz.
Solution 1 - Using Internal Timing Mode
In order to achieve both 60Hz and 50Hz AC noise rejection,
the integration time needs to be adjusted to coincide with an
integer multiple of the AC noise cycle times.
T int = i(1/60Hz) = j(1/50Hz).
The first instance of integer values at which T int rejects both
60Hz and 50Hz is when i = 6, and j= 5.
T int = 6(1/60Hz) = 5(1/50Hz)
T int = 100ms
Next, the Gain/Range needs to be determined. Based on the
application condition given, Lux(max) = 500Lux, a range of
1000Lux is desirable. This corresponds to a Gain/Range
Range1 mode. Also impose a resolution of n = 16-bit. Hence
we choose equation 10 to determine R EXT .
int
n
2
(EQ. 14)
EXT
for Internal Timing Mode and Gain/Range is set to Range3 or Range
FN6221.1
November 17, 2011
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