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ISL29003
TABLE 13. INTEGRATION TIMES FOR TYPICAL R EXT VALUES
Noise Rejection
R EXT
(k Ω)
RANGE1
RANGE2
n = 16-BIT n = 12-BIT
RANGE3
RANGE4
n = 12-BIT n=4
In general, integrating type ADC’s have 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
50
100**
200
500
100
200
400
1000
6.4
13
26
64
3.2
6.5
13
32
0.013
0.025
0.050
0.125
0ms to 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.
DESIGN EXAMPLE 1
*Integration time in milliseconds
**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 Mode1 or Mode2 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
The ISL29003 will be designed in a portable system. The
ambient light conditions that the device will be exposed to is
at most 500 lux, 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 I 2 C 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.
sync_iic command is received. Writing a logic 1 to the
sync_iic bit ends the current ADC integration and starts
t int = i ( 1 ? 60Hz ) = j ( 1 ? 50Hz )
(EQ. 14)
another one.
The first instance of integer values at which t int rejects both
For Mode3, the operation is a sequential Mode1 and Mode2.
The device needs three sync_iic commands to complete two
photodiode measurements. The 1st sync_iic command starts
the conversion of the Diode1. The 2nd sync_iic completes the
60Hz and 50Hz is when i = 6, and j = 5.
t int = 6 ( 1 ? 60Hz ) = 5 ( 1 ? 50Hz )
t int = 100ms
(EQ. 15)
t int = ----------
conversion of Diode1 and starts the conversion of Diode2.
The 3rd sync_iic pulse ends the conversion of Diode2 and
starts over again to commence conversion of Diode1.
The integration time, t int , is determined by Equation 12:
i I 2 C
(EQ. 12)
f I 2 C
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.
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,
the number of clock cycles per integration is no longer fixed
Next, the Gain/Range needs to be determined. Based on the
application condition given, lux(max) = 500 lux, a range of
1000 lux 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 .
t int × 327kHz × 100 k Ω
n
R EXT = -------------------------------------------------------------
2
(EQ. 16)
R EXT = 50k Ω
for Internal Timing Mode and Gain/Range is set to Range3 or Range4 only
The Full Scale Range, FSR, needs to be determined from
Equation 3:
100k Ω
50k Ω
at 2 n . The number of clock cycles varies with the chosen
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.
FSR = 1000 lux ------------------
FSR = 2000 lux
(EQ. 17)
t int < ------------------
E = ------------- × 2000 lux
65,535
f OSC
(EQ. 13)
The effective transfer function becomes:
data
16
2
(EQ. 18)
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.
9
FN7464.6
November 17, 2011
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