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ISL29004IROZ-EVALZ Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL29004IROZ-EVALZ Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 17 page  10 FN6221.0 December 21, 2006 Since fosc is dual speed depending on the Gain/Range bit, Tint is dual time. The integration time as a function of REXT and n is: Tint1 is the integration time when the the device is configured for Internal Timing Mode and Gain/Range is set to Range1 or Range2. Tint2 is the integration time when the the device is configured for Internal Timing Mode and Gain/Range is set to Range3 or Range4. 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 I2C 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, Tint, is determined by the following equation: iI2C is the number of I 2C clock cycles to obtain the T int. fI2C is the I 2C operating frequency The internal oscillator, fOSC, operates identically in both the internal and external timing modes, with the same dependence on REXT. However, in External Timing Mode, the number of clock cycles per integration is no longer fixed at 2n. The number of clock cycles varies with the chosen integration time, and is limited to 216 = 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. fosc = 327kHz*100kΩ/REXT. When Range/Gain is set to Range1 or Range2. fosc = 655kHz*100kΩ/REXT. When Range/Gain is set to Range3 or Range4. Noise Rejection 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 k*16.66ms (k = 1,2...ki) 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 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. Tint = i(1/60Hz) = j(1/50Hz). The first instance of integer values at which Tint rejects both 60Hz and 50Hz is when i = 6, and j= 5. Tint = 6(1/60Hz) = 5(1/50Hz) Tint = 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 REXT. TABLE 13. INTEGRATION TIMES FOR TYPICAL REXT VALUES REXT (k Ω) RANGE1 RANGE2 RANGE3 RANGE4 n = 16-BIT n = 12-BIT n = 12-BIT n = 4 50 100 6.4 13 0.013 100** 200 13 26 0.025 200 400 26 52 0.050 500 1000 64 128 0.125 *Integration time in milliseconds **Recommended REXT resistor value T int 12 n R EXT 327kHz 100k Ω × ---------------------------------------------- × = (EQ. 10) T int 22 n R EXT 655kHz 100k Ω × ---------------------------------------------- × = (EQ. 11) T int i I2C f I2C ---------- = (EQ. 12) T int 65,535 f OSC ------------------ < (EQ. 13) R EXT T int 327kHz 100 × × k Ω 2 n ----------------------------------------------------------------- = R EXT 50k Ω = for Internal Timing Mode and Gain/Range is set to Range3 or Range (EQ. 14) ISL29004 |
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