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ISL29003IROZ-EVAL Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL29003IROZ-EVAL Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 14 page 9 FN7464.6 November 17, 2011 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 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 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 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, tint, is determined by Equation 12: iI 2 C is the number of I 2C clock cycles to obtain the t int. fI 2 C 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 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 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 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. The first instance of integer values at which tint rejects both 60Hz and 50Hz is when i = 6, and j = 5. 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 REXT. The Full Scale Range, FSR, needs to be determined from Equation 3: The effective transfer function becomes: 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 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 REXT resistor value t int i I 2 C f I 2 C ---------- = (EQ. 12) t int 65,535 f OSC ------------------ < (EQ. 13) t int i1 60Hz ⁄ () j1 50Hz ⁄ () = = (EQ. 14) t int 61 60Hz ⁄ () 51 50Hz ⁄ () = = (EQ. 15) t int 100ms = for Internal Timing Mode and Gain/Range is set to Range3 or Range4 only R EXT t int 327kHz 100 × × k Ω 2 n ------------------------------------------------------------- = R EXT 50k Ω = (EQ. 16) FSR 1000 lux 100k Ω 50k Ω ------------------ = FSR 2000 lux = (EQ. 17) E data 2 16 ------------- 2000 lux × = (EQ. 18) ISL29003 |
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