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ISL29011 Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL29011 Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 15 page 9 FN6467.2 May 14, 2009 The transfer function used for n-bit ADC becomes Equation 3: Here, n = 4, 8, 12 or 16. This is the number of ADC bits programmed in the command register. 2n represents the maximum number of counts possible from the ADC output. Data is the ADC output stored in the data registers (02 hex and 03 hex). Integration and Conversion Time The ADC resolution and fOSC determines the integration time, tint as shown in Equation 4. where n is the number of bits of resolution and n = 4, 8, 12 or 16. 2n, therefore, is the number of clock cycles. n can be programmed at the command register 01(hex) bits 3 and 2. External Scaling Resistor REXT for fOSC and Range The ISL29011 uses an external resistor REXT to fix its internal oscillator frequency, fOSC and the light sensing range, Range. fOSC and Range are inversely proportional to REXT. For user simplicity, the proportionality constant is referenced to 499k Ω as shown in Equations 5 and 6: 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 conversion rate. 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. ADC Output in IR Sensing The ISL29011’s ADC output codes, DATA, are directly proportional to the IR intensity received in the IR sensing. Here, EIR is the received IR intensity. The constant β changes with the spectrum of background IR noise like sunlight and incandescent light. The β also changes with the ADC’s range and resolution selections. ADC Output in Proximity Sensing In the proximity sensing, the ADC output codes, DATA, are directly proportional to the total IR intensity from the background IR noise and from the IR LED driven by the ISL29011. Here, β and EIR have the same meanings as in Equation 7. The constant γ depends on the spectrum of the used IR LED and the ADC’s range and resolution selections. ELED is the IR intensity which is emitted from the IR LED and reflected by a specific objector to the ISL29011. ELED depends on the current to the IR LED and the surface of the object. ELED decreases with the square of the distance between the object and the sensor. If background IR noise is small, EIR can be neglected, and the ADC output directly decreases with the distance. If there is significant background IR noise, ISL29011 offers two schemes to reduce the effect. The first way is do a proximity sensing using Scheme 0, immediately followed by an IR sensing. The differential reading of ADC outputs from the proximity and IR sensing will then reduce the effect of background IR noise and directly decrease with the distance between the object and the sensor. The second way is to do a proximity sensing using Scheme 1 to do on-chip background IR noise subtraction. While Scheme 0 has wider dynamic range, Scheme 1 proximity detection is faster but with half the resolution. Please refer to “Typical Performance Curves” on page 12 for ADC output versus distance using Scheme 0 detection. Figure 9 shows ISL29011 configured at 12-bit ADC resolution and sensitivity range select at 16000 (range 3) for the proximity reading. A 12.5mA external LED current at 360kHz modulation frequency detects three different sensing objects: 92% brightness paper, 18% gray card and ESD black foam. Figure 10 shows ISL29011 configured at 12-bit ADC resolution and sensitivity range select at 1000 (range 1) for the proximity reading, with a programmed external LED at 360kHz modulation frequency, detecting the same sensing object: 18% gray card under four different external LED current: 12.5mA, 25mA, 50mA and 100mA to compare the proximity readout versus distance. ISL29011 Proximity sensing relies on the amount of IR reflected back from the objects to be detected. Clearly, it can not detect an optically black object that reflects no light. However, ISL29011 is sensitive enough to detect a black ESD foam, which reflects slightly less than 1% of IR, as shown in Figure 9 on page 12. For biological objects, blonde hair reflects more than brunette hair, as expected and shown in TABLE 11. INTEGRATION TIME OF n-BIT ADC REXT (k Ω) n = 16-BIT (ms) n = 12-BIT (ms) n = 8-BIT (µs) n = 4-BIT (µs) 250 45 2.812 175.5 10.8µs 499** 90 5.63 351 21.6µs **Recommended REXT resistor value (EQ. 3) Ecal Range k () 2 n --------------------------- DATA × = tint 2 n 1 fOSC -------------- × 2 n REXT 725kHz 499k Ω × ---------------------------------------------- × == (EQ. 4) (EQ. 5) Range 499k Ω REXT ------------------ Range k () × = (EQ. 6) fOSC 499k Ω REXT ------------------ 725 × kHz = DATAIR β E IR × = (EQ. 7) DATAPROX β E IR ×γ E LED × + = (EQ. 8) ISL29011 |
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