A Low Cost 4Mbps IrDA Receiver

The LT1328, in the MS8 and SO-8 packages, contains all the necessary circuitry to convert current pulses from an external photodiode to a digital TTL output while rejecting unwanted lower frequency interference. The LT1328 plus fi ve external components is all that is required to make the IrDA-compatible receiver shown in Figure 1. An IrDA-compatible transmitter can also be implemented with only six components, as shown in Figure 2. Power requirements for the LT1328 are minimal: a single 5V supply and 2mA of quiescent current.
A Low Cost 4Mbps IrDA Receiver - schematic

LT1328 Functional Description Figure 3 is a block diagram of the LT1328. Photodiode current from D1 is transformed into a voltage by feedback resistor RFB. The DC level of the preamp is held at VBIAS by the servo action of the transconductance amplifier’s gm. The servo action only suppresses frequencies below the Rgm/CFILT pole. This highpass filtering attenuates interfering signals, such as sunlight or incandescent or fluorescent lamps, and is selectable at Pin 7 for low or high data rates. For high data rates, Pin 7 should be held low. The highpass filter breakpoint is set by the capacitor C4 at f = 25/(2π • Rgm • C4), where Rgm = 60k. The 330pF capacitor (C4) sets a 200kHz corner frequency and is used for data rates above 115kbps. For low data rates (115kbps and below), the capacitance at Pin 2 is increased by taking Pin 7 to a TTL high. This switches C1 in parallel with C4, lowering the highpass filter breakpoint. A 10nF capacitor (C1) produces a 6.6kHz corner. Signals processed by the preamp/gm amplifier combination cause the comparator output to swing low. IrDA SIR The LT1328 circuit in Figure 1 operates over the full 1cm to 1 meter range of the IrDA standard at the stipulated light levels. For IrDA data rates of 115kbps and below, a 1.6μs pulse width is used for a zero and no pulse for a one. Light levels are 40mW/sr (Watts per steradian) to 500mW/sr. Figure 4 shows a scope photo for a...

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