LT1328CS8 datasheet(5/12 Pages) LINER

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LT1328

APPLICATIONS INFORMATION

The LT1328 is a low noise, high speed, high gain infrared

receiver designed to easily provide IR communications with

portable computers, PDAs, desktop computers and periph-

erals. The receiver takes the photodiode current from an

infrared photodiode (typically Temic BPV22NF) and per-

forms a current-to-voltage conversion. Figure 1 is a block

diagram of the LT1328. Photodiode current is transformed

into a voltage by feedback resistor RFB. The DC level of the

preamp is held at VBIAS due to the servo action of the

transconductance amplifier gm. The servo action only sup-

presses frequencies below the Rgm, CFILT pole.

This highpass filtering attenuates interfering signals such

as sunlight, incandescent and fluorescent lamps. For

higher data rates, the rejection band can be increased to

include higher frequencies (Pin 7 LOW). If this option is

not used, Pin 7 is grounded and Pin 3 is open. The rejection

band of the filter is then determined by the lowest data rate;

C4 equals 10nF for IrDA (SIR, FIR and 4ppm). The high-

pass filter break point is set by the capacitor from Pin 2 to

ground at f(Hz) = 25/(2

π • Rgm • C) where Rgm = 60k. The

330pF capacitor in Figure 1 sets a 200kHz corner fre-

quency and is used for data rates above 115kbps. For low

data rates (115kbps and below) Pin 7 should be set to TTL

HIGH which grounds Pin 3 and reduces the break fre-

quency with the addition of the capacitor between Pins 2

and 3. A 10nF capacitor produces a 6.6kHz corner.

Signals passed by the preamp/gm amplifier combination

cause the comparator output to swing low.

Performance of the LT1328 circuit in Figure 1 extends

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 1.6

µspulsewidthisusedforalogiczeroand

no pulse for a logic one. Light levels are 40mW/SR to

500mW/SR.

Figure 2 shows a scope photo for a transmitter input

(bottom trace) and the LT1328 output (top trace). Note

that the input to the transmitter is inverted, i.e., a high at

the input is light transmitted, which is a zero at the output

of the transmitter (Pin 5). The MODE pin (Pin 7) should be

high for these data rates.

The second fastest tier of the IrDA standard addresses

576kbps and 1.152Mbps data rates with pulse widths of

1/4 of the bit interval for zero and no pulse for one. The

Figure 2. IrDA-SIR Modulation

TRANSMITTER

INPUT

RECEIVER

OUTPUT

RECEIVER

OUTPUT

TRANSMITTER

INPUT

IrDA-SIR (1m)

IrDA-SIR (1cm)

1328 F02a

NRZ DATA

IrDA-SIR HIGH = LIGHT TRANSMITTED

DATA RATES: 2400BAUD TO 115kbps

BIT INTERVAL: 417

µs TO 8.7µs

PULSE WIDTH: 3/16 OF INTERVAL OR 1.63

µs

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