LT1328CMS8 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1328CMS8' PDF : 12 Pages View PDF
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/(2π • 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 1.6µs pulse width is used for a logic zero and
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
IrDA-SIR (1m)
NRZ DATA
0
1
0
0
1
1
0
1
0
IrDA-SIR HIGH = LIGHT TRANSMITTED
DATA RATES: 2400BAUD TO 115kbps
BIT INTERVAL: 417µs TO 8.7µs
PULSE WIDTH: 3/16 OF INTERVAL OR 1.63µs
1328 F02a
IrDA-SIR (1cm)
RECEIVER
OUTPUT
TRANSMITTER
INPUT
RECEIVER
OUTPUT
TRANSMITTER
INPUT
Figure 2. IrDA-SIR Modulation
5
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