參數(shù)資料
型號: LT1328CS8
廠商: LINEAR TECHNOLOGY CORP
元件分類: 通信及網(wǎng)絡
英文描述: FASTENERS, TIES CABLE
中文描述: SPECIALTY TELECOM CIRCUIT, PDSO8
封裝: PLASTIC, SO-8
文件頁數(shù): 5/12頁
文件大?。?/td> 274K
代理商: LT1328CS8
5
LT1328
APPLICATIO
S I
N
FOR
ATIO
U
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 R
FB
. The DC level of the
preamp is held at V
BIAS
due to the servo action of the
transconductance amplifier g
m
. The servo action only sup-
presses frequencies below the R
gm
, C
FILT
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
R
gm
C) where R
gm
= 60k.
The
330pF capacitor in Figure 1 sets a 200kHz corner fre-
quency and is used for data rates above 115kbps. For low
W
U
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/g
m
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
μ
s 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
Figure 2. IrDA-SIR Modulation
TRANSMITTER
INPUT
RECEIVER
OUTPUT
RECEIVER
OUTPUT
TRANSMITTER
INPUT
IrDA-SIR (1m)
IrDA-SIR (1cm)
0
1
0
0
1
1
0
1
0
1328 F02a
NRZ DATA
IrDA-SIR HIGH = LIGHT TRANSMITTED
DATA RATES: 2400BAUD TO 115kbps
BIT INTERVAL: 417
μ
s TO 8.7
μ
s
PULSE WIDTH: 3/16 OF INTERVAL OR 1.63
μ
s
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