LT1319 Linear Technology, LT1319 Datasheet - Page 7

• Manufacturer Part Number: LT1319
• Description: Multiple Modulation Standard Infrared Receiver
• Manufacturer: Linear Technology

APPLICATIONS
tion, the loops around the gain stages provide an accurate
DC threshold setting for the comparators. At DC, the loops
force the differential voltages at the output of the gain
stages to zero. The comparator threshold is set by the
currents provided by the V
resistors R
the current into Pin 11. For 100 A into Pin 11, the compara-
tor thresholds are nominally 200mV.
Power Supply Rejection and Biasing
The LT1319 has very high gain and bandwidth so great care
is taken to reduce false output transitions due to power
supply noise. As a first step the V
to approximately 4V to power all the analog sections of the
circuit which are also tied to Analog Ground (Pin 1) as is the
substrate of the die. Additionally, the internal 4V is by-
passed at Pin 16. The digital circuitry (the comparators and
shutdown logic) is powered directly off of V
returned to Digital Ground (Pin 12). To provide a clean bias
point for the preamp, filter buffers and the gain stages, a
1.9V reference is generated from the 4V rail and is by-
passed at Pin 5. The gain stages are pure differential
designs which inherently reject supply variations.
Filtering
Filtering is needed for two main reasons: sensitivity and
ambient rejection. Lowpass filtering is needed to limit the
bandwidth in order to minimize the noise. Low noise
permits reliable detection of smaller input signals over a
larger distance. Highpass filtering is used to reject interfer-
ing ambient signals. Interference includes low frequency
sources of infrared light such as sunlight, incandescent
lights, and ordinary fluorescent lights, as well as high
frequency sources such as TV remote controls (40kHz) and
high frequency fluorescent lighting (40kHz to 80kHz).
The circuit topology allows for filtering between the pream-
plifier and the filter buffers as well as filtering with the three
internal highpass loops. With two channels the filtering can
be optimized for different modulation schemes. The high
speed channel (with a 25ns comparator) is ideal for modu-
lation schemes using frequencies above 1MHz. Carrier-
based methods as well as narrow pulse schemes can have
superior ambient rejection by adding in a dedicated high-
C1
and R
C3
U
. These currents are equal to 4 times
INFORMATION
U
TH
generator through the 500
CC
input is regulated down
W
U
CC
and is
pass filter network. The application on the first page of the
data sheet is repeated in the Block Diagram and can be used
to illustrate the filtering for IRDA-SIR and Sharp/Newton.
The preamp highpass zero is set by GM1 and C
frequency is located at:
On the low speed channel there is a lowpass filter at 800kHz
set by R
by GM2 and C
channel has an LC tank circuit at 500kHz with Q = 3 set by
R
istic set by GM3 and C
These filters are suitable for the 1.6 s pulses and up to
115kBd data rates of IRDA-SIR on the slow channel. The
fast channel is used for Sharp/Newton ASK Modulation
with 500kHz bursts at data rates up to 38.4kBd.
A second circuit is shown in the Typical Applications
section for IRDA SIR/FIR and Sharp. This circuit is Demo
Board 54. The first filter is a preamp highpass loop set at
600Hz by C
modulation is run on the low speed channel and is next
filtered by a tank circuit formed by R
centered at 500kHz. L
filter buffer input. A final highpass for the lower speed
channel is set by C
used by IRDA SIR and FIR which use 1.6 s and 220ns wide
pulses. A lowpass formed by R
bandwidth. The final highpass is set by C
FIR) or C
Q3 and R
and will be discussed later.
In designing custom filters for different applications, the
following guidelines should be used.
1. Limit the noise bandwidth with a lowpass filter that has
2. Limit the maximum highpass to 1/(4 • pulse width). For
3. In setting the highpass filters, space the filters apart by
F1
f = (15k /10k )/(2 • 4k • 10nF) = 6kHz
a rise time equal to half the pulse width. For example, for
1 s pulses a 700kHz lowpass filter has a 10% to 90%
rise time of 0.35/700kHz = 500ns.
1 s pulses, 1/4 s = 250kHz.
a factor of 5 to 10 to reduce overshoot due to filter
. The high speed gain stage has a highpass character-
F2
F6
C1
and C
(450kHz). The squelch circuit formed by Q1, Q2,
F7
to R
for IRDA or 180kHz by C
F4
F3
at approximately 500kHz. The high speed
C6
. The gain stage has a highpass filter set
F4
extends the short range performance
at 130kHz. The high speed channel is
F5
F1
with a break frequency of 1.1kHz.
also provides the DC bias for the
F1
and C
F2
F1
, L
F2
for Sharp. Sharp
F1
F5
limit the noise
F1
and C
(2.5MHz for
LT1319
. The break
F3
7
and