LT1319 Просмотр технического описания (PDF) - Linear Technology

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LT1319

Multiple Modulation Standard Infrared Receiver

Linear Technology 

LT1319

APPLICATIONS INFORMATION

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 VTH generator through the 500Ω

resistors RC1 and RC3. These currents are equal to 4 times

the current into Pin 11. For 100μA 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 VCC input is regulated down

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 VCC and is

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-

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 CF1. The break

frequency is located at:

f = (15kΩ/10kΩ)/(2π • 4kΩ • 10nF) = 6kHz

On the low speed channel there is a lowpass filter at 800kHz

set by RF2 and CF3. The gain stage has a highpass filter set

by GM2 and CF4 at approximately 500kHz. The high speed

channel has an LC tank circuit at 500kHz with Q = 3 set by

RF1. The high speed gain stage has a highpass character-

istic set by GM3 and CF5 with a break frequency of 1.1kHz.

These filters are suitable for the 1.6μs 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 CF7 for IRDA or 180kHz by CF1 for Sharp. Sharp

modulation is run on the low speed channel and is next

filtered by a tank circuit formed by RF2, LF1 and CF3 and

centered at 500kHz. LF1 also provides the DC bias for the

filter buffer input. A final highpass for the lower speed

channel is set by CF4 at 130kHz. The high speed channel is

used by IRDA SIR and FIR which use 1.6μs and 220ns wide

pulses. A lowpass formed by RF1 and CF2 limit the noise

bandwidth. The final highpass is set by CF5 (2.5MHz for

FIR) or CF6 (450kHz). The squelch circuit formed by Q1, Q2,

Q3 and RC1 to RC6 extends the short range performance

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

a rise time equal to half the pulse width. For example, for

1μs pulses a 700kHz lowpass filter has a 10% to 90%

rise time of 0.35/700kHz = 500ns.

2. Limit the maximum highpass to 1/(4 • pulse width). For

1μs pulses, 1/4μs = 250kHz.

3. In setting the highpass filters, space the filters apart by

a factor of 5 to 10 to reduce overshoot due to filter

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