7

LT1319

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

the current into Pin 11. For 100

µAintoPin11,thecompara-

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

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

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.

frequency is located at:

f = (15k

Ω/10kΩ)/(2π • 4kΩ • 10nF) = 6kHz

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

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

These filters are suitable for the 1.6

µs 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

modulation is run on the low speed channel and is next

filter buffer input. A final highpass for the lower speed

used by IRDA SIR and FIR which use 1.6

µs and 220ns wide

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

1

µs 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

1

µs 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