9

LT1319

first trace in the photograph shows the current with the

photodiode 10cm from a source with 100mW/sr intensity.

At 200mV/div, there is about 40

µA of peak current and the

decay is consistent with the 210ns time constant. The

lower trace shows the current with the photodiode 2cm

from the LEDs where the photodiode current is theoreti-

cally 25 times greater than at 10cm. The voltage is clamped

by the photodiode to nearly 0.4V, but what is now notice-

able is that there is a tail with a time constant a bit greater

than 1

µs. If the signal is AC coupled and has a low duty

cycle, the waveform will be centered at the very bottom

which can result in very wide output pulses. This issue will

be discussed later in more detail and a method to circum-

vent it will be shown.

transients at the comparator outputs parasitically coupling

to the FILTIN or FILTINL inputs and is layout dependent.

Extending Short Range Performance

The short range performance of the LT1319 is normally

limited by the photocurrent tail, but in some instances the

peak current level cannot be supported by the output of the

preamplifier and the input will sag at Pin 2. Typically the

maximum input current is 6mA. To increase this current to

20mA or more, place an NPN transistor with its emitter tied

to Pin 2, the base to Pin 4 and collector to the 5V supply.

The choice of transistor is dependent on the bandwidth

required for the preamp. The base-emitter capacitance of

resistor of the preamplifier and performs a lowpass filter-

ing function. For modest data rates such as IRDA-SIR and

Sharp/Newton a 2N3904 limits the bandwidth to 2MHz

which is ample. For the highest data rates, a transistor with

Another issue with large input signals is the photocurrent

tail. When this tail is AC coupled and the data has a low duty

cycle, the output pulse width can become so wide that it

extends into the next bit interval. A highpass filter can reject

this tail, but for the case of IRDA-SIR, rejecting the 1

µstime

constant can cause rejection of the 1.6

µspulsewhichleads

to a loss of sensitivity and reduced maximum link distance.

The circuit on the front page of the data sheet uses a 500kHz

highpass that trades off some sensitivity for rejection of

this tail. Unfortunately both maximum and minimum dis-

tance are compromised. An alternative is shown in the

IRDA-SIR/FIR application. In this instance the final highpass

filter for SIR is moved into 450kHz, but a clamp/squelch

is used as described above to clamp the input, but the input

current level at which the clamp engages has been modified

Without the resistors, Q1 would turn on when the voltage

across the 15k resistor in the preamp reaches about 0.7V

(a current of 0.7V/15k

reduces this voltage by about 365mV. The drop is set by the

Photocurrent Waveforms

10cm

2cm

1319 AI01

Threshold Adjustment

The comparator thresholds are set by the current into Pin

11. The simplest method of setting this current is by a

should be bypassed. The current is given by:

I

VV

Rk

TH

CC

T

=

−

()

+

()

09

2

1

.

Ω

The threshold referred to the input of the filter buffer is:

V

I

VV

TH

TH

=

••

/

4 500

400

Ω