8

LT1319

interaction. Overshoot becomes especially important

for high input levels because it can cause false pulses

which may not be tolerated in certain modulation

schemes. It is also more of a problem in modulation

schemes such as IRDA-SIR and FIR where the duty

cycle can get very low (i.e., transmitting data with lots of

ones which are signaled with the absence of pulses). AC

coupled receivers when faced with low duty cycle data

set their thresholds close to the baseline DC level of the

data stream which converts small overshoots into erro-

neously received pulses.

4. As a general rule, place the lowest frequency highpass

around the preamp and the highest highpass around the

gain stage or between the preamp and gain stage. The

reason for this is again due to high signal levels where

there can be slow photocurrent tails. The tail response

can be filtered out by high enough frequency filters.

5. In all cases with custom filtering, or when modifying one

of the applications presented in this data sheet, try the

system over the full distance range with a full range of

duty cycle data streams. Modulation methods with fixed

or limited duty cycle are superior because they have little

or no data dependent problems.

Dynamic Range

The calculation of dynamic range can only be made in the

context of a specific modulation scheme and with the

system variations taken into account. The required infor-

mation includes: minimum signal-to-noise ratio (or BER,

Bit Error Rate requirement), photodiode capacitance at

1.9V back bias, preamp noise spectrum, preamp output

filtering, AC loop cutoff frequencies, modulation method,

demodulation method including allowable pulse widths

and the effect of missing or extra pulses, photodiode rise

and fall times, and ambient interference. The best solution

is to experimentally determine the maximum and minimum

distances at which a desired BER is obtained. This measure

of dynamic range is more meaningful in terms of the overall

system than any analytic solution.

Using the IRDA-SIR modulation scheme as an example,

however, we can illustrate how some limits on the required

receiver/photodiode combination can be obtained. The

minimum light intensity in the angular range is 40mW/sr

which translates to a photodiode current as follows (using

the BPW34FA data sheet specs):

ImW sr

mm

mm

AW

nA

PD MIN

















(

40

7

1000

0 65

0 95 0 95

164

2

2

/•

./

.

.

distance from the light source. The 0.65A/W is the spectral

sensitivity at 880nm wavelength. The first 0.95 term is the

relative sensitivity at 850nm wavelength and the second

term is the sensitivity at 15

° off axis. Similar calculations

are detailed in the Infrared Data Association Serial Infrared

(SIR) Physical Layer Link Specification, version 1.0. This

minimum photocurrent implies that the input-referred

noise current of the receiver be less than 13.7nA rms for a

bit error rate of 1E-9. With an 800kHz lowpass filter on the

preamp output the LT1319 has approximately 3.6nA rms of

input-referred current noise. The maximum photodiode

current at 20mm, on-axis with 500mW/sr intensity:

ImW sr

mm

mm

PD MAX

















500

7

20

2

2

/•

./

.

.

0 65A W 0 95

5 4mA

so we see that the dynamic range requirement is 90.4dB.

What is not obvious, however, is that the photodiode

output current is not simply a pulse of current, there is a

significant tail at high current levels that has a time constant

of more than 1

µs which can cause distortion in the output

pulse width of the LT1319. This tail can be shown in the

following photograph which shows the voltage across a 5k

resistor that is connected between the anode of a photo-

diode and ground. The cathode of the photodiode is

connected to 2V. There is a 2pF Schottky diode across the

resistor to clamp the voltage swing to less than 0.5V. With

about 30pF photodiode capacitance and 10pF for an oscil-

loscope probe, any tail observed with a time constant

greater than 210ns is due to decaying photocurrent. The