LT1319 Linear Technology, LT1319 Datasheet - Page 8

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

APPLICATIONS
LT1319
4. As a general rule, place the lowest frequency highpass
5. In all cases with custom filtering, or when modifying one
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
8
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.
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.
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.
U
INFORMATION
U
W
U
which translates to a photodiode current as follows (using
the BPW34FA data sheet specs):
The 7mm
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:
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 s 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
I
I
PD MIN
PD MAX
2
term is the photodiode area. The 1000mm is the
0 65
40
500
0 65A W 0 95
.
.
mW sr
A W
mW sr
/
/
/
/
0 95 0 95
.
1000
.
7
20
7
mm
mm
mm
mm
.
5 4mA
2
.
2
2
2
164
•
•
nA