4

LT1328

GND (Pin 4): Ground.

DATA (Pin 5): TTL Output of the Comparator. A TTL LOW

at this pin indicates that the photodiode has received a

pulse of light. This pin will remain HIGH when the light

signal and noise are below the attenuation band of the DC

servo.

close as possible from Pin 6 to ground is recommended

(4.7

µF).

MODE (Pin 7): A TTL HIGH on this pin will cause the open

collector at Pin 3 to switch to ground. This is an option that

switches in extra capacitance for lower frequency data

rates. If this pin is not used, select a capacitor (C4) to

accommodate the lowest data rate (for IrDA C4 = 10nF).

is required.

IN (Pin 1): Connect this pin to the photodiode cathode.

This is the input to the preamplifier and is held at 1.5V by

the output of the DC servo. To maintain the 1.5V on this

pin, the DC servo can source up to 2mA or sink 40

µAtothis

pin.

FILTER (Pin 2): Connect a capacitor from this pin to

ground. The value of the capacitor will set the band of

frequencies that will be attenuated by the servo. The pole

is set by the formula f = 25/(2

π • 60k • C). A 330pF

capacitor will attenuate photodiode signals below 200kHz

when operating at high data rates (FIR and 4ppm).

FILTER SWITCH (Pin 3): Collector of a Transistor Switch.

When the MODE switch (Pin 7) is TTL HIGH this pin will be

switched to ground. This pin is used to increase the filter

capacitor for slower data rates (SIR). A 10nF capacitor

from Pin 2 to Pin 3 will bring the highpass frequency down

to 6.6kHz for SIR.

Figure 1. LT1328 Block Diagram

50k

1.5V

2k

D1

PHOTODIODE

TEMIC

BPV22NF

C1

10nF

IN

1

8

6

5V

7

MODE

5

1328 BD

DATA

FILTER

2

FILTER SWITCH

3

GND

4

PREAMP

DC SERVO

FILTER

60k

+

–

–

+

COMPARATOR

BIAS

C4

330pF

C6

1000pF

HIGH: SIR

LOW: FIR AND 4ppm

C2

4.7

µF