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APPLICATION BULLETIN
FIBER OPTIC TRANSMISSION
By Christian Henn, Burr-Brown International GmbH
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
Fiber optic transmission is assuming an increasingly impor-
tant role in systems for wide-band analog signals and digital
signals with high data rates. Although the number of appli-
cations for digital networks and telecommunications sys-
tems is skyrocketing, analog transmission is still vital to
many applications. Analog systems with bandwidths of up
to 150MHz are used for wide-band RGB signal distribution,
HDTV video signal transmission, and many types of EMI-
and EMC-disturbed environments. Also important are medi-
cal applications, which demand the precision of fiber optic
technology for safety reasons. The many features of fiber
optic cables make them vital for all of these types of
applications. Fiber optic cables enable transmission over
long distances, ensure low damping vs frequency, are light
and flexible, and provide high immunity against distur-
bances from magnetic and electric fields. State-of-the-art
fiber optic transmission systems are now available even for
data networks with transmission rates of up to 1.2Gbit/s, and
gallium arsenide technology is used for their transmitter and
receiver circuits.
The fiber optic transmission interface presented here uses
new complementary bipolar integrated circuits from Burr-
Brown. The OPA660, which is used as an LED driver and
AGC multiplier, contains an operational transconductance
amplifier and a buffer in an 8-pin package. The OPA621 is
a low-noise, wide-band op amp in classical configuration,
which functions as an amplifier in the I/V conversion section
behind the photodiode and as an I/V converter behind the
AGC multiplier. The current-feedback amplifier OPA623
provides additional gain in the AGC section and drives the
75鈩?output. A discrete differential amplifier functions as an
AGC error amp and controls the quiescent current of the
OPA660 together with a FET. The CA3080 stabilizes the
DC performance, and the LM1881 functions as a sync
separator. The interface uses the IA184A as LED and the
SFH202 as pin diode.
FIBER OPTIC INTERFACE BASICS
A fiber optic interface generally consists of five major
functions as shown in Figure 1. On the transmitter side, a
circuit processes the input signal in order to drive the
electro-optical converter. This converter, which can be an
LED or a laser diode, generates the signal-dependent light
intensity modulation, and its mechanical case eases trans-
mission of the signal into the fiber. At the fiber end, a pin
diode converts the optical signal back into a low electrical
current. The low-noise transimpedance preamplifier con-
verts the current signal into a voltage and also amplifies it to
an acceptable level.
Because the photodiode input signal can vary in amplitude,
and AGC amplifier adjusts the peak-to-peak signal level to
1.4Vp-p and restores the DC level for no signal to 0V.
The quality of a fiber optic interface is characterized by
several factors such as signal-to-noise ratio, linearity, band-
width, power consumption, and transmission distance. The
S/N ratio should be at least 50dB for analog systems to
achieve an image that is free of noise. In conventional
designs, there are basically two ways to improve the S/N
ratio. One is to increase the diode drive current, which,
though, leads to higher harmonic distortion. The second is to
use a very low-noise transimpedance amplifier as a receiver.
Both alternatives increase the component count and add
manufacturing costs. The circuits presented here, however,
are a new approach to simplifying and minimizing the
design of an analog fiber optic interface and to provide an
interface that is more integrated and offers lower power
consumption. Table I summarizes the parameters of this new
type of wide-band analog fiber optic interface.
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3
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5
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V
IN
75鈩?/div>
Pre-equalizer
Electro/
Optical
Converter
Fiber
LED
Transimpedance
Amplifier
AGC
Amplifier
Output
Driver
75鈩?/div>
V
OUT
PIN-diode
DC
Restoration
FIGURE 1. Block Diagram of a Fiber Optic Transmission Interface.
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1993 Burr-Brown Corporation
AB-192
Printed in U.S.A. December, 1993
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