Datasheet

__________Applications Information
Converting Average Optical Power to
Signal Amplitude
Many of the MAX3761/MAX3762’s specifications relate
to input-signal amplitude. When working with fiber optic
receivers, the input is usually expressed in terms of
average optical power and extinction ratio. The rela-
tions given in Table 1 are helpful for converting optical
power to input signal when designing with the
MAX3761/MAX3762.
In an optical receiver, the input voltage to the limiting
amplifier can be found by multiplying the relationship in
Table 1 with the photodiode responsivity (p) and tran-
simpedance amplifier gain (G).
Optical Hysteresis
Power and hysteresis are often expressed in decibels.
By definition, decibels are always 10log (power). At the
inputs to the MAX3761/MAX3762 limiting amplifier, the
power is V
IN
2
/R. If a receiver’s optical input power (x)
increases by a factor of two, and the preamplifier is lin-
ear, then the voltage input to the MAX3761/MAX3762
also increases by a factor of two.
The optical power change is 10log(2x / x) = 10log(2) =
+3dB
At the MAX3761/MAX3762, the voltage change is:
In an optical receiver the dB change at the MAX3761/
MAX3762 will always equal 2x the optical dB change.
The MAX3761/MAX3762’s typical voltage hysteresis is
3.6dB. This provides an optical hysteresis of 1.8dB.
Input Sensitivity
The receiver’s gain sensitivity defines the smallest signal
input that results in fully limited PECL-compatible data
outputs. Smaller signals result in nonlimited outputs. The
MAX3761/MAX3762’s input sensitivity (S
GAIN
) is 4mVp-p:
SGAIN = 4mV
Optical gain sensitivity (in dBm) is:
In a receiver with G = 6kΩ, r
e
= 10, and ρ = 0.8A/W,
gain sensitivity is 510nW, or -32.9dBm.
622Mbps ATM Component Selection
As an example, a preamplifier with a 150kHz low-
frequency cutoff and a 950mVp-p maximum output has
the best performance with the following selections:
C
IN
= 5.6nF, so that f
C
= 15kHz (one decade below the
150kHz cutoff)
C
AZ
= 150pF, so that f
OC
< 1.5kHz (one decade
below f
C
)
C
FILTER
= 100pF, so that the integration frequency
equals 3MHz.
These selections should provide data-dependent jitter
less than 110ps p-p when the input consists of PRBS
data with no more than 72 consecutive bits.
10log
S
2G
xx
GAIN
ρ
r
r
e
e
+
−
⎡
⎣
⎢
⎤
⎦
⎥
1
1
1000
10 10 2 20
2
2
2
log log( ) log
2V / R
V/R
IN
IN
()
==(()26=+dB
MAX3761/MAX3762
Low-Power, 622Mbps Limiting Amplifiers
with Chatter-Free Power Detect for LANs
_______________________________________________________________________________________ 9
Table 1. Optical-Power Relations*
TIME
P0
P1
P
AVE
Figure 4. Optical-Power Relations
SYMBOL RELATION
Average
Power
P
AVE
PARAMETER
Extinction
Ratio
r
e
Optical Power
of a “1”
P1
Optical Power
of a “0”
P0
Signal
Amplitude
P
IN
P=P0+P1
AVE
()
/2
r= 1/P0
e
P
PP
r
r
AVE
e
e
12
1
=
+
PP r
AVE e
02 1=+
()
/
PPP P
r
r
IN AVE
e
e
=− =
−
()
+
102
1
1
OPTICAL
POWER
* Assuming a 50% average input data duty cycle (true for SONET/ATM data).