MAX3664 Просмотр технического описания (PDF) - Maxim Integrated
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MAX3664 Datasheet PDF : 12 Pages
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622Mbps, Ultra-Low-Power, 3.3V
Transimpedance Preamplifier for SDH/SONET
In a system where the photodiode responsivity is
0.9A/W and the extinction ratio is 10, the MAX3664/
MAX3675 receiver with 670nA gain sensitivity will deliv-
er a fully limited output for signals of average optical
power larger than:
(600nA / 0.9A/W)(1 / 2)(11 / 9) = 407nW ⇒ -33.9dBm
Sensitivity is a key specification of the receiver module.
The ITU/Bellcore specifications for SDH/SONET
receivers require a link sensitivity of -27dBm with a bit
error rate (BER) of 1E - 10. There is an additional 1dB
power penalty to accommodate various system losses;
therefore, the sensitivity of a 622Mbps receiver must be
better than -28dBm.
Although several parameters affect sensitivity (such as
the quantizer sensitivity and preamplifier gain, as previ-
ously discussed), most fiber optic receivers are designed
so that noise is the dominant factor. Noise from the high-
gain transimpedance amplifier, in particular, determines
the sensitivity. The noise generated by the MAX3664 can
be modeled with a Gaussian distribution. In this case, a
BER of 1E - 10 corresponds to a peak-to-peak signal
amplitude to RMS noise ratio (SNR) of 12.7. The
MAX3664’s typical input-referred noise, in, (bandwidth-
limited to 470MHz) is 55nARMS. Therefore, the minimum
input for a BER of 1E - 10 is (12.7 x 55nA) = 700nAp-p.
Rearranging the previous equations in these terms
results in the following relation:
Optical Sensitivity (dBm) =
-10log[(in / ρ)(SNR)(1/2)(re + 1) / (re - 1)(1000)]
POWER
P1
PAVE
P0
TIME
At room temperature, with re = 10, SNR = 12.7, in =
55nA, and ρ = 0.9A/W, the MAX3664 sensitivity is
-33.2dBm. At +85°C, noise increases to 62nA and sen-
sitivity decreases to -32.7dBm. The MAX3664 provides
4.7dB margin over the SDH/SONET specifications, even
at +85°C.
The largest allowable input to an optical receiver is called
the input overload. The MAX3664’s largest input current
(Imax) is 300µAp-p, with 200ps of pulse-width distortion.
The pulse-width distortion and input current are closely
related (see Typical Operating Characteristics). If the
clock recovery circuit can accept more pulse-width dis-
tortion, a higher input current might be acceptable. For
worst-case responsivity and extinction ratio, ρ = 1A/W
and re = ∞, the input overload is:
Overload (dBm) = -10log (Imax)(1 / 2)(1000)
For Imax = 300µA, the MAX3664 overload is -8.2dBm.
Step 2: Selecting Time Constants
A receiver built with the MAX3664 will have a bandpass
frequency response. The low-frequency cutoff causes
unwanted data-dependent jitter and sensitivity loss.
Because SDH/SONET data streams contain scrambled
data, certain data sequences may generate continuous
successions of 1s or 0s. The low-frequency cutoff
forces the output of such sequences to zero, ultimately
causing a sensitivity reduction. The SDH specifications
state that a receiver must be able to handle up to 72
consecutive bits of the same value within the data.
Therefore, choose the low-frequency cutoff to ensure
an acceptable amount of data-dependent jitter and
sensitivity loss.
Determine the reduction in signal-to-noise ratio due to a
transitionless sequence of duration t as follows:
SNRloss = 1 - e-t / τ = 1-e-(2πfct)
where τ is the time constant of the offset correction, fc
is the low-frequency cutoff, and t is the time for 72 bits
(116ns for a 622Mbps data rate).
Suppose that the receiver should not have more than
0.25dB (6%) of sensitivity loss due to a 72-bit transition-
less sequence. This means that:
(1 - e-(2πfc)(116ns)) < 0.06
fc = (ln 0.94) / [(-2π)(116ns)] = 85kHz (max)
The loss of sensitivity is a concern only when the SNR is
small (close to 12.7), which occurs with input currents
less than 3µAp-p.
Figure 2. Optical Power Definitions
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