SY88353BL_07 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – 3.3V, 3.2 Gbps Limiting Post Amplifier with Programmable Decision Threshold

Micrel, Inc.
Functional Block Diagram
SY88353BL
Detailed Description
The SY88353L limiting post amplifier operates from a
single +3.3V power supply, over temperatures from
–40°C to +85°C. Signals with data rates from 155Mbps
up to 3.2Gbps, and as small as 10mV_pp, can be
amplified. Figure 1 shows the allowed input voltage
swing. The SY88353L generates a LOS output that can
be feedback to /EN for output stability in the absence of
a signal at the input. LOS_LVL sets the sensitivity of the
input amplitude detection. The amplifier features a
signal crossing adjustment for BER optimization in
optical links using optical amplifiers such in WDM
applications.
Input Amplifier/Buffer
Figure 2 shows a simplified schematic of the input
stage. The input amplifier allows signals as small as
10mV_pp to be detected and amplified. The input amplifier
allows input signals as large as 1800mV_pp. Input signals
are linearly amplified with a typical 38dB differential
voltage gain. Since it is a limiting amplifier, the
SY88353BL outputs typically 800mV_pp voltage-limited
waveforms for input signals that are greater than
12mV_pp. Applications requiring the SY88353L to operate
with high gain should have the upstream TIA placed as
close as possible to the SY88353BL’s input pins to
ensure the best performance of the device.
Output Buffer
The SY88353BL’s CML output buffer is designed to
drive 50Ω lines and is internally terminated with 50Ω to
V_CC. Figure 3 shows a simplified schematic of the output
stage.
Loss-of-Signal
The SY88353L generates a chatter-free loss-of-signal
(LOS) open-collector, TTL output, as shown in Figure 4.
LOS is used to determine if the input amplitude is too
small to be considered a valid input. LOS asserts high if
the input amplitude falls below the threshold set by
LOSLVL and de-asserts low otherwise. LOS can be fed
back to the enable (/EN) input to maintain output
stability under a loss of signal condition. /EN de-asserts
low the true output signal without removing the input
signals. Typically, 3.3dB LOS hysteresis is provided to
prevent chattering.
Loss-of-Signal-Level Set
A programmable LOS level set pin (LOS_LVL) sets the
threshold of the input amplitude detection. Connecting
an external resistor between V_CC and LOS_LVL sets the
voltage at LOS_LVL. This voltage ranges from V_CC to V_CC-
1.3V. The external resistor creates a voltage divider
between V_CC and V_CC-1.3V, as shown in Figure 5.
Hysteresis
The SY88353L typically provides 3.3dB LOS electrical
hysteresis, which is defined as 20log (VIN_{LOS-De-assert}/
VIN_LOS-Assert). Since the relationship between the voltage
out of the ROSA and optical power at its input is linear,
the optical hysteresis will be typically half of the
electrical hysteresis reported in the datasheet. In
practice, the ratio between electrical and optical
hysteresis is found to be within the range 1.5 to 1.8dB.
Thus, 3.3dB electrical hysteresis will correspond to an
optical hysteresis within the range 1.8 to 2.2dB.
February 2007
8
M9999-021707-B
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