SY88903AL Ver la hoja de datos (PDF) - Micrel
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SY88903AL Datasheet PDF : 9 Pages
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Micrel, Inc.
Functional Block Diagram
SY88903AL
Detailed Description
The SY88903AL high-sensitivity limiting post amplifier
operates from a single +3.3V power supply, over
temperatures from –40°C to +85°C. Signals with data
rates from 622Mbps up to 1.25Gbps, and as small as
5mVpp, can be amplified. Figure 1 shows the allowed
input voltage swing. The SY88903AL generates an LOS
output, allowing feedback to /EN for output stability.
LOSLVL sets the sensitivity of the input amplitude
detection.
Input Amplifier/Buffer
Figure 2 shows a simplified schematic of the input
stage. The high-sensitivity of the input amplifier allows
signals as small as 5mVpp to be detected and amplified.
The input amplifier allows input signals as large as
1800mVpp. Input signals are linearly amplified with a
typically 42dB differential voltage gain. Since it is a
limiting amplifier, the SY88903AL outputs typically
1500mVpp voltage-limited waveforms for input signals
that are greater than 12mVpp. Applications requiring the
SY88903AL to operate with high-gain should have the
upstream TIA placed as close as possible to the
SY88903AL’s input pins to ensure the best performance
of the device.
Output Buffer
The SY88903AL’s PECL output buffer is designed to
drive 50Ω lines. The output buffer requires appropriate
termination for proper operation. An external 50Ω
resistor to VCC–2V for each output pin provides this.
Figure 3 shows a simplified schematic of the output
stage.
Loss-of-Signal
The SY88903AL generates a chatter-free loss-of-signal
(LOS) open-collector TTL output with internal 4.75kΩ
pull-up resistor as shown in Figure 4. LOS is used to
determine that 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, 3.4dB LOS hysteresis is provided to prevent
chattering.
Loss-of-Signal-Level Set
A programmable LOS level set pin (LOSLVL) sets the
threshold of the input amplitude detection. Connecting
an external resistor between VCC and LOSLVL sets the
voltage at LOSLVL. This voltage ranges from VCC to VREF.
The external resistor creates a voltage divider between
VCC and VREF, as shown in Figure 5.
Hysteresis
The SY88903AL provides typically 3.4dB LOS electrical
hysteresis. By definition, a power ratio measured in dB
is 10log (power ratio). Power is calculated as V2IN/R for
an electrical signal. Hence, the same ratio can be stated
as 20log (voltage ratio). While in linear mode, the
electrical voltage input changes linearly with the optical
power and hence, the ratios change linearly. Therefore,
the optical hysteresis in dB is half the electrical
hysteresis in dB given in the data sheet. The
SY88903AL is an electrical device, this data sheet refers
to hysteresis in electrical terms. With 3.4dB LOS
hysteresis, a voltage factor of 1.5 is required to assert or
de-assert LOS.
February 2007
7
M9999-021707-D
hbwhelp@micrel.com or (408) 955-1690
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