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SY88289CLMGTR Datasheet(PDF) 7 Page - Micrel Semiconductor |
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SY88289CLMGTR Datasheet(HTML) 7 Page - Micrel Semiconductor |
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7 / 10 page  Micrel, Inc. SY88289CL February 2007 7 M9999-021207-B hbwhelp@micrel.com or (408) 955-1690 Functional Block Diagram Detailed Description The SY88289CL high-sensitivity limiting post amplifier operates from a single +3.3V power supply, over temperatures from –40 oC to +85oC. Signals with data rates up to 3.2Gbps and as small as 5mVPP can be amplified. Figure 1 shows the allowed input voltage swing. The SY88289CL generates a LOS output. LOSLVL sets the sensitivity of the input amplitude detection. Input Amplifier/Buffer Figure 2 shows a simplified schematic of the SY88289CL’s input stage. The high-sensitivity of the input amplifier allows signals as small as 5mVPP to be detected and amplified. The input amplifier also allows input signals as large as 1800mVPP. Input signals are linearly amplified with a typical 38dB differential voltage gain. Since it is a limiting amplifier, the SY88289CL outputs typically 800mVPP voltage-limited waveforms for input signals that are greater than 12mVPP. Applications requiring the SY88289CL to operate with high-gain should have the upstream TIA placed as close as possible to the SY88289CL’s input pins to ensure the best performance of the device. Output Buffer The SY88289CL’s CML output buffer is designed to drive 50 Ω lines. The output buffer requires appropriate termination for proper operation. An external 50 Ω resistor to VCC for each output pin provides this. Figure 3 shows a simplified schematic of the output stage. Loss-of-signal The SY88289CL generates a chatter-free LOS open- collector TTL output with an internal 4.75k Ω pull -up resistor, as shown in Figure 4. LOS is used to determine that the input amplitude is large enough to be considered a valid input. LOS asserts high if the input amplitude falls below the threshold sets by LOSLVL and de-asserts low otherwise. LOS can be fed back to the enable bar (/EN) input to maintain output stability under a loss of signal condition. /EN de- asserts the true output signal without removing the input signals. Typically, 3.5dB 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 SY88289CL typically provides 3.5dB LOS electrical hysteresis. By definition, a power ratio measured in dB is 10log (power ratio). Power is calculated as V 2 IN/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 therefore, the ratios change linearly. Thus, the optical hysteresis in dB is half the electrical hysteresis in dB given in the data sheet. Since the SY88289CL is an electrical device, this data sheet refers to hysteresis in electrical terms. With 3.5dB LOS hysteresis, a voltage factor of 1.5 is required to assert or de-assert LOS. |
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