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SY88973BLMGTR Datasheet(PDF) 8 Page - Micrel Semiconductor |
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SY88973BLMGTR Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 11 page Micrel, Inc. SY88973BL February 2007 8 M9999-020207-A hbwhelp@micrel.com or (408) 955-1690 Description The SY88973BL low-power, limiting post amplifier operates from a single +3.3V ±10% power supply, over an industrial temperature range of –40°C to +85°C. Signals with data rates up to 3.2Gbps and as small as 10mVPP can be amplified. Figure 1 shows the allowed input voltage swing. The SY88973BL generates a LOS output, providing feedback to /EN for output stability. LOSLVL sets the sensitivity of the input amplitude detection. Input Amplifier/Buffer The SY88973BL’s inputs are internally terminated with 50Ω-to-REF. Unless unaffected by this internal termination scheme, upstream devices need to be AC-coupled to the SY88973BL’s inputs. Figure 2 shows a simplified schematic of the input structure. The high sensitivity of the input amplifier detects and amplifies signals as small as 10mVPP. The input amplifier 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 SY88973BL outputs, typically 800mVPP, voltage-limited waveforms for input signals that are greater than 10mVPP. Applications requiring the SY88973BL to operate with high gain should have the upstream TIA placed as close as possible to the SY88973BL’s input pins to ensure the device’s best performance. Output Buffer The SY88973BL’s CML output buffer is designed to drive 50Ω lines. The output buffer requires appropriate termination for proper operation. An external 50Ω resistor- to-VCC or equivalent for each output pin provides this. Figure 3 shows a simplified schematic of the output structure and includes an appropriate termination method. Of course, driving a downstream device with a CML input that is internally terminated with 50Ω-to-VCC eliminates the need for external termination. As noted in the previous section, the amplifier outputs, typically, 800mVPP, waveforms across 25Ω total loads. The output buffer, thus, switches typically 16mA tail-current. Figure 4 shows the power supply current measurement which excludes the 16mA tail-current. Loss-of-Signal (LOS) The SY88973BL incorporates a chatter-free, LOS open- collector TTL output. For proper operation, an external 4.75kΩ to 10kΩ pull-up resistor between LOS and Vcc is required. 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-assert low the true output signal without removing the input signals. Typically, 4.6dB LOS hysteresis is provided to prevent chattering. Loss-of-Signal Level Set A programmable, loss-of-signal level set pin 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 REF as shown in Figure 6. If desired, an appropriate external voltage may be applied rather than using a resistor. The relationship between VLOSLVL and RLOSLVL is given by: 8 . 2 3 . 1 + − = LOSLVL LOSLVL CC LOSLVL R R V V where voltages are in volts and resistances are in kΩ. The smaller the external resistor, which implies a smaller voltage difference from LOSLVL to VCC, the lower the LOS sensitivity. Hence, larger input amplitude is required to de- assert LOS. The “Typical Operating Characteristics” section contains graphs showing the relationship between the input amplitude detection sensitivity and VLOSLVL or RLOSLVL. Hysteresis The SY88973BL provides typically 4.6dB 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 the ratios change linearly as well. Therefore, the optical hysteresis in dB is half the electrical hysteresis in dB given in the datasheet. The SY88973BL provides typically 2.3dB LOS optical hysteresis. As the SY88973BL is an electrical device, this datasheet refers to hysteresis in electrical terms. With 4.6dB LOS hysteresis, a voltage factor of 1.7 is required to de-assert LOS. Hysteresis and Sensitivity Improvement If increased LOS sensitivity and hysteresis are required, an application note entitled “Notes on Sensitivity and Hysteresis in Micrel Post Amplifiers” is available at: http://www.micrel.com/_PDF/HBW/App-Notes/an-45.pdf. |
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