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SY88403BLMGTR Datasheet(PDF) 8 Page - Micrel Semiconductor |
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SY88403BLMGTR Datasheet(HTML) 8 Page - Micrel Semiconductor |
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8 / 12 page  Micrel, Inc. SY88403BL November 2008 8 M9999-111808-F hbwhelp@micrel.com or (408) 955-1690 Detailed Description The SY88403BL low-power limiting post amplifier operates from a single +3.3V power supply, over temperatures from –40°C to +85°C. Signals with data rates up to 4.25Gbps and as small as 5mVPP can be amplified. Figure 1 shows the allowed input voltage swing. The SY88403BL generates an LOS output, allowing feedback to /EN for output stability. LOSLVL sets the sensitivity of the input amplitude detection. Input Amplifier/Buffer The SY88403BL’s inputs are internally terminated with 50 Ω to REF. If not affected by this internal termination scheme, upstream devices need to be AC-coupled to the SY88403BL's inputs. 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 38dB differential voltage gain. Since it is a limiting amplifier, the SY88403BL outputs typically 800mVPP voltage-limited waveforms for input signals that are greater than 20mVPP. Applications requiring the SY88403BL to operate with high-gain should have the upstream TIA placed as close as possible to the SY88403BL’s input pins to ensure the best performance of the device. Output Buffer The SY88403BL’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 or equivalent for each output pin provides this. Figure 3 shows a simplified schematic of the output stage and includes an appropriate termination method. Of course, driving a downstream device with a CML input that is internally terminated with 50 Ω to V CC eliminates the need for external termination. As noted in the previous section, the amplifier outputs typically 800mVPP waveforms across 25 Ω 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 The SY88403BL incorporates a chatter-free loss-of- signal (LOS) open-collector TTL output with internal 4.75k Ω pull-up resistor as shown in Figure 5. 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 3.5dB LOS hysteresis is provided to prevent chattering. Loss-of-Signal Level Set A programmable loss-of-signal 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 6. If desired, an appropriate external voltage may be applied rather than using a resistor. The relationship between VLOSLVL and RLOSLVL is given by: + − = 2.8 R R 1.3 V V LOSLVL LOSLVL CC LOSLVL where voltages are in volts and resistances are in k Ω. The smaller the external resistor, implying a smaller voltage difference from LOSLVL to VCC, lowers the LOS sensitivity. Hence, larger input amplitude is required to de-assert LOS. “Typical Operating Characteristics” contains graphs showing the relationship between the input amplitude detection sensitivity and RLOSLVL. Hysteresis The SY88403BL provides typically 3.5dB LOS electrical hysteresis. By definition, a power ratio measured in dB is 10log (power ratio). Power is calculated as VIN 2/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 datasheet. The SY88403BL provides typically 1.75dB LOS optical hysteresis. As the SY88403BL is an electrical device, this datasheet refers to hysteresis in electrical terms. With 3.5dB LOS hysteresis, a voltage factor of 1.5 is required to de-assert LOS. |
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