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SY88803V Datasheet(PDF) 5 Page - Micrel Semiconductor |
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SY88803V Datasheet(HTML) 5 Page - Micrel Semiconductor |
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5 SY88803V Micrel, Inc M9999-051706 hbwhelp@micrel.com or (408) 955-1690 DETAILED DESCRIPTION The SY88803V low-power limiting post-amplifier operates from a single +3.3V or +5V power supply, over temperatures from –40 °C to +85°C. Signals with data rates up to 155Mbps and as small as 5mV PP can be amplified. Figure 1 shows the allowed input voltage swing. The SY88803V generates an LOS output. LOS LVL sets the sensitivity of the input amplitude detection. Input Amplifier/Buffer Figure 2 shows a simplified schematic of the SY88803V's input stage. The high-sensitivity of the input amplifier allows signals as small as 5mV PP to be detected and amplified. The input amplifier allows input signals as large as 1800mV PP. Input signals are linearly amplified with a typically 38dB differential voltage gain. Since it is a limiting amplifier, the SY88803V outputs typically 1500mV PP voltage-limited waveforms for input signals that are greater than 18mV PP. Applications requiring the SY88803V to operate with high- gain should have the upstream TIA placed as close as possible to the SY88803V’s input pins to ensure the best performance of the device. Output Buffer The SY88803V’s PECL output buffer is designed to drive 50 Ω lines. The output buffer requires appropriate termination for proper operation. An external 50 Ω resistor to V CC–2V for each output pin provides this. Figure 3 shows a simplified schematic of the output stage and includes an appropriate termination method. Loss-of-Signal The SY88803V generates a chatter-free TTL LOS. A recommended 2.5k Ω pullup resistor to V CC is required for proper operation, 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 LOS LVL and deasserts low otherwise. LOS can be fed back to the enable (/EN) input to maintain output stability under a loss of signal condition. /EN deasserts 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 (LOSLVL) sets the threshold of the input amplitude detection. Setting a voltage on LOSLVL between VCC and VREF sets this threshold. If desired, a resistor divider between V CC and V REF, as shown in Figure 5, also creates this threshold. The smaller the voltage difference from LOS LVL to VCC, the smaller the LOS sensitivity. Hence, larger input amplitude is required to deassert LOS. “Typical Operating Characteristics” shows the relationship between the input amplitude detection sensitivity and the LOSLVL voltage. Hysteresis The SY88803V provides typically 4.6dB LOS electrical hysteresis. By definition, a power ratio measured in dB is 10log(power ratio). Power is calculated as V2 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 hence the ratios change linearly. Therefore, the optical hysteresis in dB is half the electrical hysteresis in dB given in the datasheet. The SY88803V provides typically 2.3dB LOS optical hysteresis. As the SY88803V is an electrical device, this datasheet refers to hysteresis in electrical terms. With 6dB LOS hysteresis, a voltage factor of two is required to assert or deassert LOS. |
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