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OQ2538HP Datasheet(PDF) 6 Page - NXP Semiconductors |
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OQ2538HP Datasheet(HTML) 6 Page - NXP Semiconductors |
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6 / 24 page  1997 Nov 26 6 Philips Semiconductors Preliminary specification SDH/SONET main amplifier OQ2538HP FUNCTIONAL DESCRIPTION The OQ2538HP is comprised of four DC-coupled amplifier stages along with additional circuitry for offset compensation and level detection. The first amplifier stage contains a modified Cherry/Hooper amplifying cell with high gain (approximately 20 dB) and a wide bandwidth. Special attention is paid to minimizing the equivalent input noise at this stage, thus reducing the overall noise level. Additional feedback is applied at the second and third stages, improving isolation and reducing the gain to 14 dB per stage. The last stage is an output buffer, a unity gain amplifier, with an output impedance of 100 Ω. The total gain of the OQ2538HP amounts to 48.5 dB, thus providing a constant CML-compatible output signal over a wide input signal range. Two rectifier circuits are used to measure the input signal level. Two separate RF pre-amplifiers are used to generate the voltage gain needed to obtain a suitable rectifier output voltage. For rectifier A the gain is approximately 18 dB, for rectifier B it is about 14 dB. The output of rectifier A can be used for AGC at the pre-amplifier stage in front of the OQ2538HP. The output of rectifier B can be used for LOS detection. There is a linear relationship between the rectifier output voltage and the input signal level provided the amplifiers are not saturated. Because the four gain stages are DC-coupled and provide a high overall gain, the effect of the input offset can be considerable. The OQ2538HP features an internal offset compensation circuit for eliminating the input offset. The bandwidth of the offset control loop is determined by an external capacitor. COFF and COFFQ offset compensation Automatic offset compensation eliminates the input offset of the OQ2538HP. This offset cancellation influences the low frequency gain of the amplifier stages. With a capacitance of 100 nF between COFF and COFFQ the loop bandwidth will be less than 1.5 kHz, small enough to have no influence on amplifier gain over the frequencies of interest. If the capacitor were omitted, the loop bandwidth would be greater than 30 MHz, which would influence the input signal gain. The loop bandwidth can be calculated from the following formula: (1) f loop 1 2 π 1250 Ω × C ext × ------------------------------------------------ = where Cext is the capacitance connected between COFF and COFFQ. REF and CAPA band gap output and decoupling capacitance To reduce band gap noise levels, a 1 nF decoupling capacitor on CAPA is recommended. Since the band gap is referenced to the negative supply, VEE, the decoupling capacitor should be connected between CAPA and VEE. The band gap voltage is present for test purposes only. It is not intended to serve as an external reference. RF input and output connections Striplines, or microstrips, with an odd mode characteristic impedance of Zo,odd =50 Ω must be used for the differential RF connections on the PCB. This applies to both the input signal pair IN and INQ and to the output signal pair OUT and OUTQ. The two lines in each pair should be the same length. RF input matching circuit The input circuit for pins IN and INQ contains internal 100 Ω resistors decoupled to ground via an internal common mode 6 pF capacitor. The topology is depicted in Fig.3. An external 200 Ω resistor between IN and INQ is recommended in order to match the inputs to a differential transmission line, coupled microstrip or stripline with an odd mode impedance Zo,odd of 50 Ω. Fig.3 RF input topology. handbook, halfpage MGM114 IN INQ GND 100 Ω 6 pF 100 Ω |
Similar Part No. - OQ2538HP_97 |
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Similar Description - OQ2538HP_97 |
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