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AD8328ARQ-EVAL Scheda tecnica(PDF) 10 Page - Analog Devices |
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AD8328ARQ-EVAL Scheda tecnica(HTML) 10 Page - Analog Devices |
10 / 20 page AD8328 Rev. A | Page 10 of 20 APPLICATIONS GENERAL APPLICATIONS The AD8328 is primarily intended for use as the power amplifier (PA) in Data Over Cable Service Interface Specification (DOCSIS)-certified cable modems and CATV set-top boxes. The upstream signal is either a QPSK or QAM signal generated by a DSP, a dedicated QPSK/QAM modulator, or a DAC. In all cases, the signal must be low-pass filtered before being applied to the PA to filter out-of-band noise and higher order harmonics from the amplified signal. Due to the varying distances between the cable modem and the head-end, the upstream PA must be capable of varying the output power by applying gain or attenuation. The ability to vary the output power of the AD8328 ensures that the signal from the cable modem has the proper level once it arrives at the head-end. The upstream signal path commonly includes a diplexer and cable splitters. The AD8328 has been designed to overcome losses associated with these passive components in the upstream cable path. CIRCUIT DESCRIPTION The AD8328 is composed of three analog functions in the power-up or forward mode. The input amplifier (preamp) can be used single-ended or differentially. If the input is used in the differential configuration, it is imperative that the input signals be 180° out of phase and of equal amplitude. A vernier is used in the input stage for controlling the fine 1 dB gain steps. This stage then drives a DAC, which provides the bulk of the AD8328’s attenuation. The signals in the preamp and DAC gain blocks are differential to improve the PSRR and linearity. A differential current is fed from the DAC into the output stage. The output stage maintains 300 Ω differential output impedance, which maintains proper match to 75 Ω when used with a 2:1 balun transformer. VIN+ VIN– VCC GND AD8328 BYP RL 5V VIN VIN VOUT+ VOUT– 1 2 1 2 Figure 19. Characterization Circuit SPI PROGRAMMING AND GAIN ADJUSTMENT The AD8328 is controlled through a serial peripheral interface (SPI) of three digital data lines: CLK, DATEN, and SDATA. Changing the gain requires eight bits of data to be streamed into the SDATA port. The sequence of loading the SDATA register begins on the falling edge of the DATEN pin, which activates the CLK line. With the CLK line activated, data on the SDATA line is clocked into the serial shift register on the rising edge of the CLK pulses, MSB first. The 8-bit data-word is latched into the attenuator core on the rising edge of the DATEN line. This provides control over the changes in the output signal level. The serial interface timing for the AD8328 is shown in Figure 3 and Figure 4. The programmable gain range of the AD8328 is −28 dB to +31 dB with steps of 1 dB per least significant bit (LSB). This provides a total gain range of 59 dB. The AD8328 was characterized with a differential signal on the input and a TOKO 458PT-1087 2:1 transformer on the output. The AD8328 incorporates supply current scaling with gain code, as shown in Figure 18. This allows reduced power consumption when operating in lower gain codes. INPUT BIAS, IMPEDANCE, AND TERMINATION The VIN+ and VIN− inputs have a dc bias level of VCC/2; therefore, the input signal should be ac-coupled as shown in Figure 20. The differential input impedance of the AD8328 is approximately 1.6 kΩ, while the single-ended input is 800 Ω. The high input impedance of the AD8328 allows flexibility in termination and properly matching filter networks. The AD8328 exhibits optimum performance when driven with a pure differential signal. OUTPUT BIAS, IMPEDANCE, AND TERMINATION The output stage of the AD8328 requires a bias of 5 V. The 5 V power supply should be connected to the center tap of the output transformer. In addition, the VCC applied to the center tap of the transformer should be decoupled as seen in Figure 20. |
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