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AD6121 bảng dữ liệu(PDF) 10 Page - Analog Devices |
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AD6121 bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 16 page AD6121 –10– REV. B X-AMP is a trademark of Analog Devices, Inc. THEORY OF OPERATION The AD6121 consists of high dynamic range IF amplifiers with voltage controlled gain, a divide-by-two quadrature generator, an I and Q demodulator, a low dropout regulator and power- down control inputs (Figure 27). The AD6121 accommodates both the desired CDMA signal and an interferer 42 dB larger—approximately 6 mV p-p for the desired signal and 700 mV p-p for the interferer—as specified in the CDMA system. IF Amplifiers and Gain Control The IF gain is provided by two sections: a CDMA or FM input stage followed by three cascaded IF amplifiers. The CDMA and FM input stages use differential, continuously-variable attenua- tors based on Analog Devices’ patented X-AMP™ topology. These low noise attenuators consist of a differential R-2R ladder network, linear interpolator, and a fixed gain amplifier. The bulk of the IF gain is provided by three cascaded, wideband 2 I Q ROOFING FILTER IF OUTPUT DEMODULATOR INPUT IF AMPLIFIERS INPUT STAGE QUADRATURE DEMODULATOR CDMA INPUT FM INPUT CDMA/FM SELECT GAIN CONTROL VOLTAGE INPUT 1.23V REFERENCE OUTPUT GAIN CONTROL VOLTAGE REFERENCE INPUT POWER- DOWN 2 POWER- DOWN 1 IOUT IOUT QOUT QOUT LOCAL OSCILLATOR INPUT VPOS VREG LOW DROPOUT REGULATOR GAIN CONTROL SCALE FACTOR PTAT TEMPERATURE COMPENSATION AD6121 Figure 27. Functional Block Diagram VGAIN – Volts 16 0.5 2.5 1 1.5 2 8 10 4 14 TA = –40 C TA = +25 C 6 12 2 0 TA = +85 C Figure 26. Current Consumption vs. VGAIN, TA = –40 °C, +25 °C and +85°C amplifiers. The gain and input bandwidth of the AD6121 are identical for both the FM and CDMA operating modes. When the CDMA/FM pin is high, CDMA mode is enabled. When the pin is low, FM mode is enabled. The IF amplifiers operate in two different configurations, one with the I and Q demodulator powered up and another with the I and Q demodulator powered down. The I and Q demodulator power setting is configured with pin PD2. The power-down control is further discussed in the section of this data sheet entitled Power-Down Control. When the demodulator is powered up, the outputs of the IF amplifiers are internally dc-biased and there is no need for exter- nal pull-up inductors. A roofing filter is required (see section entitled Roofing Filter in this data sheet) when using the IF amplifiers with the I and Q demodulator powered up. Under these conditions, the IF amplifiers and the low noise attenuator input stage has +41.4 dB of gain. When the I and Q demodulator is powered down, the IF ampli- fiers have open collector outputs resulting in the need for pull- up inductors. Under this configuration, and with the output of the IF amplifiers loaded with 1 k Ω, the gain of the IF amplifiers and low noise attenuator input stage is +47 dB. The pull-up inductors should be chosen so that the parasitic capacitance seen at the output of the IF amplifiers is resonated at the fre- quency of interest. Figure 28 shows how to configure the pull-up inductors at the output of the IF amplifiers. The 10 nF capaci- tors are used for ac coupling. In order to resonate the parasitic capacitors, rearrange Equation 1 to solve for L. f LCPAR 0 1 2 = π (1) where f0 is the IF frequency in Hertz, CPAR is the total parasitic capacitance in Farads, and L is the total shunt inductor value in henrys. |
Số phần tương tự - AD6121_15 |
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Mô tả tương tự - AD6121_15 |
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