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AD8345ARE-REEL7 Datasheet(PDF) 11 Page - Analog Devices |
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AD8345ARE-REEL7 Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page AD8345 Rev. B | Page 11 of 20 CIRCUIT DESCRIPTION OVERVIEW The AD8345 can be divided into the following sections: local oscillator (LO) interface, mixer, differential voltage-to-current (V-to-I) converter, differential-to-single-ended (D-to-S) converter, and bias. A block diagram of the part is shown in Figure 27. OUT LOIP LOIN IBBP IBBN QBBP QBBN PHASE SPLITTER Σ Figure 27. AD8345 Block Diagram The LO interface generates two LO signals at 90° of phase difference with each other, to drive two mixers in quadrature. Baseband signals are converted into current form in the differential V-to-I converters, feeding into the two mixers. The outputs of the mixers are combined to feed the differential-to- single-ended converter, which provides a 50 Ω output interface. Bias currents to each section are controlled by the enable (ENBL) signal. A detailed description of each section follows. LO INTERFACE The LO interface consists of interleaved stages of polyphase phase splitters and buffer amplifiers. The polyphase phase splitter contains resistors and capacitors connected in a circular manner to split the LO signal into I and Q paths in precise quadrature with each other. The signal on each path goes through a buffer amplifier to make up for the loss and high frequency roll-off. The two signals then go through another polyphase network to enhance the quadrature accuracy. The broad operating frequency range (140 MHz to 1000 MHz) is achieved by staggering the RC time constants of each stage of the phase splitters. The outputs of the second phase splitter are fed into the driver amplifiers for the mixers’ LO inputs. DIFFERENTIAL VOLTAGE-TO-CURRENT CONVERTER In this circuit, each baseband input pin is connected to an op amp driving a transistor connected as an emitter follower. A resistor between the two emitters maintains a varying current proportional to the differential input voltage through the transistor. These currents are fed to the two mixers in differential form. MIXERS There are two double-balanced mixers, one for the in-phase channel (I channel) and one for the quadrature channel (Q channel). Each mixer uses the Gilbert-cell design with four cross-connected transistors. The bases of the transistors are driven by the LO signal of the corresponding channel. The output currents from the two mixers are summed together in two load resistors. The signal developed across the load resistors is sent to the differential-to-single-ended converter. DIFFERENTIAL-TO-SINGLE-ENDED CONVERTER The differential-to-single-ended converter consists of two emitter followers driving a totem-pole output stage whose output impedance is established by the emitter resistors in the output transistors. The output of this stage is connected to the output pin (VOUT). BIAS A band gap reference circuit based on the Δ-VBE principle generates the proportional-to-absolute temperature (PTAT) as well as temperature-stable currents used by the different sections as references. When the band gap reference is disabled by pulling down the voltage at the ENBL pin, all other sections are shut off accordingly. |
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