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AD8345ARE-REEL Datasheet(PDF) 8 Page - Analog Devices |
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AD8345ARE-REEL Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page REV. 0 AD8345 –8– CIRCUIT DESCRIPTION Overview The AD8345 can be divided into the following sections: Local Oscillator (LO) Interface, Mixer, Differential Voltage-to-Cur- rent (V-to-I) Converter, Differential-to-Single-Ended (D-to-S) Converter, and Bias. A block diagram of the part is shown in Figure 2. OUT LOIP LOIN IBBP IBBN QBBP QBBN PHASE SPLITTER Figure 2. AD8345 Block Diagram The LO Interface generates two LO signals at 90 degrees of phase difference with each other, to drive two mixers in quadra- ture. 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. 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 quadra- ture 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 oper- ating frequency range (250 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 V-to-I 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 propor- tional 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 D-to-S stage. Differential to Single-Ended Converter The differential-to-single-ended converter consists of two emit- ter 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 (VOUT) pin. Bias A bandgap reference circuit based on the ∆-VBE principle gen- erates the Proportional-To-Absolute-Temperature (PTAT) as well as temperature-stable currents used by the different sec- tions as references. When the bandgap reference is disabled by pulling down the voltage at the ENBL pin, all other sections are shut off accordingly. LO LEVEL – dBm –150 –10 –152 –160 –151 –153 –154 –155 VS = 5V –156 –157 –158 –159 –9 –8 –7 –6 –5 –4 –3 –2 –1 012 TPC 19. Noise Floor vs. LO Input Power. (TA = 25 °C, FLO = 800 MHz, VS = 5 V, All I and Q Inputs are DC-Biased to 0.7 V) Noise Measured at 20 MHz Offset from Carrier LO LEVEL – dBm –36 –10 VS = 5.5V –40 –50 –38 –42 –44 –46 –48 –9 –8 –7 –6 –5 –4 –3 –2 –1 012 TPC 20. LO Feedthrough vs. LO Input Power. (TA = 25 °C, LO = 800 MHz, VS = 5.5 V) |
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