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AD8346-EVAL Datasheet(PDF) 7 Page - Analog Devices |
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AD8346-EVAL Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page REV. 0 AD8346 –7– CIRCUIT DESCRIPTION OVERVIEW The AD8346 can be divided into the following sections: Local Oscillator (LO) Interface, Mixer, Voltage-to-Current (V-to-I) Converter, Differential-to-Single-ended (D-to-S) Converter, and Bias. A detailed block diagram of the part is shown in Fig- ure 20. The LO Interface generates two LO signals, with 90 degrees of phase difference between them, to drive two mixers in quadra- ture. Baseband voltage signals are converted into current form in the V-to-I converters, feeding into two mixers. The output of the mixers are combined to feed the D-to-S converter which provides the 50 Ω output interface. Bias currents to each section are controlled by the Enable (ENBL) signal. Detailed descrip- tion of each section follows. LO Interface The differential LO inputs allow the user to drive the LO differ- entially in order to achieve maximum performance. The LO can be driven single-endedly but the LO feedthrough performance will be degraded, especially towards the higher end of the fre- quency range. The LO Interface consists of interleaved stages of polyphase network phase-splitters and buffer amplifiers. The 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 of 0.8 GHz to 2.5 GHz is achieved by staggering the RC time constants in each stage of the phase-splitters. The outputs of the second phase-splitter are fed into the driver amplifiers for the mixers’ LO inputs. V-to-I Converter Each baseband input pin is connected to an op amp driving an emitter follower. Feedback at the emitter maintains a current proportional to the input voltage through the transistor. This current is 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 resistors in series with two coupled on-chip inductors. The signal developed across the R-L loads are 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. Output imped- ance 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 ∆-V BE principle gener- ates the Proportional-To-Absolute-Temperature (PTAT) cur- rents used by the different sections as references. The bandgap voltage is also used to generate a temperature-stable current in the V-to-I converters to produce a temperature independent slew rate. When the bandgap reference is disabled by pulling down the ENBL pin, all other sections are shut off accordingly. MIXER MIXER V-TO-I V-TO-I V-TO-I V-TO-I D-TO-S BIAS CELL AD8346 LOIN LOIP ENBL QBBP QBBN VOUT IBBN IBBP PHASE SPLITTER 1 PHASE SPLITTER 2 Figure 20. Detailed Block Diagram |
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