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AD8345AREZ-REEL7 参数 Datasheet PDF下载

AD8345AREZ-REEL7图片预览
型号: AD8345AREZ-REEL7
PDF下载: 下载PDF文件 查看货源
内容描述: [140MHz - 1000MHz RF/MICROWAVE QUADRAPHASE MODULATOR, MO-153ABT, TSSOP-16]
分类和应用: 射频微波
文件页数/大小: 20 页 / 359 K
品牌: ADI [ ADI ]
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AD8345  
CIRCUIT DESCRIPTION  
OVERVIEW  
DIFFERENTIAL VOLTAGE-TO-CURRENT  
CONVERTER  
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.  
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.  
LOIP  
LOIN  
PHASE  
SPLITTER  
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.  
IBBP  
IBBN  
Σ
OUT  
QBBP  
QBBN  
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.  
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  
LO INTERFACE  
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.  
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.  
Rev. B | Page 11 of 20