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ADA4927-1 PDF预览

ADA4927-1

更新时间: 2024-10-28 06:36:15
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亚德诺 - ADI 驱动器
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24页 840K
描述
Ultralow Distortion Current Feedback Differential ADC Driver

ADA4927-1 数据手册

 浏览型号ADA4927-1的Datasheet PDF文件第17页浏览型号ADA4927-1的Datasheet PDF文件第18页浏览型号ADA4927-1的Datasheet PDF文件第19页浏览型号ADA4927-1的Datasheet PDF文件第21页浏览型号ADA4927-1的Datasheet PDF文件第22页浏览型号ADA4927-1的Datasheet PDF文件第23页 
ADA4927-1/ADA4927-2  
R
F
RTS = RTH = RS||RT = 26.5 ꢁ. Note that VTH is greater than  
1 V p-p, which was obtained with RT = 50 ꢁ. The modified  
circuit with the Thevenin equivalent (closest 1% value used for  
RTH) of the terminated source and RTS in the lower feedback  
loop is shown in Figure 53.  
357  
+V  
1V p-p  
S
R
R
S
G
50Ω  
348Ω  
R
T
V
S
56.2Ω  
V
OUT, dm  
1.01V p-p  
V
OCM  
ADA4927  
R
2V p-p  
L
R
F
R
G
348Ω  
+V  
348Ω  
R
TS  
26.7Ω  
S
R
R
–V  
S
TH  
G
R
F
26.7Ω  
348Ω  
V
TH  
1.06V p-p  
357Ω  
V
OCM  
V
OUT, dm  
ADA4927  
R
L
Figure 54. Terminated Single-Ended-to-Differential System with G = 1  
R
G
348Ω  
R
TS  
26.7Ω  
INPUT COMMON-MODE VOLTAGE RANGE  
–V  
S
The ADA4927 input common-mode range is centered between the  
two supply rails, in contrast to other ADC drivers with level-shifted  
input ranges, such as the ADA4937. The centered input common-  
mode range is best suited to ac-coupled, differential-to-differential,  
and dual supply applications.  
R
F
348Ω  
Figure 53. Thevenin Equivalent and Matched Gain Resistors  
Figure 53 presents a tractable circuit with matched  
feedback loops that can be easily evaluated.  
For operation with 5 V supplies, the input common-mode  
range at the summing nodes of the amplifier is specified as  
−3.5 V to +3.5 V and is specified as +1.3 V to +3.7 V with a  
single +5 V supply. To avoid nonlinearities, the voltage swing  
at the +IN and −IN terminals must be confined to these ranges.  
It is useful to point out two effects that occur with a  
terminated input. The first is that the value of RG is increased  
in both loops, lowering the overall closed-loop gain. The  
second is that VTH is a little larger than 1 V p-p, as it is  
when RT = 50 ꢁ. These two effects have opposite impacts  
on the output voltage, and for large resistor values in the  
feedback loops (~1 kꢁ), the effects essentially cancel each  
other out. For small RF and RG, or high gains, however, the  
diminished closed-loop gain is not canceled completely by the  
increased VTH. This can be seen by evaluating Figure 53.  
INPUT AND OUTPUT CAPACITIVE AC COUPLING  
Input ac coupling capacitors can be inserted between the source  
and RG. This ac coupling blocks the flow of the dc common-  
mode feedback current and causes the ADA4927 dc input  
common-mode voltage to equal the dc output common-mode  
voltage. These ac coupling capacitors must be placed in both  
loops to keep the feedback factors matched.  
The desired differential output in this example is 1 V p-p  
because the terminated input signal is 1 V p-p and the  
closed-loop gain = 1. The actual differential output voltage,  
however, is equal to (1.06 V p-p)(348/374.7) = 0.984 V p-p.  
To obtain the desired output voltage of 1 V p-p, a final gain  
adjustment can be made by increasing RF without modifying  
any of the input circuitry. This is discussed in Step 4.  
ꢀutput ac coupling capacitors can be placed in series between  
each output and its respective load. See Figure 58 for an example  
that uses input and output capacitive ac coupling.  
SETTING THE OUTPUT COMMON-MODE VOLTAGE  
The VꢀCM pin of the ADA4927 is internally biased with a voltage  
divider comprising two 10 kꢁ resistors at a voltage approximately  
equal to the midsupply point, [(+VS) + (−VS)]/2. Because of this  
internal divider, the VꢀCM pin sources and sinks current, depending  
on the externally applied voltage and its associated source resistance.  
Relying on the internal bias results in an output common-mode  
voltage that is within about 100 mV of the expected value.  
In cases where accurate control of the output common-mode level  
is required, it is recommended that an external source or resistor  
divider be used with source resistance less than 100 ꢁ. The output  
common-mode offset listed in the Specifications section presumes  
that the VꢀCM input is driven by a low impedance voltage source.  
It is also possible to connect the VꢀCM input to a common-mode  
level (CML) output of an ADC; however, care must be taken to  
ensure that the output has sufficient drive capability. The input  
impedance of the VꢀCM pin is approximately 10 kꢁ. If multiple  
ADA4927 devices share one ADC reference output, a buffer may  
be necessary to drive the parallel inputs.  
4. The feedback resistor value is modified as a final gain  
adjustment to obtain the desired output voltage.  
To make the output voltage VꢀUT = 1 V p-p, RF must be  
calculated using the following formula:  
RF =  
(
Desired VOUT,dm  
)
(
RG + RTS  
)
(
)(  
)
= 35  
1V p p 374.7Ω  
=
VTH  
1.06V p p  
The closest standard 1% values to 353 ꢁ are 348 ꢁ and  
357 ꢁ. Choosing 357 ꢁ for RF gives a differential output  
voltage of 1.01 V p-p. The closed-loop bandwidth is  
diminished by a factor of approximately 348/357 from  
what it would be with RF = 348 ꢁ due to the inversely  
proportional relationship between RF and closed-loop  
gain that is characteristic of current feedback amplifiers.  
The final circuit is shown in Figure 54.  
Rev. 0 | Page 20 of 24  
 
 
 

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