RTS = RTH
參數(shù)資料
型號(hào)： ADA4927-2YCPZ-RL
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 13/24頁(yè)
文件大?。?/td> 0K
描述： IC OPAMP CF DIFF DUAL LN 24LFCSP
標(biāo)準(zhǔn)包裝： 5,000
放大器類型： 電流反饋
電路數(shù)： 2
輸出類型： 差分
轉(zhuǎn)換速率： 5000 V/µs
-3db帶寬： 2.3GHz
電流 - 輸入偏壓： 500nA
電壓 - 輸入偏移： 300µV
電流 - 電源： 20mA
電流 - 輸出 / 通道： 65mA
電壓 - 電源，單路/雙路(±)： 4.5 V ~ 11 V，±2.25 V ~ 5.5 V
工作溫度： -40°C ~ 105°C
安裝類型： 表面貼裝
封裝/外殼： 24-VFQFN 裸露焊盤(pán)，CSP
供應(yīng)商設(shè)備封裝： 24-LFCSP-VQ（4x4）
包裝： 帶卷 (TR)
ADA4927-1/ADA4927-2
Rev. A | Page 20 of 24
RTS = RTH = RS||RT = 26.5 Ω. Note that VTH is greater than
1 V p-p, which was obtained with RT = 50 Ω. 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.
ADA4927
RL VOUT, dm
+VS
–VS
RTH
26.7
RG
348
RG
348
RF
348
RF
348
VOCM
VTH
1.06V p-p
RTS
26.7
07
57
4-
05
3
()()
Figure 53. Thevenin Equivalent and Matched Gain Resistors
Figure 53 presents a tractable circuit with matched
feedback loops that can be easily evaluated.
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 Ω. These two effects have opposite impacts
on the output voltage, and for large resistor values in the
feedback loops (~1 kΩ), 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.
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.
4.
The feedback resistor value is modified as a final gain
adjustment to obtain the desired output voltage.
To make the output voltage VOUT = 1 V p-p, RF must be
calculated using the following formula:
()(
)
35
06
.
1
Ω
374.7
1
,
=
=
+
p
V
p
V
R
V
Desired
TH
TS
G
dm
OUT
F =
R
The closest standard 1% values to 353 Ω are 348 Ω and
357 Ω. Choosing 357 Ω 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 Ω 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.
ADA4927
RL
VOUT, dm
1.01V p-p
+VS
–VS
RS
50
RG
348
RG
348
RF
357
RF
357
VOCM
VS
2V p-p
1V p-p
RT
56.2
RTS
26.7
07
574
-05
4
Figure 54. Terminated Single-Ended-to-Differential System with G = 1
INPUT COMMON-MODE VOLTAGE RANGE
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.
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.
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.
Output 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 VOCM pin of the ADA4927 is internally biased with a voltage
divider comprising two 10 kΩ resistors at a voltage approximately
equal to the midsupply point, [(+VS) + (VS)]/2. Because of this
internal divider, the VOCM 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 Ω. The output
common-mode offset listed in the Specifications section presumes
that the VOCM input is driven by a low impedance voltage source.
It is also possible to connect the VOCM 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 VOCM pin is approximately 10 kΩ. If multiple
ADA4927 devices share one ADC reference output, a buffer may
be necessary to drive the parallel inputs.
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