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参数资料
| 型号： | ADA4939-2YCP-EBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 13/24页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADA4939-2YCP |
| 标准包装： | 1 |
| 每 IC 通道数： | 2 - 双 |
| 放大器类型： | 差分 |
| 板类型： | 裸（未填充） |
| 已供物品： | 板 |
| 已用 IC / 零件： | 24-LFCSP 封装 |
ADA4939-1/ADA4939-2
Rev. 0 | Page 20 of 24
2.
In order to match the 50 Ω source resistance, the termi-
nation resistor, RT, is calculated using RT||300 Ω = 50 Ω.
The closest standard 1% value for RT is 60.4 Ω.
ADA4939
RL
VOUT, dm
+VS
–VS
RS
50
RG
200
RG
200
RF
400
RF
400
VOCM
VS
2V p-p
RIN
50
RT
60.4
07
42
9-
0
54
Figure 47. Adding Termination Resistor RT
3.
upper feedback loop is now greater than the RG in the
lower loop due to the addition of the termination resistors.
To compensate for the imbalance of the gain resistors,
a correction resistor (RTS) is added in series with RG in the
lower loop. RTS is equal to the Thevenin equivalent of the
source resistance RS and the termination resistance RT and
is equal to RS||RT.
RS
50
VS
2V p-p
RT
60.4
RTH
27.4
VTH
1.09V p-p
0
74
29
-05
5
Figure 48. Calculating the Thevenin Equivalent
RTS = RTH = RS||RT = 27.4 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 of the terminated source
ADA4939
RL VOUT, dm
+VS
–VS
RTH
27.4
RG
200
RG
200
RF
400
RF
400
VOCM
VTH
1.09V p-p
RTS
27.4
07
42
9-
0
56
Figure 49. Thevenin Equivalent and Matched Gain Resistors
Figure 49 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 would
be if 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, however, the diminished closed-loop
gain is not canceled completely by the increased VTH. This
can be seen by evaluating Figure 49.
The desired differential output in this example is 2 V p-p
because the terminated input signal was 1 V p-p and the
closed-loop gain = 2. The actual differential output voltage,
however, is equal to (1.09 V p-p)(400/227.4) = 1.92 V p-p.
To obtain the desired output voltage of 2 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 = 2 V p-p, RF must be
calculated using the following formula:
()() ()(
)
Ω
=
Ω
=
+
=
417
09
.
1
4
.
227
2
,
P
TH
TS
G
dm
OUT
F
V
R
V
Desired
R
The closest standard 1 % values to 417 Ω are 412 Ω and
422 Ω. Choosing 422 Ω gives a differential output voltage
of 2.02 V p-p.
The final circuit is shown in Figure 50.
ADA4939
RL
VOUT, dm
2.02V p-p
+VS
–VS
RS
50
RG
200
RG
200
RF
422
RF
422
VOCM
VS
2V p-p
1V p-p
RT
60.4
RTS
27.4
07
429
-05
7
Figure 50. Terminated Single-Ended-to-Differential System with G = 2
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