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| 型號： | ADA4940-1ARZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 19/33頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC DIFF ADC DVR 18BIT LN 8SOIC |
| 標(biāo)準(zhǔn)包裝： | 98 |
| 放大器類型： | 差分 |
| 電路數(shù)： | 1 |
| 輸出類型： | 差分 |
| 轉(zhuǎn)換速率： | 95 V/µs |
| -3db帶寬： | 260MHz |
| 電流 - 輸入偏壓： | 1.1µA |
| 電壓 - 輸入偏移： | 60µV |
| 電流 - 電源： | 1.25mA |
| 電流 - 輸出 / 通道： | 46mA |
| 電壓 - 電源，單路/雙路(±)： | 3 V ~ 6 V |
| 工作溫度： | -40°C ~ 125°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 8-SOIC（0.154"，3.90mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 8-SO |
| 包裝： | 管件 |
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Data Sheet
ADA4940-1/ADA4940-2
Rev. C | Page 25 of 32
Terminating a Single-Ended Input
This section describes how to properly terminate a single-ended
and RG = 1 k. An example using an input source with a terminated
output voltage of 1 V p-p and source resistance of 50 illustrates
the three steps that must be followed. Because the terminated
output voltage of the source is 1 V p-p, the open-circuit output
voltage of the source is 2 V p-p. The source shown in Figure 66
indicates this open-circuit voltage.
RS
50
VS
2V p-p
RIN, se
1.33k
ADA4940-1
ADA4940-2
RL VOUT, dm
+VS
–VS
RG
1k
RG
1k
RF
1k
RF
1k
VOCM
08452-
059
Figure 66. Calculating Single-Ended Input Impedance, RIN
1. The input impedance is calculated by
Ω
k
.33
1
)
1000
(
2
1000
1
1000
)
(
2
1
,
=
+
×
=
+
×
=
F
G
F
G
se
IN
R
2. To match the 50 source resistance, calculate the
termination resistor, RT, using RT||1.33 k = 50 .
The closest standard 1% value for RT is 52.3 .
ADA4940-1
ADA4940-2
RL
VOUT, dm
+VS
–VS
RS
50
RG
1k
RG
1k
RF
1k
RF
1k
VOCM
VS
2V p-p
RIN, se
50
RT
52.3
08452-
060
Figure 67. Adding Termination Resistor RT
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, add a correction resistor (RTS)
in series with RG in the lower loop. RTS is 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
52.3
RTH
25.5
VTH
1.02V p-p
08452-
061
Figure 68. Calculating the Thevenin Equivalent
RTS = RTH = RS||RT = 25.5 . 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 69.
ADA4940-1
ADA4940-2
RL VOUT, dm
+VS
–VS
RTH
25.5
RG
1k
RG
1k
RF
1k
RF
1k
VOCM
VTH
1.02V p-p
RTS
25.5
08452-
062
Figure 69. Thevenin Equivalent and Matched Gain Resistors
Figure 69 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 .
These two effects have opposite impacts on the output voltage,
and for large resistor values in the feedback loops (~1 k), the
effects essentially cancel each other out. For small RF and RG,
or high gains, however, the diminished closed-loop gain is not
cancelled completely by the increased VTH. This can be seen by
evaluating Figure 69.
The desired differential output in this example is 1 V p-p
because the terminated input signal was 1 V p-p and the
closed-loop gain = 1. The actual differential output voltage,
however, is equal to (1.02 V p-p)(1000/1025.5) = 0.996 V p-p.
This is within the tolerance of the resistors, so no change to
the feedback resistor, RF, is required.
INPUT COMMON-MODE VOLTAGE RANGE
The ADA4940-1/ADA4940-2 input common-mode range is
shifted down by approximately 1 VBE, in contrast to other ADC
drivers with centered input ranges, such as the ADA4939-1/
ADA4939-2. The downward-shifted input common-mode range
is especially suited to dc-coupled, single-ended-to-differential,
and single-supply applications.
For ±2.5 V or +5 V supply operation, the input common-mode
range at the summing nodes of the amplifier is specified as 2.7 V
to +1.3 V or 0.2 V to 3.8 V, and is specified as 0.2 V to +1.8 V
with a +3 V supply.
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