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| 型号： | ADA4938-2ACPZ-RL |
| 厂商： | Analog Devices Inc |
| 文件页数： | 14/28页 |
| 文件大小： | 0K |
| 描述： | IC ADC DRIVER DUAL DIFF 24LFCSP |
| 标准包装： | 5,000 |
| 类型： | ADC 驱动器 |
| 应用： | 数据采集 |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-VFQFN 裸露焊盘，CSP |
| 供应商设备封装： | 24-LFCSP-VQ（4x4） |
| 包装： | 带卷 (TR) |
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ADA4938-1/ADA4938-2
Rev. A | Page 21 of 28
TERMINATING A SINGLE-ENDED INPUT
Using an example with an input source of 2 V, a source
resistance of 50 Ω, and an overall gain of 1 V/V, four simple
steps must be followed to terminate a single-ended input to the
ADA4938-x.
1. The input impedance is calculated using the formula
()
Ω
267
200)
(200
2
200
1
200
2
1
=
+
×
=
+
×
=
F
G
F
G
IN
R
06
59
2-
0
8
1
ADA4938
RL
VO
+VS
–VS
RS
50
RG
200
RG
200
RF
200
RF
200
VOCM
VS
2V
RIN
267
Figure 61. Single-Ended Input Impedance
2. To provide a 50 Ω termination for the source, the Resistor RT
is calculated such that RT || RIN = 50 Ω, or RT = 61.9 Ω.
06
59
2-
08
2
ADA4938
RL
VO
+VS
–VS
RS
50
RG
200
RG
200
RF
200
RF
200
VOCM
VS
2V
50
RT
61.9
Figure 62. Adding Termination Resistor RT
3. To compensate for the imbalance of the gain resistors, a correc-
tion resistor (RTS) is added in series with the inverting Input
Gain Resistor RG. RTS is equal to the Thevenin equivalent of
the source resistance (RS||RT).
06
59
2-
08
3
RS
50
VS
2V
RT
61.9
RTH
27.4
VTH
1.1V
Figure 63. Calculating Thevenin Equivalent
RTS = RTH = RS || RT = 27.4 Ω. Note that VTH is not equal to
VS/2, which would be the case if the amplifier circuit did
not affect the termination.
06
59
2
-08
4
ADA4938
RL
VO
0.97V
+VS
–VS
RTH
27.4
RG
200
RG
200
RF
200
RF
200
VOCM
VTH
1.1V
RTS
27.4
Figure 64. Balancing Gain Resistor RG
4. Finally, the feedback resistor is recalculated to adjust the
output voltage to the desired level.
a.
To make the output voltage VO = 1 V, RF is calculated
using
Ω
207
1.1
27.4)
(200
1
)
(
=
+
×
=
+
×
=
TH
TS
G
O
F
V
R
V
R
b.
To return the overall gain to 1 V/V (VO = VS = 2 V), RF
should be
Ω
414
1.1
27.4)
(200
2
)
(
=
+
×
=
+
×
=
TH
TS
G
O
F
V
R
V
R
06
59
2-
0
85
ADA4938
RL
VO
+VS
–VS
RS
50
RG
200
RG
200
RF
VOCM
VS
2V
RT
61.9
RTS
27.4
Figure 65. Complete Single-Ended-to-Differential System
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The VOCM pin of the ADA4938-x is internally biased at a voltage
approximately equal to the midsupply point (average value of
the voltages on V+ and V). Relying on this internal bias results
in an output common-mode voltage that is within about 100 mV of
the expected value.
In cases where more accurate control of the output common-
mode level is required, it is recommended that an external
source or resistor divider (10 kΩ or greater resistors) be used.
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
ADA4938-x devices share one reference output, it is recommended
that a buffer be used.
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