ada4937-2 Analog Devices, Inc., ada4937-2 Datasheet - Page 20

• Manufacturer Part Number: ada4937-2
• Description: Ultralow Distortion Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

ADA4937-1/ADA4937-2
2.
3.
4.
V
2V
In order for the source termination to be 50 Ω, the termi-
nation resistor (R
which makes R
To compensate for the imbalance of the gain resistors,
a correction resistor (R
inverting input gain resistor R
Thevenin equivalent of the source resistance R
R
V
affected by the amplifier circuit.
The feedback resistor is calculated to adjust the output
voltage.
a.
S
R
TS
S
F
/2, which would be the case if the termination were not
1.1V
V
=
= R
50Ω
TH
To make the output voltage V
calculated using the following formula:
R
⎛
⎜
⎜
⎝
S
V
TH
V
2V
OUT
S
Figure 55. Calculating Thevenin Equivalent
50Ω
= R
Figure 54. Adding Termination Resistor R
27.4Ω
27.4Ω
R
Figure 56. Balancing Gain Resistor R
R
TH
×
TS
50Ω
S
R
||R
V
61.9Ω
(
T
S
R
TH
equal to 61.9 Ω.
T
R
G
T
200Ω
V
200Ω
T
R
R
) is calculated using R
= 27.4 Ω. Note that V
OCM
+
G
G
R
61.9Ω
T
R
200Ω
V
200Ω
TS
TS
R
R
OCM
) is added in series with the
G
G
)
⎞
⎟
⎟
⎠
=
200Ω
200Ω
ADA4937
R
R
⎛
⎜
⎝
G
F
F
+V
–V
1
. R
S
S
×
200Ω
200Ω
ADA4937
TS
1.1V
V
OUT
R
R
(
TH
200
F
F
+V
–V
is equal to the
1
S
S
= 1 V, R
1 .
27.4Ω
+
TH
T
R
||R
TH
G
27
is not equal to
IN
T
R
.
) 4
L
F
= 50 Ω,
S
||R
must be
V
0.97V
⎞
= ⎟
⎠
O
R
T
207
L
.
V
Ω
O
Rev. A | Page 20 of 28
INPUT COMMON-MODE VOLTAGE RANGE IN
SINGLE-SUPPLY APPLICATIONS
The ADA4937 is optimized for level-shifting ground-referenced
input signals. As such, the center of the input common-mode
range is shifted approximately 1 V down from midsupply. For
5 V single-supply operation, the input common-mode range at
the summing nodes of the amplifier is 0.3 V to 3.0 V, and 0.3 V
to 1.9 V with a 3.3 V supply. To avoid clipping at the outputs,
the voltage swing at the +IN and −IN terminals must be confined
to these ranges.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
approximately equal to the midsupply point, [(+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.
The output common-mode offset listed in the Specifications
section assumes that the V
voltage source.
It is also possible to connect the V
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 V
ADA4937 devices share one reference output, it is recommended
that a buffer be used.
Table 10 and Table 11 list several common gain settings,
associated resistor values, input impedance and output noise
density for both balanced and unbalanced input configurations.
b.
R
OCM
V
2V
F
S
Figure 57. Complete Single-Ended-to-Differential System
To make V
input termination, R
=
pin of the ADA4937 is internally biased at a voltage
⎛
⎜
⎜
⎝
50Ω
R
V
S
OUT
61.9Ω
27.4Ω
×
OCM
R
V
O
R
(
TS
T
R
TH
= V
G
pin is approximately 10 kΩ. If multiple
+
OCM
200Ω
V
200Ω
S
R
R
OCM
= 2 V to recover the loss due to the
R
G
G
TS
input is driven by a low impedance
F
)
should be
⎞
⎟
⎟
⎠
=
OCM
ADA4937
⎛
⎜
⎝
R
R
2
F
F
input to a common-mode
+V
–V
×
S
S
(
200
1
1 .
+
27
S
) + (−V
.
) 4
R
L
⎞
⎟
⎠
=
V
O
S
414
)]/2.
Ω