Datasheet
ADA4937-1/ADA4937-2 Data Sheet
Rev. D | Page 20 of 28
2. For the source termination to be 50 Ω, the termination
resistor (R
T
) is calculated using R
T
||R
IN
= 50 Ω, which
makes R
T
equal to 61.9 Ω.
06591-082
ADA4937
R
L
V
O
+V
S
–V
S
R
S
50Ω
R
G
200Ω
R
G
200Ω
R
F
200Ω
R
F
200Ω
V
OCM
V
S
2V
50Ω
R
T
61.9Ω
Figure 57. Adding Termination Resistor R
T
3. To compensate for the imbalance of the gain resistors,
a correction resistor (R
TS
) is added in series with the
inverting Input Gain Resistor R
G
. R
TS
is equal to the
Thevenin equivalent of the Source Resistance R
S
||R
T
.
06591-083
R
S
50Ω
V
S
2V
R
T
61.9Ω
R
TH
27.4Ω
V
TH
1.1V
Figure 58. Calculating Thevenin Equivalent
R
TS
= R
TH
= R
S
||R
T
= 27.4 Ω. Note that V
TH
is not equal to V
S
/2,
which would be the case if the termination were not affected by
the amplifier circuit.
06591-084
ADA4937
R
L
V
O
0.97V
+V
S
–V
S
R
TH
27.4Ω
R
G
200Ω
R
G
200Ω
R
F
200Ω
R
F
200Ω
V
OCM
V
TH
1.1V
R
TS
27.4Ω
Figure 59. Balancing Gain Resistor R
G
4. The feedback resistor is calculated to adjust the output
voltage.
a.
To make the output voltage V
OUT
= 1 V, R
F
must be
calculated using the following formula:
207
1.1
)4.27200(1
)(
TH
TS
G
OUT
F
V
RRV
R
b.
To make V
O
= V
S
= 2 V to recover the loss due to the
input termination, R
F
should be
414
1.1
)4.27200(2
)(
TH
TS
G
OUT
F
V
RRV
R
06591-085
ADA4937
R
L
V
O
+V
S
–V
S
R
S
50Ω
R
G
200Ω
R
G
200Ω
R
F
R
F
V
OCM
V
S
2V
R
T
61.9Ω
R
TS
27.4Ω
Figure 60. Complete Single-Ended-to-Differential System
INPUT COMMON-MODE VOLTAGE RANGE IN
SINGLE-SUPPLY APPLICATIONS
The ADA4937-x 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 1.2 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
OCM
pin of the ADA4937-x is internally biased at a voltage
approximately equal to the midsupply point, [(+V
S
) + (−V
S
)]/2.
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 Table 2 and Table 4 assumes
that the V
OCM
input is driven by a low impedance voltage source.
It is also possible to connect the V
OCM
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 V
OCM
pin is approximately 10 kΩ. If multiple
ADA4937-x devices share one reference output, it is recommended
that a buffer be used.
Table 10 and Table 11 list several common gain settings, asso-
ciated resistor values, input impedances, and output noise density
values for both balanced and unbalanced input configurations.
POWER-DOWN OPERATION
The ADA4937-x power-down pin features an internal 25 k
pull-up resistor to the positive supply (+V
S
). This ensures that,
with the power-down pin left unconnected (floating), the
ADA4937-x turns on. Applying a voltage of ≤1 V turns the
ADA4937-x off.