Datasheet
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
()
267
200)(2002
200
1
200
2
1
=
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
+×
−
=
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
+×
−
=
F
G
F
G
IN
RR
R
R
R
06592-081
ADA4938
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
R
IN
267Ω
Figure 61. Single-Ended Input Impedance
2. To provide a 50 Ω termination for the source, the Resistor R
T
is calculated such that R
T
|| R
IN
= 50 Ω, or R
T
= 61.9 Ω.
06592-082
ADA4938
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 62. Adding Termination Resistor R
T
3. To compensate for the imbalance of the gain resistors, a correc-
tion 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
).
06592-083
R
S
50Ω
V
S
2V
R
T
61.9Ω
R
TH
27.4Ω
V
TH
1.1V
Figure 63. 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 amplifier circuit did
not affect the termination.
06592-084
ADA4938
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 64. Balancing Gain Resistor R
G
4. Finally, the feedback resistor is recalculated to adjust the
output voltage to the desired level.
a.
To make the output voltage V
O
= 1 V, R
F
is calculated
using
207
1.1
27.4)(2001
)(
=
⎟
⎠
⎞
⎜
⎝
⎛
+×
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×
=
TH
TS
GO
F
V
RRV
R
b.
To return the overall gain to 1 V/V (V
O
= V
S
= 2 V), R
F
should be
414
1.1
27.4)(2002
)(
=
⎟
⎠
⎞
⎜
⎝
⎛
+×
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×
=
TH
TS
GO
F
V
RRV
R
06592-085
ADA4938
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 65. Complete Single-Ended-to-Differential System
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
OCM
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 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
ADA4938-x devices share one reference output, it is recommended
that a buffer be used.