Datasheet

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
input to the ADA4940-1/ADA4940-2 with a gain of 1, R
F
= 1 kΩ
and R
G
= 1 kΩ. An example using an input source with a terminated
output voltage of 1 V p-p and source resistance of 50 Ω 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.
R
S
50Ω
V
S
2V p-p
R
IN, se
1.33kΩ
ADA4940-1
ADA4940-2
R
L
V
OUT, dm
+V
S
–V
S
R
G
1kΩ
R
G
1kΩ
R
F
1kΩ
R
F
1kΩ
V
OCM
08452-059
Figure 66. Calculating Single-Ended Input Impedance, R
IN
1. The input impedance is calculated by
Ωk.331
)10001000(2
1000
1
1000
)(2
1
,
=












+
×
−
=












+×
−
=
F
G
F
G
seIN
RR
R
R
R
2. To match the 50 Ω source resistance, calculate the
termination resistor, R
T
, using R
T
||1.33 kΩ = 50 Ω.
The closest standard 1% value for R
T
is 52.3 Ω.
ADA4940-1
ADA4940-2
R
L
V
OUT, dm
+V
S
–V
S
R
S
50Ω
R
G
1kΩ
R
G
1kΩ
R
F
1kΩ
R
F
1kΩ
V
OCM
V
S
2V p-p
R
IN, se
50Ω
R
T
52.3Ω
08452-060
Figure 67. Adding Termination Resistor R
T
3. Figure 67 shows that the effective R
G
in the upper feedback
loop is now greater than the R
G
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 (R
TS
)
in series with R
G
in the lower loop. R
TS
is the Thevenin
equivalent of the source resistance, R
S
, and the termination
resistance, R
T
, and is equal to R
S
||R
T
.
R
S
50Ω
V
S
2V p-p
R
T
52.3Ω
R
TH
25.5Ω
V
TH
1.02V p-p
08452-061
Figure 68. Calculating the Thevenin Equivalent
R
TS
= R
TH
= R
S
||R
T
= 25.5 Ω. Note that V
TH
is greater than
1 V p-p, which was obtained with R
T
= 50 Ω. The modified
circuit with the Thevenin equivalent (closest 1% value used for
R
TH
) of the terminated source and R
TS
in the lower feedback
loop is shown in Figure 69.
ADA4940-1
ADA4940-2
R
L
V
OUT, dm
+V
S
–V
S
R
TH
25.5Ω
R
G
1kΩ
R
G
1kΩ
R
F
1kΩ
R
F
1kΩ
V
OCM
V
TH
1.02V p-p
R
TS
25.5Ω
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 R
G
is increased in both loops,
lowering the overall closed-loop gain. The second is that V
TH
is a little larger than 1 V p-p, as it would be if R
T
= 50 Ω.
These two effects have opposite impacts on the output voltage,
and for large resistor values in the feedback loops (~1 kΩ), the
effects essentially cancel each other out. For small R
F
and R
G
,
or high gains, however, the diminished closed-loop gain is not
cancelled completely by the increased V
TH
. 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, R
F
, is required.
INPUT COMMON-MODE VOLTAGE RANGE
The ADA4940-1/ADA4940-2 input common-mode range is
shifted down by approximately 1 V
BE
, 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.