ADA4940-1 AD [Analog Devices], ADA4940-1 Datasheet - Page 24

• Manufacturer Part Number: ADA4940-1
• Description: Ultralow Power, Low Distortion
• Manufacturer: AD [Analog Devices]

ADA4940-1/ADA4940-2
Terminating a Single-Ended Input
This section describes how to properly terminate a single-ended
input to the
and R
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.
1.
2.
3.
2V p-p
2V p-p
V
S
The input impedance is calculated by
To match the 50 Ω source resistance, calculate the
termination resistor, R
The closest standard 1% value for R
Figure 67 shows that the effective R
loop is now greater than the R
addition of the termination resistors. To compensate for the
imbalance of the gain resistors, add a correction resistor (R
in series with R
equivalent of the source resistance, R
resistance, R
V
R
G
S
IN
= 1 kΩ. An example using an input source with a terminated
Figure 66. Calculating Single-Ended Input Impedance, R
,
50Ω
se
R
50Ω
=
S
R
ADA4940-1/ADA4940-2
S






1
R
50Ω
−
IN, se
1.33kΩ
R
Figure 67. Adding Termination Resistor R
IN, se
2
52.3Ω
T
×
, and is equal to R
(
R
R
R
G
T
G
R
G
in the lower loop. R
V
1kΩ
1kΩ
F
R
R
OCM
+
G
G
R
V
1kΩ
1kΩ
R
R
F
OCM
)
T
G
G
, using R






=






ADA4940-1
ADA4940-2
1
1kΩ
1kΩ
−
R
R
G
ADA4940-1
ADA4940-2
F
F
+V
–V
2
S
with a gain of 1, R
in the lower loop due to the
||R
1kΩ
1kΩ
T
×
R
S
R
S
||1.33 kΩ = 50 Ω.
(
F
F
+V
–V
T
1000
G
T
.
S
S
1000
TS
S
is 52.3 Ω.
in the upper feedback
, and the termination
1000
is the Thevenin
+
1000
T
R
L
)






F
V
R
=
= 1 kΩ
OUT, dm
IN
L
1
.33
V
OUT, dm
Rev. B | Page 24 of 32
TS
k
Ω
)
INPUT COMMON-MODE VOLTAGE RANGE
The
shifted down by approximately 1 V
drivers with centered input ranges, such as the ADA4939-x. 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.
1.02V p-p
ADA4940-1/ADA4940-2
R
1 V p-p, which was obtained with R
circuit with the Thevenin equivalent (closest 1% value used for
R
loop is shown in Figure 69.
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
lowering the overall closed-loop gain. The second is that V
is a little larger than 1 V p-p, as it would be if R
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
or high gains, however, the diminished closed-loop gain is not
cancelled completely by the increased V
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
TS
TH
V
Figure 69. Thevenin Equivalent and Matched Gain Resistors
TH
) of the terminated source and R
= R
2V p-p
TH
Figure 68. Calculating the Thevenin Equivalent
25.5Ω
V
25.5Ω
= R
R
S
R
TH
TS
S
||R
50Ω
R
S
V
1kΩ
1kΩ
R
R
T
OCM
G
G
= 25.5 Ω. Note that V
R
52.3Ω
T
F
, is required.
ADA4940-1
ADA4940-2
input common-mode range is
1kΩ
1kΩ
R
R
F
F
+V
–V
BE
1.02V p-p
S
S
, in contrast to other ADC
G
TS
is increased in both loops,
V
T
TH
in the lower feedback
= 50 Ω. The modified
TH
. This can be seen by
TH
25.5Ω
R
Data Sheet
TH
is greater than
R
L
T
V
= 50 Ω.
OUT, dm
F
and R
G
TH
,