ada4950-1 Analog Devices, Inc., ada4950-1 Datasheet - Page 21

• Manufacturer Part Number: ada4950-1
• Description: Low Power, Selectable Gain Differential Adc Driver, G = 1, 2, 3
• Manufacturer: Analog Devices, Inc.

Terminating a Single-Ended Input
This section describes how to properly terminate a single-ended
input to the ADA4950-x with a gain of 1, R
500 Ω. An example using an input source with a terminated output
voltage of 1 V p-p and source resistance of 50 Ω illustrates the
steps that must be followed. Note that 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 56
indicates this open-circuit voltage.
1.
2.
2V p-p
2V p-p
V
S
The input impedance is calculated using the following
formula:
To match the 50 Ω source resistance, calculate the
termination resistor, R
closest standard 1% value for R
R
V
IN
S
,
Figure 56. Calculating Single-Ended Input Impedance, R
se
50Ω
R
=
S
50Ω
R
⎛
⎜
⎜
⎜
⎜
⎝
S
1
R
50Ω
IN, se
Figure 57. Adding Termination Resistor, R
−
R
667Ω
2
53.6Ω
IN, se
×
R
(
T
R
R
G
R
G
500Ω
V
500Ω
F
R
R
OCM
+
G
G
500Ω
500Ω
V
R
R
R
OCM
T
G
G
F
, using R
)
⎞
⎟
⎟
⎟
⎟
⎠
=
ADA4950-x
500Ω
500Ω
⎛
⎜
⎜
⎜
⎜
⎝
R
R
1
ADA4950-x
F
F
500Ω
500Ω
+V
–V
T
−
R
R
T
is 53.6 Ω.
S
S
||667 Ω = 50 Ω. The
F
F
+V
–V
2
×
S
S
(
F
500
= 500 Ω, and R
500
500
+
T
R
L
500
R
V
L
OUT, dm
IN
)
V
⎞
⎟
⎟
⎟
⎟
⎠
OUT, dm
=
G
Rev. 0 | Page 21 of 28
667
=
Ω
3.
Figure 59 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
a little larger than 1 V p-p, as it would be if R
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
gains, however, the diminished closed-loop gain is not canceled
completely by the increased V
Figure 59.
The desired differential output in this example is 1 V p-p
because the terminated input signal is 1 V p-p and the closed-
loop gain = 1. The actual differential output voltage, however, is
equal to (1.03 V p-p)(500/525.5) = 0.98 V p-p.
1.03V p-p
Figure 57 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
R
Note that V
with R
equivalent (closest 1% value used for R
source and R
Figure 59.
TS
Figure 59. Thevenin Equivalent and Matched Gain Resistors
V
= R
TH
2V p-p
T
TH
Figure 58. Calculating the Thevenin Equivalent
= 50 Ω. The modified circuit with the Thevenin
V
= R
25.5Ω
25.5Ω
S
R
TH
R
TH
T
TS
TS
S
, and is equal to R
||R
is greater than 1 V p-p, which was obtained
50Ω
R
in the lower feedback loop is shown in
G
S
T
500Ω
500Ω
in the lower loop. R
V
R
R
OCM
= 25.9 Ω
G
G
R
53.6Ω
T
ADA4950-1/ADA4950-2
TH
ADA4950-x
500Ω
500Ω
. This can be seen by evaluating
R
R
F
F
+V
–V
G
1.03V p-p
S
G
in the lower loop due to the
S
S
||R
is increased in both loops,
T
V
G
.
TH
TS
S
in the upper feedback
, and the termination
TH
T
is the Thevenin
) of the terminated
F
= 50 Ω. These two
25.9Ω
R
and R
TH
R
L
V
G
OUT, dm
, or high
TH
TS
is
)