ada4932 Analog Devices, Inc., ada4932 Datasheet - Page 22

• Manufacturer Part Number: ada4932
• Description: Low Power Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

ADA4932-1/ADA4932-2
input terminals due to negative voltage feedback and is in phase
with the input signal, thus reducing the effective voltage across
R
Terminating a Single-Ended Input
This section describes how to properly terminate a single-ended
input to the ADA4932-x with a gain of 1, R
499 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 four
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 59
indicates this open-circuit voltage.
1.
2.
2V p-p
G
in the upper loop and partially bootstrapping R
2V p-p
V
S
The input impedance is calculated using the formula
To match the 50 Ω source resistance, calculate the
termination resistor, R
closest standard 1% value for R
R
V
IN
S
Figure 59. Calculating Single-Ended Input Impedance, R
50Ω
,
R
se
S
=
50Ω
R






S
R
50Ω
1
IN, se
Figure 60. Adding Termination Resistor, R
−
R
665Ω
53.6Ω
IN, se
2
R
×
T
(
R
R
G
R
499Ω
V
499Ω
G
R
R
OCM
F
499Ω
499Ω
G
G
V
+
R
R
OCM
G
G
R
T
, using R
F
)






ADA4932-x
=
499Ω
499Ω
R
R
ADA4932-x






499Ω
499Ω
F
F
+V
1
–V
R
T
R
T
F
F
−
+V
–V
is 53.6 Ω.
S
S
||665 Ω = 50 Ω. The
2
S
S
×
(
F
499
= 499 Ω, and R
499
499
G
.
+
T
R
L
R
499
L
V
OUT, dm
IN
V
)
OUT, dm






=
G
Rev. 0 | Page 22 of 28
=
665
Ω
3.
1.03V p-p
Figure 60 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
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 62.
Figure 62 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 termi-
nated input. The first is that the value of R
both loops, lowering the overall closed-loop gain. The
second is that V
be if R
the output voltage, and for large resistor values in the feedback
loops (~1 kΩ), the effects essentially cancel each other out.
For small R
closed-loop gain is not canceled completely by the increased
V
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 (1.03 V p-p)(499/524.5) = 0.98 V p-p.
To obtain the desired output voltage of 1 V p-p, a final gain
adjustment can be made by increasing R
any of the input circuitry. This is discussed in Step 4.
TS
TH
TH
Figure 62. Thevenin Equivalent and Matched Gain Resistors
V
) of the terminated source and R
. This can be seen by evaluating Figure 62.
= R
TH
2V p-p
T
TH
Figure 61. Calculating the Thevenin Equivalent
= 50 Ω. These two effects have opposite impacts on
V
= R
25.5Ω
25.5Ω
S
R
F
R
TH
and R
T
TS
, and is equal to R
S
||R
50Ω
R
G
S
TH
499Ω
499Ω
V
in the lower loop. R
T
R
R
OCM
G
= 25.9 Ω. Note that V
G
G
is a little larger than 1 V p-p, as it would
, or high gains, however, the diminished
R
53.6Ω
T
ADA4932-x
499Ω
499Ω
R
R
F
F
+V
–V
G
1.03V p-p
S
S
S
in the lower loop due to the
||R
TS
T
V
G
.
T
TH
TS
in the lower feedback
in the upper feedback
S
= 50 Ω. The modified
, and the termination
F
is the Thevenin
without modifying
G
TH
25.9Ω
R
is increased in
TH
is greater than
R
L
V
OUT, dm
TS
)