Datasheet

ADA4932-1/ADA4932-2 Data Sheet
Rev. B | Page 22 of 28
input terminals due to negative voltage feedback and is in phase
with the input signal, thus reducing the effective voltage across
R
G
in the upper loop and partially bootstrapping R
G
.
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
F
= 499 Ω, and R
G
=
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. The input impedance is calculated using the formula
Ω665
)499499(2
499
1
499
)(2
1
,
=












+×
−
=












+×
−
=
F
G
F
G
seIN
RR
R
R
R
R
S
50Ω
V
S
2V p-p
R
IN, se
665Ω
ADA4932-x
R
L
V
OUT, dm
+V
S
–V
S
R
G
499Ω
R
G
499Ω
R
F
499Ω
R
F
499Ω
V
OCM
07752-050
Figure 59. Calculating Single-Ended Input Impedance, R
IN
2. To match the 50 Ω source resistance, calculate the
termination resistor, R
T
, using R
T
||665 Ω = 50 Ω. The
closest standard 1% value for R
T
is 53.6 Ω.
ADA4932-x
R
L
V
OUT, dm
+V
S
–V
S
R
S
50Ω
R
G
499Ω
R
G
499Ω
R
F
499Ω
R
F
499Ω
V
OCM
V
S
2V p-p
R
IN, se
50Ω
R
T
53.6Ω
07752-051
Figure 60. Adding Termination Resistor, R
T
3. Figure 60 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
53.6Ω
R
TH
25.9Ω
V
TH
1.03V p-p
07752-052
Figure 61. Calculating the Thevenin Equivalent
R
TS
= R
TH
= R
S
||R
T
= 25.9 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 62.
ADA4932-x
R
L
V
OUT, dm
+V
S
–V
S
R
TH
25.5Ω
R
G
499Ω
R
G
499Ω
R
F
499Ω
R
F
499Ω
V
OCM
V
TH
1.03V p-p
R
TS
25.5
Ω
07752-053
Figure 62. Thevenin Equivalent and Matched Gain Resistors
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
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 canceled completely by the increased
V
TH
. This can be seen by evaluating Figure 62.
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
F
without modifying
any of the input circuitry. This is discussed in Step 4.