Datasheet

ADA4939-1/ADA4939-2
Rev. 0 | Page 20 of 24
2. In order to match the 50 Ω source resistance, the termi-
nation resistor, R
T
, is calculated using R
T
||300 Ω = 50 Ω.
The closest standard 1% value for R
T
is 60.4 Ω.
ADA4939
R
L
V
OUT, dm
+V
S
–V
S
R
S
50Ω
R
G
200Ω
R
G
200Ω
R
F
400Ω
R
F
400Ω
V
OCM
V
S
2V p-p
R
IN
50Ω
R
T
60.4Ω
07429-054
Figure 47. Adding Termination Resistor R
T
3. It can be seen from Figure 47 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,
a correction resistor (R
TS
) is added in series with R
G
in the
lower loop. R
TS
is equal to 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
2
V p-
p
R
T
60.4Ω
R
TH
27.4Ω
V
TH
1.09V p-p
0
7429-055
Figure 48. Calculating the Thevenin Equivalent
R
TS
= R
TH
= R
S
||R
T
= 27.4 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 of the terminated source
and R
TS
in the lower feedback loop is shown in Figure 49.
ADA4939
R
L
V
OUT, dm
+V
S
–V
S
R
TH
27.4Ω
R
G
200Ω
R
G
200Ω
R
F
400Ω
R
F
400Ω
V
OCM
V
TH
1.09V p-p
R
TS
27.4Ω
07429-056
Figure 49. Thevenin Equivalent and Matched Gain Resistors
Figure 49 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
, however, the diminished closed-loop
gain is not canceled completely by the increased V
TH
. This
can be seen by evaluating
Figure 49.
The desired differential output in this example is 2 V p-p
because the terminated input signal was 1 V p-p and the
closed-loop gain = 2. The actual differential output voltage,
however, is equal to (1.09 V p-p)(400/227.4) = 1.92 V p-p.
To obtain the desired output voltage of 2 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.
4.
The feedback resistor value is modified as a final gain
adjustment to obtain the desired output voltage.
To make the output voltage V
OUT
= 2 V p-p, R
F
must be
calculated using the following formula:
()
()
()( )
Ω=
Ω
=
+
=
−
−
417
09.1
4.2272
,
PP
PP
TH
TS
G
dmOUT
F
V
V
V
RRVDesired
R
The closest standard 1 % values to 417 Ω are 412 Ω and
422 Ω. Choosing 422 Ω gives a differential output voltage
of 2.02 V p-p.
The final circuit is shown in
Figure 50.
ADA4939
R
L
V
OUT, dm
2.02V p-p
+V
S
–V
S
R
S
50Ω
R
G
200Ω
R
G
200Ω
R
F
422Ω
R
F
422Ω
V
OCM
V
S
2V p-p
1V p-p
R
T
60.4Ω
R
TS
27.4Ω
07429-057
Figure 50. Terminated Single-Ended-to-Differential System with G = 2