ADA4932-1/ADA4932-2
Data Sheet
with the input signal, thus reducing the effective voltage across
RG in the upper loop and partially bootstrapping RG.
Terminating a Single-Ended Input
This section describes how to properly terminate a single-ended
499 . An example using an input source with a terminated output
voltage of 1 V p-p and source resistance of 50 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 i
n Figure 59indicates this open-circuit voltage.
1. The input impedance is calculated using the formula
Ω
665
)
499
(
2
499
1
499
)
(
2
1
,
=
+
×
=
+
×
=
F
G
F
G
se
IN
R
RS
50
VS
2V p-p
RIN, se
665
ADA4932-x
RL VOUT, dm
+VS
–VS
RG
499
RG
499
RF
499
RF
499
VOCM
07752-
050
Figure 59. Calculating Single-Ended Input Impedance, RIN
2. To match the 50 source resistance, calculate the
termination resistor, RT, using RT||665 = 50 . The
closest standard 1% value for RT is 53.6 .
ADA4932-x
RL
VOUT, dm
+VS
–VS
RS
50
RG
499
RG
499
RF
499
RF
499
VOCM
VS
2V p-p
RIN, se
50
RT
53.6
07752-
051
Figure 60. Adding Termination Resistor, RT
3. Figure 60 shows that the effective RG in the upper feedback loop is now greater than the RG 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 (RTS)
in series with RG in the lower loop. RTS is the Thevenin
equivalent of the source resistance, RS, and the termination
resistance, RT, and is equal to RS||RT.
RS
50
VS
2V p-p
RT
53.6
RTH
25.9
VTH
1.03V p-p
07752-
052
Figure 61. Calculating the Thevenin Equivalent
RTS = RTH = RS||RT = 25.9 . Note that VTH is greater than
1 V p-p, which was obtained with RT = 50 . The modified
circuit with the Thevenin equivalent (closest 1% value used for
RTH) of the terminated source and RTS in the lower feedback
ADA4932-x
RL VOUT, dm
+VS
–VS
RTH
25.5
RG
499
RG
499
RF
499
RF
499
VOCM
VTH
1.03V p-p
RTS
25.5
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 RG is increased in
both loops, lowering the overall closed-loop gain. The
second is that VTH is a little larger than 1 V p-p, as it would
be if RT = 50 . These two effects have opposite impacts on
the output voltage, and for large resistor values in the feedback
loops (~1 k), the effects essentially cancel each other out.
For small RF and RG, or high gains, however, the diminished
closed-loop gain is not canceled completely by the increased
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 RF without modifying
any of the input circuitry. This is discussed in Step 4.
Rev. C | Page 22 of 28