ADA4930-1/ADA4930-2
Rev. A | Page 20 of 28
4.
Set RF1 = RF2 = RF to maintain a balanced system.
Compensate the imbalance caused by RTH. There are two
methods available to compensate, which follow:
Terminating a Single-Ended Input
This section describes the five steps that properly terminate a
single-ended input to the ADA4930-1/ADA4930-2. Assume a
system gain of 1, RF1 = RF2 = 301 Ω, an input source with an open-
circuit output voltage of 2 V p-p, and a source resistance of 50 Ω.
Add RTH to RG2 to maintain balanced gain resistances
and increase RF1 and RF2 to RF =
TH
S
V
Gain(RG + RTH) to
maintain the system gain.
1.
Calculate the input impedance.
Decrease RG2 to RG2 =
Gain
V
R
S
TH
F
×
to maintain system
gain and decrease RG1 to (RG2 RTH) to maintain
balanced gain resistances.
β1 = β2 = 301/602 = 0.5 and RIN = 401.333 Ω
RS
50
VS
2V p-p
RIN
401.333
ADA4930
RL VOUT, dm
+VS
–VS
RG1
301
RG2
RF2
301
RF1
301
VOCM
09
20
9-
0
53
The first compensation method is used in the
Diff AmpCalculator tool. Using the second compensation method,
RG2 = 160.498 Ω and RG1 = 160.498 26.66 = 133.837 Ω.
ADA4930
RL VOUT, dm
+VS
–VS
RTH
26.661
RG1
133.837
RG2
160.498
RF2
301
RF1
301
VOCM
VTH
1.066V p-p
09
20
9-
05
6
Figure 46. Single-Ended Input Impedance RIN
2.
Add a termination resistor, RT. To match the 50 Ω source
resistance, RT is added. Because RT||401.33 Ω = 50 Ω,
RT = 57.116 Ω.
ADA4930
RL VOUT, dm
+VS
–VS
RS
50
RG1
301
RG2
301
RF2
301
RF1
301
VOCM
VS
2V p-p
RIN
50
RT
57.116
0
92
09
-05
4
Figure 49. Thevenin Equivalent with Matched Gain Resistors
Figure 49 presents an easily manageable circuit with matched
feedback loops that can be easily evaluated.
5.
The modified gain resistor, RG1, changes the input impedance.
Repeat Step 1 through Step 4 several times using the modified
value of RG1 from the previous iteration until the value of
RT does not change from the previous iteration. After three
additional iterations, the change in RG1 is less than 0.1%.
0.5% resistor values.
Figure 47. Adding Termination Resistor RT
3.
Replace the source-termination resistor combination with
its Thevenin equivalent. The Thevenin equivalent of the
source resistance RS and the termination resistance RT is
RTH = RS||RT = 26.66 Ω. TheThevenin equivalent of the
source voltage is
ADA4930
RL
VOUT, dm
1.990V p-p
+VS
–VS
RS
50
RG
142
VP
VN
RG2
169
RF1
301
RF2
301
VOCM
VS
2V p-p
0.998V p-p
RT
64.2
0
920
9-
0
57
VTH = VS
T
S
T
R
+
= 1.066 V p-p
RS
50
VS
2V p-p
RT
57.116
RTH
26.661
VTH
1.066V p-p
0
92
09
-05
5
Figure 48. Thevenin Equivalent Circuit
Figure 50. Terminated Single-Ended-to-Differential System with G = 1