Datasheet

ADA4930-1/ADA4930-2
Rev. A | Page 20 of 28
4.
Set R
F1
= R
F2
= R
F
to maintain a balanced system.
Compensate the imbalance caused by R
TH
. 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, R
F1
= R
F2
= 301 Ω, an input source with an open-
circuit output voltage of 2 V p-p, and a source resistance of 50 Ω.
Figure 46 shows this circuit.
•
Add R
TH
to R
G2
to maintain balanced gain resistances
and increase R
F1
and R
F2
to R
F
=
TH
S
V
V
Gain(R
G
+ R
TH
) to
maintain the system gain.
1.
Calculate the input impedance.
•
Decrease R
G2
to R
G2
=
GainV
VR
S
TH
F
×
×
to maintain system
gain and decrease R
G1
to (R
G2
− R
TH
) to maintain
balanced gain resistances.
β1 = β2 = 301/602 = 0.5 and R
IN
= 401.333 Ω
R
S
50
V
S
2V p-p
R
IN
401.333
ADA4930
R
L
V
OUT, dm
+V
S
–V
S
R
G1
301
R
G2
301
R
F2
301
R
F1
301
V
OCM
09209-053
The first compensation method is used in the Diff Amp
Calculator™ tool. Using the second compensation method,
R
G2
= 160.498 Ω and R
G1
= 160.498 − 26.66 = 133.837 .
The modified circuit is shown in Figure 49.
ADA4930
R
L
V
OUT, dm
+V
S
–V
S
R
TH
26.661
R
G1
133.837
R
G2
160.498
R
F2
301
R
F1
301
V
OCM
V
TH
1.066V p-p
09209-056
Figure 46. Single-Ended Input Impedance R
IN
2. Add a termination resistor, R
T
. To match the 50 Ω source
resistance, R
T
is added. Because R
T
||401.33 Ω = 50 Ω,
R
T
= 57.116 .
ADA4930
R
L
V
OUT, dm
+V
S
–V
S
R
S
50
R
G1
301
R
G2
301
R
F2
301
R
F1
301
V
OCM
V
S
2V p-p
R
IN
50
R
T
57.116
0
9209-054
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, R
G1
, changes the input impedance.
Repeat Step 1 through Step 4 several times using the modified
value of R
G1
from the previous iteration until the value of
R
T
does not change from the previous iteration. After three
additional iterations, the change in R
G1
is less than 0.1%.
The final circuit is shown in Figure 50 with the closest
0.5% resistor values.
Figure 47. Adding Termination Resistor R
T
3. Replace the source-termination resistor combination with
its Thevenin equivalent. The Thevenin equivalent of the
source resistance R
S
and the termination resistance R
T
is
R
TH
= R
S
||R
T
= 26.66 . TheThevenin equivalent of the
source voltage is
ADA4930
R
L
V
OUT, dm
1.990V p-p
+V
S
–V
S
R
S
50
R
G
142
V
P
V
N
R
G2
169
R
F1
301
R
F2
301
V
OCM
V
S
2V p-p
0.998V p-p
R
T
64.2
09209-057
V
TH
= V
S
T
S
T
RR
R
+
= 1.066 V p-p
R
S
50
V
S
2
V p-
p
R
T
57.116
R
TH
26.661
V
TH
1.066V p-p
0
9209-055
Figure 48. Thevenin Equivalent Circuit
Figure 50. Terminated Single-Ended-to-Differential System with G = 1