ada4930-1 Analog Devices, Inc., ada4930-1 Datasheet - Page 20

• Manufacturer Part Number: ada4930-1
• Description: Ultralow Noise Drivers For Low Voltage Adcs Ada4930-1/ada4930-2
• Manufacturer: Analog Devices, Inc.

ADA4930-1/ADA4930-2
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
circuit output voltage of 2 V p-p, and a source resistance of 50 Ω.
Figure 46 shows this circuit.
1.
2.
3.
2V p-p
2V p-p
Calculate the input impedance.
β1 = β2 = 301/602 = 0.5 and R
its Thevenin equivalent. The Thevenin equivalent of the
source resistance R
R
source voltage is
V
Add a termination resistor, R
resistance, R
R
Replace the source-termination resistor combination with
V
V
S
TH
TH
T
S
= 57.116 Ω.
= R
= V
50Ω
2V p-p
R
50Ω
S
R
S
S
S
||R
V
Figure 46. Single-Ended Input Impedance R
R
S
401.333Ω
50Ω
57.116Ω
Figure 47. Adding Termination Resistor R
R
S
T
IN
Figure 48. Thevenin Equivalent Circuit
R
R
+
T
= 26.66 Ω. TheThevenin equivalent of the
F1
IN
T
50Ω
R
is added. Because R
R
R
= R
S
T
T
301Ω
301Ω
V
F2
R
R
= 1.066 V p-p
S
OCM
G1
G2
= 301 Ω, an input source with an open-
301Ω
V
301Ω
and the termination resistance R
R
57.116Ω
R
R
OCM
T
G1
G2
301Ω
ADA4930
301Ω
R
R
T
301Ω
301Ω
1.066V p-p
. To match the 50 Ω source
ADA4930
IN
F1
F2
R
R
+V
–V
F1
F2
= 401.333 Ω
+V
–V
S
S
T
S
S
||401.33 Ω = 50 Ω,
V
TH
26.661Ω
R
TH
T
R
IN
R
L
L
V
V
OUT, dm
OUT, dm
T
is
Rev. A | Page 20 of 28
4.
5.
1.066V p-p
2V p-p
Figure 50. Terminated Single-Ended-to-Differential System with G = 1
•
•
Figure 49 presents an easily manageable circuit with matched
feedback loops that can be easily evaluated.
The modified gain resistor, R
Repeat Step 1 through Step 4 several times using the modified
value of R
R
additional iterations, the change in R
The final circuit is shown in Figure 50 with the closest
0.5% resistor values.
Set R
Compensate the imbalance caused by R
methods available to compensate, which follow:
The first compensation method is used in the
Calculator™ tool. Using the second compensation method,
R
The modified circuit is shown in Figure 49.
V
T
G2
S
V
Figure 49. Thevenin Equivalent with Matched Gain Resistors
does not change from the previous iteration. After three
TH
= 160.498 Ω and R
Add R
and increase R
maintain the system gain.
Decrease R
gain and decrease R
balanced gain resistances.
F1
50Ω
R
= R
S
26.661Ω
R
G1
F2
TH
TH
0.998V p-p
64.2Ω
from the previous iteration until the value of
= R
to R
R
160.498Ω
T
G2
133.837Ω
F
R
G2
V
to maintain a balanced system.
to R
G2
R
F1
OCM
G1
142Ω
169Ω
V
to maintain balanced gain resistances
R
and R
R
OCM
G2
G
G2
G1
=
G1
= 160.498 − 26.66 = 133.837 Ω.
F2
V
V
to (R
P
N
V
G1
R
to R
301Ω
301Ω
ADA4930
R
R
S
, changes the input impedance.
F
F1
F2
301Ω
301Ω
×
ADA4930
+V
–V
×
R
R
G2
F
F1
F2
Gain
V
S
S
+V
–V
=
− R
TH
S
S
V
V
G1
TH
TH
S
to maintain system
is less than 0.1%.
TH
) to maintain
Gain(R
. There are two
R
L
Diff Amp
R
L
G
V
OUT, dm
+ R
V
1.990V p-p
OUT, dm
TH
) to