Datasheet

ADA4930-1/ADA4930-2
Rev. A | Page 22 of 28
Input Common-Mode Adjustment with Resistors
Calculate the following:
1.
β1 and β2. For the circuit shown in Figure 54, β1 = 0.5 and
β2 = 0.5.
The circuit shown in Figure 54 shows an alternate method to
bias the amplifier inputs, eliminating the dc source.
2.
R
CM
for V
P min
= 0.3 V and V
IN min
= −0.5 V. R
CM
= 9933 Ω.
ADA4930
R
L
V
OUT, dm
+V
S
–V
S
3.3V
3.3
V
R
S
50
R
G1
301
R
G2
301
R
F2
301
R
F1
301
R
CM
V
OCM
R
CM
V
S
V
S
V
SOURCE
2V p-p
R
T
09209-152
V
IN
3.
The new values for β1 and β2. β1 = 0.4925 and β2 = 0.4925.
4.
The input impedance using the following:
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
−+
+
=
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
−
=
−
β1β2
R
R
β2β1
β2β1
R
V
V
RR
G1
F1
G1
INP
P
G1SEIN
1
1
R
IN-SE
= 399.35 .
5.
R
T
, R
TH
, and V
TH
. R
T
= 57.16 , R
TH
= 26.67 , and
V
TH
= 1.067 V.
6.
The new values for R
G1
and R
G2
. R
G2
= 160.55  and
R
G1
= 133.88 .
7.
The new values for β1 and β2. β1 = 0.284 and β2 = 0.317.
Figure 54. Single-Supply Biasing Scheme with Resistors
8.
The new value of R
CM
. R
CM
= 4759.63 .
Define β1 = R
P
/R
F1
and β2 = R
N
/R
F2
, where R
P
= R
G1
||R
CM
||R
F1
and R
N
= R
G2
||R
CM
||R
F2
.
9.
Repeat Step 3 through Step 8 until the values of R
G1
and R
G2
remain constant between iterations. After four iterations,
the final circuit is shown in Figure 55.
Set R
F1
= R
F2
= R
F
to maintain a balanced system, as shown.
Write a nodal equation at V
P
and solve for V
P
.
ADA4930
R
L
V
OUT, dm
+V
S
–V
S
R
S
50
R
G1
142
R
G2
170
R
F2
301
R
F1
301
R
CM
1.87k
R
CM
1.87k
V
OCM
+
V
S
+V
S
V
S
2V p-p
R
T
65.1
09209-153
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
++
+
=
CM
F
SOCM
IN
G
F
P
R
R
VVV
R
R
β2β1
β1β2
V
2
2
1
Determine V
P min
. This is the minimum input common-mode
voltage from the Specifications section. For a 3.3 V supply,
V
P min
= 0.3 V.
Determine the minimum input voltage, V
IN min
at the output of
the source. Recognize that once properly terminated, the source
voltage is ½ of its open circuit value. Therefore, V
IN min
= −0.5 V.
Rearrange the V
P
equation for R
CM
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−−
+
=
OCM
minIN
G
F
P
F
SCM
VV
R
R
V
β1β2
β2β1
RVR
2
2
11
1
min
Figure 55. Single-Supply, Single-Ended Input System with Bias Resistors