Datasheet
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H
d
(f) +
ȧ
ȧ
ȡ
Ȣ
K
–
ǒ
f
FSF x fc
Ǔ
2
)
1
Q
jf
FSF x fc
) 1
ȧ
ȧ
ȣ
Ȥ
x
ȧ
ȡ
Ȣ
Rt
2R4 ) Rt
1 )
j2πfR4RtC3
2R4 ) Rt
ȧ
ȣ
Ȥ
Where K +
R2
R1
(3)
FSF x fc +
1
2π 2 x R2R3C1C2
Ǹ
and Q +
2 x R2R3C1C2
Ǹ
R3C1 ) R2C1 ) KR3C1
(4)
FSF + Re
2
)
|
Im
|
2
Ǹ
and Q +
Re
2
)
|
Im
|
2
Ǹ
2Re
(5)
FSF x fc +
1
2πRC 2 x mn
Ǹ
and Q +
2 x mn
Ǹ
1 ) m
(
1 ) K
)
(6)
PRINCIPLES OF OPERATION
THEORY OF OPERATION
R
f
R
(g)
R
(g)
R
f
_
+
Differential Amplifier
V
OCM
_
+
_
+
V
DD
V
IN−
V
IN+
V
O+
V
O−
THS412x
Fully Differential Amplifier
GND
Input voltage definition V
ID
+
ǒ
V
I)
Ǔ
–
ǒ
V
I–
Ǔ
V
IC
+
ǒ
V
I)
Ǔ
)
ǒ
V
I–
Ǔ
2
(7)
Output voltage definition V
OD
+
ǒ
V
O)
Ǔ
–
ǒ
V
O–
Ǔ
V
OC
+
ǒ
V
O)
Ǔ
)
ǒ
V
O–
Ǔ
2
(8)
Transfer function V
OD
+ V
ID
x A
ǒ
f
Ǔ
(9)
Output common−mode voltage V
OC
(10)
THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
APPLICATION INFORMATION (continued)
The transfer function for this filter circuit is:
K sets the pass-band gain, fc is the cutoff frequency for the filter, FSF is a frequency scaling factor, and Q is the
quality factor.
Where Re is the real part, and Im is the imaginary part of the complex pole pair. Setting R2 = R, R3 = mR,
C1 = C, and C2 = nC results in:
Start by determining the ratios, m and n, required for the gain and Q of the filter type being designed, then select
C and calculate R for the desired fc.
The THS412x is a fully differential amplifier. Differential amplifiers are typically differential in/single out, whereas
fully differential amplifiers are differential in/differential out.
Figure 23. Differential Amplifier Versus a Fully Differential Amplifier
To understand the THS412x fully differential amplifiers, the definition for the pinouts of the amplifier are
provided.
13
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