Datasheet

V
IN
−
V
IN
+
+
−
+
−
V
OCM
V
OCM
V
IN
−
V
IN
+
V
CC
−
THS1050
THS413x
C3
C3
R4
R
(t)
R2
R4
+
C1
+
V
CC
C1
R2
R3
R3
C2
R1
R1
Vs
V
IC
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
FSF x fc +
1
2π 2 x R2R3C1C2
Ǹ
and Q +
2 x R2R3C1C2
Ǹ
R3C1 ) R2C1 ) KR3C1
FSF + Re
2
)
|
Im
|
2
Ǹ
and Q +
Re
2
)
|
Im
|
2
Ǹ
2Re
FSF x fc +
1
2πRC 2 x mn
Ǹ
and Q +
2 x mn
Ǹ
1 ) m
(
1 ) K
)
THS4130
THS4131
SLOS318H –MAY 2000–REVISED MAY 2011 www.ti.com
ACTIVE ANTIALIAS FILTERING
For signal conditioning in ADC applications, it is important to limit the input frequency to the ADC. Low-pass
filters can prevent the aliasing of the high-frequency noise with the frequency of operation. Figure 34 presents a
method by which the noise may be filtered in the THS413x.
Figure 34. Antialias Filtering
The transfer function for this filter circuit is:
(3)
(4)
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.
(5)
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:
(6)
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.
16 Copyright © 2000–2011, Texas Instruments Incorporated