THS4131 Datasheet, PDF (16/37 Pages) National Semiconductor (TI) – HIGH-SPEED, LOW-NOISE, FULLY-DIFFERENTIAL I/O AMPLIFIERS

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.
R1
V_IN−
Vs
C2
V_IN+
R1
V_IC
R2
R3
R3
R2
C1
V_CC
+
−+
THS413x
+−
V_OCM
V_CC−
+
C1
R4
R_(t)
R4
C3
V_IN+
THS1050
V_IN−
V_OCM
C3
Figure 34. Antialias Filtering
The transfer function for this filter circuit is:
ȧȧȡȢ _{ǒ Ǔ} ȧȧȣȤ _{ȧȡȢ ȧȣȤ Hd(f) +}
K
x
–
f
FSF x fc
2
)
1
Q
jf
FSF x fc
)
1
Rt
2R4 ) Rt
1
)
j2πfR4RtC3
2R4 ) Rt
Where K +
R2
R1
(3)
FSF x fc
+
1
2π ^Ǹ2 x R2R3C1C2
and
Q
+
R3C1
Ǹ_{2 x R2R3C1C2}
) R2C1 ) KR3C1
(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.
Ǹ ^Ǹ _{FSF +}
Re² ) |Im|² and Q +
Re² ) |Im|²
2Re
(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:
FSF x fc
+
2πRC
1
Ǹ₂
x
mn
and Q
+
1
Ǹ_{2 x mn}
) m(1 ) K)
(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.
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