THS4150 Datasheet, PDF (17/27 Pages) Texas Instruments – HIGH-SPEED DIFFERENTIAL I/O AMPLIFIERS

THS4150, THS4151
HIGHĆSPEED DIFFERENTIAL I/O AMPLIFIERS
SLOS321D − MAY 2000 − REVISED JANUARY 2004
APPLICATION INFORMATION
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. The following figure
presents a method by which the noise may be filtered in the THS415x.
R2
C1
R1
R3
VIN−
Vs
C2
VIN+
R1
R3
VIC
R2
VCC
+
−+
THS415x
+−
VOCM
VCC−
+
C1
R4
R(t)
R4
C3
VIN+
THS1050
VIN− _VOCM
C3
Figure 35. Antialias Filtering
The transfer function for this filter circuit is:
ȧȧȡȢ _{ǒ Ǔ} ȧȧȣȤ _{ȧȡȢ ȧȣȤ Hd(f) +}
K
–
f
FSF x fc
2
)
1
Q
jf
FSF x fc
)
1
x
Rt
2R4 ) Rt
1
)
j2πfR4RtC3
2R4 ) Rt
Where K +
R2
R1
FSF x fc
+
1
2π ^Ǹ2 x R2R3C1C2
and
Q
+
R3C1
Ǹ_{2 x R2R3C1C2}
) R2C1 ) KR3C1
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
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)
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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