THS4121CD Datasheet, PDF (13/30 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

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THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
APPLICATION INFORMATION (continued)
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.
PRINCIPLES OF OPERATION
THEORY OF OPERATION
The THS412x is a fully differential amplifier. Differential amplifiers are typically differential in/single out, whereas
fully differential amplifiers are differential in/differential out.
Differential Amplifier
R_f
R_(g)
_
+
R_(g)
R_f
THS412x
Fully Differential Amplifier
V_DD
V_IN−
V_IN+
_
+
_
+
V_O+
V_O−
V_OCM
GND
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.
ǒ Ǔ ǒ Ǔ _{Input voltage definition VID + VI) – VI–}
VIC
+
ǒ_VI)Ǔ
)
2
ǒ_VI–Ǔ
(7)
ǒ Ǔ ǒ Ǔ Output voltage definition VOD + VO₎ – VO–
ǒ Ǔ ǒ Ǔ _{VOC +}
VO₎
)
2
VO–
(8)
Transfer function
VOD + VID x A_ǒfǓ
(9)
Output common−mode voltage VOC
(10)
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