THS4120 Datasheet, PDF (14/23 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
PRINCIPLES OF OPERATION
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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 + VOCM
(10)
Differential Structure Rejects
Coupled Noise at the Input
V_DD
Differential Structure Rejects
Coupled Noise at the Output
V_IN-
_
+
V_O+
V_IN+
_
+
V_O-
Differential Structure Rejects
Coupled Noise at the Power Supply
V_OCM
GND
Figure 24. Definition of the Fully Differential Amplifier
The following schematics depict the differences between the operation of the THS412x, fully differential amplifier,
in two different modes. Fully differential amplifiers can work with differential input or can be implemented as
single in/differential out.
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