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THS4121IDGN Datasheet, PDF (14/30 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS
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THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
PRINCIPLES OF OPERATION (continued)
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Differential Structure Rejects
Coupled Noise at the Input
VDD
VIN-
VIN+
_
+
_
+
Differential Structure Rejects
Coupled Noise at the Output
VO+
VO-
Differential Structure Rejects
Coupled Noise at the Power Supply
VOCM
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.
Rf
VIN-
R(g)
VDD
Vs
VIN+
-+
VO+
+-
VO-
Note: For proper operation, maintain
symmetry by setting
Rf1 = Rf2 = Rf and R(g)1 = R(g)2 = R(g)
⇒ A = Rf/R(g)
R(g)
VOCM
GND
Rf
Figure 25. Amplifying Differential Signals
VIN- R(g)
VIN+
Vs
R(g)
Rf
VDD
-+
+-
VOCM
GND
VO+
VO-
RECOMMENDED RESISTOR VALUES
GAIN
1
R(g) Ω
150
Rf Ω
150
Rf
Figure 26. Single In With Differential Out
If each output is measured independently, each output is one-half of the input signal when gain is 1. The
following equations express the transfer function for each output:
VO
+
1
2
VI
(11)
The second output is equal and opposite in sign:
VO
+
–
1
2
VI
(12)
14
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