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THS4120 Datasheet, PDF (15/23 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS
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PRINCIPLES OF OPERATION (continued)
THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
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+
R(g)
Vs
Rf
VDD
-+
+-
VOCM
GND
VO+
VO-
RECOMMENDED RESISTOR VALUES
GAIN R(g) Ω Rf Ω
1
150
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)
Fully differential amplifiers may be viewed as two inverting amplifiers. In this case, the equation of an inverting
amplifier holds true for gain calculations. One advantage of fully differential amplifiers is that they offer twice as
much dynamic range compared to single-ended amplifiers. For example, a 1-VPP ADC can only support an input
signal of 1 VPP. If the output of the amplifier is 2 VPP, then it is not practical to feed a 2-VPP signal into the
targeted ADC. Using a fully differential amplifier enables the user to break down the output into two 1-VPP signals
with opposite signs and feed them into the differential input nodes of the ADC. In practice, the designer has been
able to feed a 2-V peak-to-peak signal into a 1-V differential ADC with the help of a fully differential amplifier. The
final result indicates twice as much dynamic range. Figure 27 illustrates the increase in dynamic range. The gain
factor should be considered in this scenario. The THS412x fully differential amplifier offers an improved CMRR
and PSRR due to its symmetrical input and output. Furthermore, second harmonic distortion is improved. Second
harmonics tend to cancel because of the symmetrical output.
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