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THS4141CDGNR Datasheet, PDF (16/33 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS
◁
THS4140
THS4141
SLOS320F – MAY 2000 – REVISED JANUARY 2006
www.ti.com
PRINCIPLES OF OPERATION (continued)
The following schematics depict the differences between the operation of the THS414x, 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)
VCC+
Note: For proper operation, maintain
symmetry by setting
Rf1 = Rf2 = Rf and R(g)1 = R(g)2 = R(g)
⇒ A = Rf/R(g)
Vs
VIN+
R(g)
-+
VO+
+-
VO-
VOCM
VCC-
Rf
Figure 29. Amplifying Differential Signals
VIN- R(g)
VIN+
R(g)
Vs
Rf
VCC+
RECOMMENDED RESISTOR VALUES
-+
VO+
+-
VO-
VOCM
VCC-
GAIN
1
2
5
10
R(g) Ω
390
374
402
402
Rf Ω
390
750
2010
4020
Rf
Figure 30. 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
(10)
The second output is equal and opposite in sign:
VO
+
–
1
2
VI
(11)
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 will not be 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 31 illustrates the increase in dynamic
range. The gain factor should be considered in this scenario. The THS414x 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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