THS4140 Datasheet, PDF (16/24 Pages) Texas Instruments – HIGH SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

THS4140, THS4141
HIGHĆSPEED FULLY DIFFERENTIAL I/O AMPLIFIERS
SLOS320E − MAY 2000 − REVISED JANUARY 2004
PRINCIPLES OF OPERATION
theory 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)
−+
+−
VOCM
VCC−
Rf
VO+
VO−
Figure 29. Amplifying Differential Signals
VIN− ^R(g)
VIN+
Vs
R(g)
Rf
VCC+
−+
+−
VOCM
VCC−
VO+
VO−
RECOMMENDED RESISTOR VALUES
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
The second output is equal and opposite in sign:
VO
+
–
1
2
VI
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-V_PP ADC can only support an input
signal of 1 V_PP. If the output of the amplifier is 2 V_PP, then it will not be practical to feed a 2-V_PP signal into the
targeted ADC. Using a fully differential amplifier enables the user to break down the output into two 1-V_PP 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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