THS4130 Datasheet, PDF (20/27 Pages) Texas Instruments – HIGH SPEED LOW NOISE, FULLY DIFFERENTIAL I/O AMPLIFIERS

THS4130, THS4131
HIGHĆSPEED, LOW NOISE, FULLYĆDIFFERENTIAL I/O AMPLIFIERS
SLOS318E − MAY 2000 − REVISED JANUARY 2004
PRINCIPLES OF OPERATION
theory of operation (continued)
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 39 illustrates the increase in dynamic range. The
gain factor should be considered in this scenario. The THS413x 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.
VIN−
VIN+
VCC+
_
+
_
+
VO+
VO−
a
+1
0
+1
VOD= 1−0 = 1
VOCM
VCC−
0
b
VOD = 0−1 = −1
Figure 39. Fully Differential Amplifier With Two 1-V_PP Signals
Similar to the standard inverting amplifier configuration, input impedance of a fully differential amplifier is
selected by the input resistor, R_(g). If input impedance is a constraint in design, the designer may choose to
implement the differential amplifier as an instrumentation amplifier. This configuration improves the input
impedance of the fully differential amplifier. The following schematic depicts the general format of
instrumentation amplifiers.
The general transfer function for this circuit is:
ǒ Ǔ ^VOD
VIN1 – VIN2
+
Rf
R(g)
1
)
2R2
R1
VIN1
THS4012
+
R(g)
Rf
_
R2
_
R1
+
THS413x
R2
_
VIN2
+ _{THS4012 R(g)}
Rf
Figure 40. Instrumentation Amplifier
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