THS4151 Datasheet, PDF (18/35 Pages) National Semiconductor (TI) – HIGH-SPEED DIFFERENTIAL I/O AMPLIFIERS

THS4150
THS4151
SLOS321G – MAY 2000 – REVISED MARCH 2009........................................................................................................................................................... www.ti.com
If each output is measured independently, each output is one-half of the input signal when the gain is 1. The
following equations express the transfer function for each output:
VO₎
+
V_I)
2
)
VOCM
The second output is equal and opposite in sign:
VO–
+
–V_I)
2
)
VOCM
V_OCM will be set to midrails if it is not derived by any external power source.
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 40 illustrates the increase in dynamic range. The gain factor should be considered in this scenario. The
THS415x 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.
V_IN-
V_IN+
V_CC+
_
+
_
+
a
+1
V_O+
0
V_O-
+1
V_OD= 1-0 = 1
V_OCM
V_CC-
0
b
V_OD = 0-1 = -1
Figure 40. 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.
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