AD7452BRTZ-REEL7 Ver la hoja de datos (PDF) - Analog Devices
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AD7452BRTZ-REEL7 Datasheet PDF : 24 Pages
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AD7452
DRIVING DIFFERENTIAL INPUTS
Differential operation requires that VIN+ and VIN– be simultane-
ously driven with two equal signals that are 180° out of phase.
The common mode must be set up externally and has a range
determined by VREF, the power supply, and the particular ampli-
fier used to drive the analog inputs (see Figure 23 and
Figure 24). Differential modes of operation with either an ac or
a dc input provide the best THD performance over a wide
frequency range. Since not all applications have a signal
preconditioned for differential operation, there is often a need
to perform single-ended-to-differential conversion.
Differential Amplifier
An ideal method of applying differential drive to the AD7452 is
to use a differential amplifier such as the AD8138. This part can
be used as a single-ended-to-differential amplifier or as a
differential-to-differential amplifier. In both cases, the analog
input needs to be bipolar. It also provides common-mode level
shifting and buffering of the bipolar input signal. Figure 29
shows how the AD8138 can be used as a single-ended-to-
differential amplifier. The positive and negative outputs of the
AD8138 are connected to the respective inputs on the ADC via
Data Sheet
a pair of series resistors to minimize the effects of switched
capacitance on the front end of the ADCs. The RC low-pass
filter on each analog input is recommended in ac applications to
remove high frequency components of the analog input. The
architecture of the AD8138 results in outputs that are very
highly balanced over a wide frequency range without requiring
tightly matched external components. If the analog input source
being used has zero impedance, all four resistors (RG1, RG2, RF1,
RF2) should be the same. If, for example, the source has a 50 Ω
impedance and a 50 Ω termination, the value of RG2 should be
increased by 25 Ω to balance this parallel impedance on the
input and thus ensure that both the positive and negative analog
inputs have the same gain (see Figure 29). The outputs of the
amplifier are perfectly matched, balanced differential outputs of
identical amplitude, and are exactly 180° out of phase.
The AD8138 is specified with +3 V, +5 V, and ±5 V power
supplies, but the best results are obtained when it is supplied by
±5 V. The AD8132 is a lower cost device that could also be used
in this configuration with slight differences in characteristics to
the AD8138 but with similar performance and operation.
+2.5V
GND
–2.5V
RF1
RS*
RG1
VOCM AD8138
C*
RG2
RS*
C*
RF2
3.75V
2.5V
1.25V
VIN–
AD7452
VIN+
VREF
3.75V
2.5V
1.25V
*MOUNT AS CLOSE TO THE AD7452 AS POSSIBLE
AND ENSURE HIGH PRECISION Rs AND Cs ARE USED.
EXTERNAL
VREF (2.5V)
RS = 50Ω; C = 1nF
RG1 = RF1 = RF2 = 499Ω; RG2 = 523Ω
Figure 29. Using the AD8138 as a Single-Ended-to-Differential Amplifier
Rev. C | Page 16 of 24
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