EVAL-AD7450ASDZ Analog Devices, EVAL-AD7450ASDZ Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7450ASDZ
• Description: Data Conversion IC Development Tools EVALUATION BOARD I.C.
• Manufacturer: Analog Devices
• Type: ADCr
• Series: AD7450Ar

Specifications of EVAL-AD7450ASDZ

• Rohs: yes
• Product: Evaluation Boards
• Tool Is For Evaluation Of: AD7450A
• Interface Type: Serial
• Operating Supply Voltage: 2.7 V to 5.25 V
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Factory Pack Quantity: 1

AD7440/AD7450A
DRIVING DIFFERENTIAL INPUTS
Differential operation requires V
simultaneously with two equal signals that are 180° out of
phase. The common mode must be set up externally and has a
range determined by V
amplifier used to drive the analog inputs (see Figure 28 and
Figure 29). Differential modes of operation with either an ac or
dc input provide the best THD performance over a wide
frequency range. Because 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
AD7440/AD7450A 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 34 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 a pair of series resistors to minimize the effects
REF
, the power supply, and the particular
IN+
*MOUNT AS CLOSE TO THE AD7440/AD7450A AS POSSIBLE
AND ENSURE HIGH PRECISION R
R
R
S
G
and V
–50Ω; C–1nF
1 = R
Figure 34. Using the AD8138 as a Single-Ended-to-Differential Amplifier
F
1 = R
+2.5V
–2.5V
IN–
GND
F
to be driven
2 = 499Ω; R
51Ω
G
2 = 523Ω
R
R
V
G
G
OCM
S
1
2
Rev. C | Page 18 of 28
AND C
AD8138
R
R
F
F
1
S
2
ARE USED.
R
R
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 (R
the source has a 50 Ω impedance and a 50 Ω termination, for
example, the value of R
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the same
gain (see Figure 34). 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 with a ±5 V supply.
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.
S
S
*
*
C*
C*
V
V
IN+
IN–
3.75V
1.25V
3.75V
1.25V
2.5V
AD7450A
2.5V
AD7440/
G
1, R
EXTERNAL
V
REF
V
G
REF
2, R
(2.5V)
G
2 should be increased by 25 Ω to
F
1, and R
F
2) should be the same. If