EVAL-AD7450ACB データシートの表示（PDF） - Analog Devices
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EVAL-AD7450ACB Datasheet PDF : 28 Pages
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AD7440/AD7450A
Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to the AD7440/AD7450A. The circuit configurations
shown in Figure 35 and Figure 36 show how a dual op amp can
be used to convert a single-ended signal into a differential
signal for both a bipolar and unipolar input signal, respectively.
The voltage applied to Point A sets up the common-mode
voltage. In both diagrams, it is connected in some way to the
reference, but any value in the common-mode range can be
input here to set up the common mode. The AD8022 is a
suitable dual op amp that could be used in this configuration
to provide differential drive to the AD7440/AD7450A.
Take care when choosing the op amp; the selection depends on
the required power supply and system performance objectives.
The driver circuits in Figure 35 and Figure 36 are optimized for
dc coupling applications requiring best distortion performance.
The circuit configuration shown in Figure 35 converts a
unipolar, single-ended signal into a differential signal.
The differential op amp driver circuit in Figure 36 is configured
to convert and level shift a single-ended, ground-referenced
(bipolar) signal to a differential signal centered at the VREF level
of the ADC.
2 × VREF p-p
VREF
GND
220Ω
390Ω
V+
V–
220Ω
220Ω
V+
27Ω
VDD
VIN+ AD7440/
AD7450A
VIN–
VREF
A
27Ω
V–
0.1μF
10kΩ
EXTERNAL
VREF
Figure 35. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
into a Differential Signal
220Ω
2 × VREF p-p
GND
390Ω
V+
27Ω
VDD
220Ω
V–
220Ω
220Ω
V+
VIN+ AD7440/
AD7450A
VIN–
VREF
A
27Ω
V–
0.1μF
20kΩ
10kΩ
EXTERNAL
VREF
Figure 36. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
a Differential Signal
RF Transformer
An RF transformer with a center tap offers a good solution for
generating differential inputs in systems that do not need to
be dc-coupled. Figure 37 shows how a transformer is used for
single-ended-to-differential conversion. It provides the benefits
of operating the ADC in the differential mode without contri-
buting additional noise and distortion. An RF transformer also
has the benefit of providing electrical isolation between the
signal source and the ADC. A transformer can be used for most
ac applications. The center tap is used to shift the differential
signal to the common-mode level required; in this case, it is
connected to the reference so the common-mode level is the
value of the reference.
3.75V
2.5V
1.25V
R
R
VIN+
C
R
AD7440/
AD7450A
VIN–
VREF
3.75V
2.5V
1.25V
EXTERNAL
VREF
Figure 37. Using an RF Transformer to Generate Differential Inputs
DIGITAL INPUTS
The digital inputs applied to the device are not limited by the
maximum ratings, which limit the analog limits. Instead the
digital inputs applied, CS and SCLK, can go to 7 V and are not
restricted by the VDD + 0.3 V limits as on the analog input.
The main advantage of the inputs not being restricted to the
VDD + 0.3 V limit is that power supply sequencing issues are
avoided. If CS and SCLK are applied before VDD, there is no risk
of latch-up as there would be on the analog inputs if a signal
greater than 0.3 V was applied prior to VDD.
REFERENCE
An external reference source is required to supply the reference
to the device. This reference input can range from 100 mV to
3.5 V. With a 5 V power supply, the specified reference is 2.5 V
and the maximum reference is 3.5 V. With a 3 V power supply,
the specified reference is 2 V and the maximum reference is
2.2 V. In both cases, the reference is functional from 100 mV.
Ensure that, when choosing the reference value for a particular
application, the maximum analog input range (VIN max) is
never greater than VDD + 0.3 V to comply with the maximum
ratings of the device. The following two examples calculate the
maximum VREF input that can be used when operating the
AD7440/AD7450A at a VDD of 5 V and 3 V, respectively.
Rev. C | Page 19 of 28
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