EVAL-AD7450ACB AD [Analog Devices], EVAL-AD7450ACB Datasheet - Page 19

• Manufacturer Part Number: EVAL-AD7450ACB
• Description: Differential Input, 1 MSPS 10-Bit and 12-Bit ADCs in an 8-Lead SOT-23
• Manufacturer: AD [Analog Devices]

V
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 V
of the ADC.
GND
REF
Figure 35. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
2
×
V
REF
p-p
390Ω
A
into a Differential Signal
220Ω
220Ω
220Ω
10kΩ
V+
V–
V+
V–
27Ω
27Ω
V
V
IN+
IN–
AD7450A
AD7440/
V
DD
EXTERNAL
V
V
REF
REF
REF
level
0.1µF
Rev. B | Page 19 of 28
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.
GND
Figure 36. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
2
Figure 37. Using an RF Transformer to Generate Differential Inputs
×
V
REF
p-p
R
390Ω
220Ω
20kΩ
A
a Differential Signal
R
R
220Ω
220Ω
220Ω
V+
V–
V+
V–
10kΩ
C
27Ω
27Ω
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
V
V
AD7440/AD7450A
IN+
IN–
AD7450A
AD7440/
V
V
IN+
IN–
EXTERNAL
AD7450A
AD7440/
V
V
V
DD
REF
REF
EXTERNAL
V
V
REF
REF
0.1µF