Datasheet

AD7452
Rev. B | Page 17 of 28
Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to the AD7452. The circuit configurations shown in
Figure 30 and Figure 31 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 AD7452.
Care must be taken when choosing the op amp because the
selection depends on the required power supply and the system
performance objectives. The driver circuits in Figure 30 and
Figure 31 are optimized for dc coupling applications that
require optimum distortion performance.
The differential op amp driver circuit in Figure 30 is configured
to convert and level shift a single-ended, ground-referenced
(bipolar) signal to a differential signal centered at the V
REF
level
of the ADC.
The circuit configuration shown in Figure 31 converts a
unipolar, single-ended signal into a differential signal.
GND
2 × V
REF
p-p
EXTERNAL
V
REF
V
IN+
V
IN–
AD7452
V
REF
V
DD
V+
V–
V+
V–
220Ω
220Ω
27Ω
390Ω
220Ω
20kΩ
10kΩ
220Ω
27Ω
A
0.1µF
03154-A-030
Figure 30. Dual Op Amp Circuit to Convert a Single-Ended
Bipolar Signal into a Differential Signal
V
REF
2 × V
REF
p-p
EXTERNAL
V
REF
V
IN+
V
IN–
AD7452
V
REF
V
DD
V+
V–
V+
V–
220Ω
220Ω
27Ω
390Ω
10kΩ
220Ω
27Ω
GND
A
0.1µF
03154-A-031
Figure 31. Dual Op Amp Circuit to Convert a Single-Ended
Unipolar Signal into a Differential Signal
RF Transformer
In systems that do not need to be dc-coupled, an RF trans-
former with a center tap offers a good solution for generating
differential inputs. Figure 32 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
contributing 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.
EXTERNAL
V
REF
(2.5 V)
R
R
C
V
IN+
V
IN–
AD7452
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
V
REF
R
03154-A-032
Figure 32. Using an RF Transformer to Generate Differential Inputs