AD7452 Analog Devices, AD7452 Datasheet - Page 17

• Manufacturer Part Number: AD7452
• Description: Differential Input, 555 kSPS, 12-Bit A/D Converter in 8-Lead SOT-23
• Manufacturer: Analog Devices

Specifications of AD7452

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 555kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: SOT

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
of the ADC.
The circuit configuration shown in Figure 31 converts a
unipolar, single-ended signal into a differential signal.
GND
2 × V
Figure 30. Dual Op Amp Circuit to Convert a Single-Ended
REF
p-p
Bipolar Signal into a Differential Signal
390Ω
220Ω
20kΩ
A
220Ω
220Ω
220Ω
V+
V–
V+
V–
10kΩ
27Ω
27Ω
V
V
IN+
IN–
AD7452
V
DD
EXTERNAL
V
V
REF
REF
REF
level
0.1µF
Rev. B | Page 17 of 28
V
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.
GND
REF
Figure 32. Using an RF Transformer to Generate Differential Inputs
2 × V
Figure 31. Dual Op Amp Circuit to Convert a Single-Ended
REF
p-p
R
Unipolar Signal into a Differential Signal
390Ω
A
R
R
220Ω
220Ω
220Ω
10kΩ
V+
V–
V+
V–
C
27Ω
27Ω
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
V
V
IN+
IN–
AD7452
V
V
IN+
IN–
EXTERNAL
V
AD7452
REF
V
V
DD
REF
(2.5 V)
EXTERNAL
V
V
REF
REF
AD7452
0.1µF