AD7266
Rev. B | Page 16 of 28
Using an Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to one of the analog input pairs of the AD7266. The
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 can be used in this configuration to
provide differential drive to the AD7266.
Take care when choosing the op amp; the selection depends on
the required power supply and system performance objectives.
dc coupling applications requiring best distortion performance.
The circuit configuration shown in
Figure 26 converts a
unipolar, single-ended signal into a differential signal.
The differential op amp driver circuit shown in
Figure 27 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.
GND
2 × VREF p-p
27
V+
V–
V+
V–
VREF
2.5V
3.75V
1.25V
2.5V
3.75V
1.25V
DCAPA/DCAPB
VIN+
AD72661
VIN–
440
220
0.47F
1ADDITIONAL PINS OMITTED FOR CLARITY.
220
10k
A
04603
-023
Figure 26. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
into a Differential Signal
20k
220k
2 × VREF p-p
27
V+
V–
V+
V–
GND
2.5V
3.75V
1.25V
2.5V
3.75V
1.25V
DCAPA/DCAPB
VIN+
AD72661
VIN–
440
220
0.47F
1ADDITIONAL PINS OMITTED FOR CLARITY.
220
10k
A
046
03-
024
Figure 27. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal
into a Differential Unipolar Signal
Pseudo Differential Mode
The AD7266 can have a total of six pseudo differential pairs. In
this mode, VIN+ is connected to the signal source that must have
an amplitude of VREF (or 2 × VREF, depending on the range
chosen) to make use of the full dynamic range of the part. A dc
input is applied to the VIN pin. The voltage applied to this input
provides an offset from ground or a pseudo ground for the VIN+
input. The benefit of pseudo differential inputs is that they
separate the analog input signal ground from the ADC’s ground
allowing dc common-mode voltages to be cancelled. The typical
voltage range for the VIN pin, while in pseudo differential
connection diagram for pseudo differential mode.
04603-043
VREF (V)
3.0
0
0.5
1.0
1.5
2.0
2.5
V
IN–
(V
)
1.0
0.8
0.4
0.6
0.2
–0.2
0
–0.4
TA = 25°C
Figure 28. VIN- Input Voltage Range vs. VREF in
Pseudo Differential Mode with VDD = 3 V
0
460
3-
04
4
VREF (V)
5.0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
V
IN
–(V
)
2.5
2.0
1.5
1.0
0.5
0
–0.5
TA = 25°C
Figure 29. VIN Input Voltage Range vs. VREF in
Pseudo Differential Mode with VDD = 5 V
DC INPUT
VOLTAGE
VREF
p–p
VREF
VIN+
AD72661
VIN–
0.47F
1ADDITIONAL PINS OMITTED FOR CLARITY.
04
60
3-
0
25
Figure 30. Pseudo Differential Mode Connection Diagram