AD7265
Rev. A | Page 15 of 28
04
67
4-
0
21
VREF (V)
5.0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
CO
M
O
N-
M
O
D
E
RANG
E
(
V
)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
TA = 25°C
Differential Mode
The AD7265 can have a total of six differential analog
input pairs.
Differential signals have some benefits over single-ended
signals, including noise immunity based on the device’s
common-mode rejection and improvements in distortion
performance.
Figure 23 defines the fully differential analog
input of the AD7265.
VIN+
VIN–
VREF p-p
COMMON
MODE
VOLTAGE
1ADDITIONAL PINS OMITTED FOR CLARITY.
04674-020
AD72651
Figure 24. Input Common-Mode Range vs. V
(0 to V
Range, V
Figure 23. Differential Input Definition
The amplitude of the differential signal is the difference
between the signals applied to the VIN+ and VIN pins in each
differential pair (VIN+ VIN). VIN+ and VIN should be
simultaneously driven by two signals each of amplitude VREF (or
2 × VREF, depending on the range chosen) that are 180° out of
phase. The amplitude of the differential signal is therefore
(assuming the 0 to VREF range is selected) VREF to +VREF peak-
to-peak (2 × VREF), regardless of the common mode (CM).
The common mode is the average of the two signals
(VIN+ + VIN)/2
and is therefore the voltage on which the two inputs are
centered.
This results in the span of each input being CM ± VREF/2. This
voltage has to be set up externally, and its range varies with the
reference value, VREF. As the value of VREF increases, the common-
mode range decreases. When driving the inputs with an amplifier,
the actual common-mode range is determined by the amplifier’s
output voltage swing.
typically varies with VREF for a 5 V power supply using the 0 to
VREF range or 2 × VREF range, respectively. The common mode
must be in this range to guarantee the functionality of the AD7265.
When a conversion takes place, the common mode is rejected,
resulting in a virtually noise-free signal of amplitude VREF to
+VREF corresponding to the digital codes of 0 to 4096. If the
2 × VREF range is used, then the input signal amplitude extends
from 2 VREF to +2 VREF after conversion.
REF
DD
= 5 V)
04674-022
VREF (V)
2.5
0
0.5
1.0
1.5
2.0
COMMON-MODE
RANGE
(V
)
5.0
4.0
4.5
3.0
3.5
2.0
2.5
0.5
1.0
1.5
0
TA = 25°C
Figure 25. Input Common-Mode Range vs. V
(2 × V
Range, V
REF
DD
= 5 V)
Driving Differential Inputs
Differential operation requires that V
and V
IN+
IN
be
simultaneously driven with two equal signals that are 180° out
of phase. The common mode must be set up externally. The
common-mode range is determined by VREF, the power supply,
and the particular amplifier used to drive the analog inputs.
Differential modes of operation with either an ac or dc input
provide the best THD performance over a wide frequency
range. Because not all applications have a signal preconditioned
for differential operation, there is often a need to perform
single-ended-to-differential conversion.