Datasheet

AD7265
Rev. A | Page 15 of 28
04674-021
V
REF
(V)
5.00 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
COMMON-MODE RANGE (V)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
T
A
= 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.
V
IN+
V
IN–
V
REF
p-p
V
REF
p-p
COMMON
MODE
VOLTAGE
1
ADDITIONAL PINS OMITTED FOR CLARITY.
04674-020
AD7265
1
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 V
IN+
and V
IN−
pins in each
differential pair (V
IN+
− V
IN−
). V
IN+
and V
IN−
should be
simultaneously driven by two signals each of amplitude V
REF
(or
2 × V
REF
, depending on the range chosen) that are 180° out of
phase. The amplitude of the differential signal is therefore
(assuming the 0 to V
REF
range is selected) −V
REF
to +V
REF
peak-
to-peak (2 × V
REF
), regardless of the common mode (CM).
The common mode is the average of the two signals
(V
IN+
+ V
IN−
)/2
and is therefore the voltage on which the two inputs are
centered.
This results in the span of each input being CM ± V
REF
/2. This
voltage has to be set up externally, and its range varies with the
reference value, V
REF
. As the value of V
REF
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.
Figure 24 and Figure 25 show how the common-mode range
typically varies with V
REF
for a 5 V power supply using the 0 to
V
REF
range or 2 × V
REF
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 −V
REF
to
+V
REF
corresponding to the digital codes of 0 to 4096. If the
2 × V
REF
range is used, then the input signal amplitude extends
from −2 V
REF
to +2 V
REF
after conversion.
REF REF DD
= 5 V)
04674-022
V
REF
(V)
2.50 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
T
A
= 25°C
Figure 25. Input Common-Mode Range vs. V
(2 × V Range, V
REF 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 V
REF
, 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.