Datasheet
AD7440/AD7450A
Rev. C | Page 16 of 28
TYPICAL CONNECTION DIAGRAM
Figure 26 shows a typical connection diagram for the
AD7440/AD7450A for both 5 V and 3 V supplies. In this setup,
the GND pin is connected to the analog ground plane of the
system. The V
REF
pin is connected to either a 2.5 V or a 2 V
decoupled reference source, depending on the power supply, to
set up the analog input range. The common-mode voltage has
to be set up externally and is the value on which the two inputs
are centered. The conversion result is output in a 16-bit word
with four leading zeros followed by the MSB of the 12-bit or
10-bit result. The 10-bit result of the AD7440 is followed by two
trailing zeros. For more details on driving the differential inputs
and setting up the common mode, refer to the Driving
Differential Inputs section.
03051-A-026
AD7440/
AD7450A
0.1
μ
F
0.1
μ
F
10
μ
F
V
REF
V
DD
V
IN+
SCLK
3V/5V
SUPPLY
SERIAL
INTERFACE
μ
C/
μ
PSDATA
CS
GND
V
IN–
2V/2.5V
V
REF
*CM IS THE COMMON-MODE VOLTAGE.
CM*
V
REF
p-p
CM*
V
REF
p-p
Figure 26. Typical Connection Diagram
ANALOG INPUT
The analog input of the AD7440/AD7450A is fully differential.
Differential signals have a number of benefits over single-
ended signals, including noise immunity based on the device’s
common-mode rejection, improvements in distortion perfor-
mance, doubling of the device’s available dynamic range, and
flexibility in input ranges and bias points.
Figure 27 defines the
fully differential analog input of the AD7440/AD7450A.
03051-A-027
V
REF
p-p
V
IN+
V
IN–
V
REF
p-p
AD7440/
AD7450A
COMMON-MODE
VOLTAGE
Figure 27. Differential Input Definitions
The amplitude of the differential signal is the difference
between the signals applied to the V
IN+
and V
IN–
pins
(i.e., V
IN+
– V
IN–
). V
IN+
and V
IN–
are simultaneously driven by
two signals each of amplitude V
REF
that are 180° out of phase.
The amplitude of the differential signal is therefore –V
REF
to
+V
REF
peak-to-peak (2 × V
REF
). This is true regardless of the
common mode (CM).
The common mode is the average of the two signals, that is,
(V
IN+
+ V
IN–
)/2 and is therefore the voltage that the two inputs
are centered on. 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 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 28 and Figure 29 show how the common-mode range
typically varies with V
REF
for both a 5 V and a 3 V power supply.
The common mode must be in this range to guarantee the
functionality of the AD7440/AD7450A.
For ease of use, the common mode can be set up to equal V
REF
,
resulting in the differential signal being ±V
REF
centered on V
REF
.
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 in the
case of the AD7450A and 0 to 1024 in the AD7440.
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
03051-A-028
V
REF
(V)
COMMON-MODE VOLTAGE (V)
1.75V
3.25V
COMMON-MODE RANGE
Figure 28. Input Common-Mode Range vs. V
REF
(V
DD
= 5 V and V
REF
(Max) = 3.5 V)
2.5
0.5
1.0
1.5
2.0
0
0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
03051-A-029
V
REF
(V)
COMMON-MODE VOLTAGE (V)
1V
2V
COMMON-MODE RANGE
Figure 29. Input Common-Mode Range vs. V
REF
(V
DD
= 3 V and V
REF
(Max) =2V)