EVAL-CONTROLBRD2 AD [Analog Devices], EVAL-CONTROLBRD2 Datasheet - Page 12

• Manufacturer Part Number: EVAL-CONTROLBRD2
• Description: Differential Input, 1MSPS, 12-Bit ADC in ?SO-8 and S0-8
• Manufacturer: AD [Analog Devices]

TYPICAL CONNECTION DIAGRAM
Figure 6 shows a typical connection diagram for the
AD7450 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
1.25 V decoupled reference source depending on the
power supply, to set up the analog input range. The com-
mon mode voltage has to be set up externally and is the
value that the two inputs are centered on. For more details
on driving the differential inputs and setting up the com-
mon mode, see the ‘Driving Differential Inputs’ section.
The conversion result for the ADC is output in a 16-bit
word consisting of four leading zeros followed by the
MSB of the 12-bit result. For applications where power
consumption is of concern, the power-down mode should
be used between conversions or bursts of several conver-
sions to improve power performance. See ‘Modes of
Operation’ section.
AD7450
V REF
P-to-P
V REF
P-to-P
* CM - COMMON MODE VOLTAGE
000...000
011...110
111...111
011...111
000...001
100...010
100...001
100...000
Figure 5. AD7450 Ideal Transfer Characteristic
Figure 6. Typical Connection Diagram
-V REF + 1LSB
CM*
CM*
REF
V DD
V REF
1LSB = 2xV REF /4096
V IN-
pin is connected to either a 2.5 V or a
V IN+
0.1µF
0.1µF
ANALOG INPUT
AD7450
1.25V/2.5V
(V IN+ - V IN- )
VREF
0LSB
PRELIMINARY TECHNICAL DATA
10µF
SDATA
+V REF - 1LSB
SCLK
+3V/+5V
SUPPLY
GND
SERIAL
INTERFACE
µC/µP
–12–
THE ANALOG INPUT
The analog input of the AD7450 is fully differential.
ferential signals have a number of benefits over single
ended signals including noise immunity based on the
device’s common mode rejection, improvements in distor-
tion performance, doubling of the device’s available
dynamic range and flexibility in input ranges and bias
points.
Figure 7 defines the fully differential analog input of the
AD7450.
The amplitude of the differential signal is the difference
between the signals applied to the V
V
two signals each of amplitude V
phase. The amplitude of the differential signal is therefore
-V
gardless of the common mode (CM). The common mode
is the average of the two signals, i.e. (V
is therefore the voltage that the two inputs are centered on.
This results in the span of each input being CM ± V
This voltage has to be set up externally and its range var-
ies with V
common mode range decreases.
with an amplfier, the actual common mode range will be
determined by the amplifier’s output voltage swing.
Figure 8 shows how the common mode range varies with
V
ample of the common mode range when using the
AD8138 differential amplifer to drive the analog inputs.
The common mode must be in this range to guarantee the
specifications.
Mode range is TBD.
For ease of use, the common mode can be set up to be
equal to V
±V
the common mode is rejected resulting in a virtually noise
free signal of amplitude -V
he digital codes of 0 to 4095.
IN+
REF
REF
REF
COMMON
VOLTAGE
- V
for a 5 V power supply and figure 9 shows an ex-
to +V
MODE
centered on V
IN-
REF
).
REF
Figure 7. Differential Input Definition
REF
. As the value of V
, resulting in the differential signal being
With a 3V power supply, the Common
V
peak-to-peak (i.e. 2 x V
IN+
and V
REF
. When a conversion takes place,
P-to-P
P-to-P
V REF
V REF
IN-
REF
are simultaneously driven by
to +V
REF
REF
When driving the inputs
that are 180° out of
IN+
increases, the
REF
IN+
and V
REF
V IN-
V IN+
corresponding to
). This is re-
+ V
AD7450
IN-
IN-
REV. PrJ
pins (i.e.
)/2 and
REF
Dif-
/2.