EVAL-AD7450ACB AD [Analog Devices], EVAL-AD7450ACB Datasheet - Page 20

• Manufacturer Part Number: EVAL-AD7450ACB
• Description: Differential Input, 1 MSPS 10-Bit and 12-Bit ADCs in an 8-Lead SOT-23
• Manufacturer: AD [Analog Devices]

AD7440/AD7450A
DIGITAL INPUTS
The digital inputs applied to the device are not limited by the
maximum ratings, which limit the analog limits. Instead the
digital inputs applied, CS and SCLK, can go to 7 V and are not
restricted by the V
The main advantage of the inputs not being restricted to the
V
avoided. If CS and SCLK are applied before V
of latch-up as there would be on the analog inputs if a signal
greater than 0.3 V was applied prior to V
REFERENCE
An external reference source is required to supply the reference
to the device. This reference input can range from 100 mV to
3.5 V. With a 5 V power supply, the specified reference is 2.5 V
and the maximum reference is 3.5 V. With a 3 V power supply,
the specified reference is 2 V and the maximum reference is
2.2 V. In both cases, the reference is functional from 100 mV.
Ensure that, when choosing the reference value for a particular
application, the maximum analog input range (V
never greater than V
ratings of the device. The following two examples calculate the
maximum V
AD7440/AD7450A at a V
Example 1
If V
Therefore
3 × V
V
Thus, when operating at V
from 100 mV to a maximum value of 3.5 V. When V
V
Example 2
V
V
If V
Therefore,
3 × V
V
Thus, when operating at V
from 100 mV to a maximum value of 2.2 V. When V
V
DD
REF
REF
IN
IN
REF
REF
max = V
max = V
DD
DD
+ 0.3 V limit is that power supply sequencing issues are
max = 3.5 V
max = 2.2 V
max = 2 V.
max = 3.17 V.
REF
REF
= 3 V, then V
= 5 V, then V
/2 = 5.3 V
/2 = 3.3 V
DD
REF
REF
+ 0.3
+ V
input that can be used when operating the
DD
REF
IN
IN
V
DD
+ 0.3 V limits as on the analog input.
max = 3.3 V.
max = 5.3 V.
/2
V
IN
+ 0.3 V to comply with the maximum
IN
max = V
DD
max = V
DD
DD
of 5 V and 3 V, respectively.
= 5 V, the value of V
= 3 V, the value of V
REF
DD
+ V
+ 0.3
REF
DD
/2
.
DD
, there is no risk
IN
REF
REF
max) is
DD
DD
can range
can range
= 4.75 V,
= 2.7 V,
Rev. B | Page 20 of 28
These examples show that the maximum reference applied to
the AD7440/AD7450A is directly dependent on the value
applied to V
The value of the reference sets the analog input span and the
common-mode voltage range. Errors in the reference source
result in gain errors in the AD7440/AD7450A transfer function
and add to specified full-scale errors on the part. A 0.1 µF
capacitor should be used to decouple the V
Figure 38 shows a typical connection diagram for the V
Table 6 lists examples of suitable voltage references.
Table 6. Examples of Suitable Voltage References
Reference
AD780
ADR421
ADR420
SINGLE-ENDED OPERATION
When supplied with a 5 V power supply, the AD7440/AD7450A
can handle a single-ended input. The design of these devices is
optimized for differential operation, so with a single-ended
input, performance degrades. Linearity degrades by typically
0.2 LSB, the full-scale errors degrade typically by 1 LSB, and ac
performance is not guaranteed.
To operate the AD7440/AD7450A in single-ended mode, the
V
biased to the appropriate voltage corresponding to the midscale
code transition. This voltage is the common mode, which is a
fixed dc voltage (usually the reference). The V
around this value and should have a voltage span of 2 × V
make use of the full dynamic range of the part. The input signal
therefore has peak-to-peak values of common mode ±V
the analog input is unipolar, an op amp in a noninverting unity
gain configuration can be used to drive the V
operates from a single supply, so it is necessary to level shift
ground-based bipolar signals to comply with the input
requirements. An op amp can be configured to rescale and level
shift the ground-based bipolar signal, so it is compatible with
the selected input range of the AD7440/AD7450A (Figure 39).
V
*ADDITIONAL PINS OMITTED FOR CLARITY
DD
0.1µF
IN+
input is coupled to the signal source, while the V
Figure 38. Typical V
10nF
DD
.
Output
Voltage (V)
2.5/3
2.5
2.048
0.1µF
NC
REF
NC = NO CONNECT
1
2
3
4
Connection Diagram for V
V
TEMP
GND
IN
AD780
Initial
Accuracy (%)
0.04
0.04
0.05
OPSEL
TRIM
V
OUT
8
7
6
5
2.5V
REF
NC
NC
NC
IN+
IN+
pin to GND.
pin. The ADC
input swings
DD
Operating
Current (µA)
1000
500
500
AD7450A*
AD7440/
= 5 V
IN–
0.1µF
REF
input is
REF
REF
pin.
V
. If
V
REF
DD
to