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參數(shù)資料
| 型號： | AD744 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Precision, 500 ns Settling BiFET Op Amp |
| 中文描述： | 精密，500納秒的建立BiFET運(yùn)算放大器 |
| 文件頁數(shù)： | 17/24頁 |
| 文件大?。?/td> | 914K |
| 代理商： | AD744 |
REV. PrF
PRELIMINARY TECHNICAL DATA
–17–
AD7450A/AD7440
GND
2 X VREF P-to-P
27
27
390
220
220
220
10K
EXTERNAL
VREF
..
VDD
V+
V+
V-
V-
VIN+
VIN-
VREF
AD7450A/
AD7440
220
20K
.
0.1μF
Figure 16(a). Dual Opamp Circuit to Convert a Single Ended
Bipolar Input into a Differential Input
GND
2 X VREF P-to-P
27
27
390
220
220
10K
EXTERNAL
VREF
VDD
V+
V+
V-
V-
VIN+
VIN-
VREF
AD7450A/
AD7440
220
.
.
A
VREF
0.1μF
Figure 16(b). Dual Opamp Circuit to Convert a Single Ended
Unipolar Input into a Differential Input
RF Transformer
In systems that do not need to be dc-coupled, an RF trans-
former with a center tap offers a good solution for
generating differential inputs. Figure 17 shows how a
transformer is used for single ended to differential conver-
sion. It provides the benefits of operating the ADC in the
differential mode without contributing additional noise
and distortion. An RF transformer also has the benefit of
providing electrical isolation between the signal source
and the ADC. A transformer can be used for most ac ap-
plications. The center tap is used to shift the differential
signal to the common mode level required, in this case it
is connected to the reference so the common mode level is
the value of the reference.
C
EXTERNAL
VREF (2.5V)
R
R
R
VREF
VIN+
AD7450/
AD7440
VIN-
3.75V
2.5V
1.25V
2.5V
1.25V
3.75V
Figure 17. Using an RF Transformer to Generate
Differential Inputs
REFERENCE SECTION
An external reference source is required to supply the
reference to the AD7450A/AD7440. This reference input
can range from 100 mV to 3.5 V. With a 5 V power sup-
ply, the specified reference is 2.5 V and 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 100mV.
It is important to ensure that, when chosing the reference
value for a particular application, the maximum analog
input range (VINmax) is never greater than V
DD
+ 0.3V
to comply with the maximum ratings of the device. The
following two examples calculate the maximum V
REF
in-
put that can be used when operating the AD7450A/
AD7440 at V
DD
of 5 V and 3.3 V respectively.
Example 1:
VINmax = V
DD
+ 0.3
VINmax = V
REF
+ V
REF
/2
If V
DD
= 5 V
then VINmax = 5.3 V
Therefore 3xV
REF
/2 = 5.3 V
V
REF
max = 3.5 V
Therefore, when operating at V
DD
= 5 V, the value of
V
REF
can range from 100mV to a maximum value of 3.5V.
When V
DD
= 4.75 V, V
REF
max = 3.17 V.
Example 2:
VINmax = V
DD
+ 0.3
VINmax = V
REF
+ V
REF
/2
If V
DD
= 3.3V
then VINmax = 3.6 V
Therefore 3xV
REF
/2 = 3.6 V
V
REF
max = 2.4 V
Therefore, when operating at V
DD
= 3.3 V, the value of
V
REF
can range from 100mV to a maximum value of 2.4V.
When V
DD
= 2.7 V, V
REF
max = 2 V.
These examples show that the maximum reference applied
to the AD7450A/AD7440 is directly dependant on the
value applied to V
DD
.
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