EVAL-CONTROLBRD2 AD [Analog Devices], EVAL-CONTROLBRD2 Datasheet - Page 15

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

The AD8138 is specified with 3 V, 5 V and ±5 V power
supplies but the best results are obtained when it is supplied
by ±5 V. A lower cost device that could also be used in this
configuration with slight differences in characteristics to the
AD8138 but with similar performance and operation is the
AD8132.
Opamp Pair
An opamp pair can be used to directly couple a differential
signal to the AD7450.
in figures 15(a) and 15(b) show how a dual opamp can be
used to convert a single ended signal into a differential
signal for both a bipolar and a unipolar input signal re-
spectively.
The voltage applied to point A is the Common Mode
Voltage. In both diagrams, it is connected in some way to
the reference but any value in the common mode range can
be input here to setup the common mode. Examples of
suitable dual opamps that could be used in this configura-
tion to provide differential drive to the AD7450 are the
AD8042, AD8056 and the AD8022.
Care must be taken when chosing the opamp used, as the
selection will depend on the required power supply and the
system performance objectives. The driver circuits in fig-
ures 15(a) and 15(b) are optimized for dc coupling
applications requiring optimum distortion performance.
The differential op-amp driver circuit in figure 15(a) is
configured to convert and level shift a 2.5 V p-p single
ended, ground referenced (bipolar) signal to a 5 V p-p
differential signal centered at the V
The circuit configuration shown in figure 15(b) converts a
unipolar, single ended signal into a differential signal.
*Mount as close to the AD7450 as
Figure 14. Using the AD8138 as a Single Ended to Differen-
REV. PrJ
possible and ensure high
precision Rs and Cs are used
Rs - 10R; C - 1nF;
Rg1=Rf1=Rf2= 499R; Rg2 = 523R
+2.5V
GND
-2.5V
51R
tial Amplifier
The circuit configurations shown
Rg2
Rg1
V ocm
PRELIMINARY TECHNICAL DATA
AD8138
Rf2
Rf1
REF
Rs*
Rs*
level of the ADC.
C*
C*
V IN+
V IN-
3.75V
2.5V
1.25V
AD7450
3.75V
2.5V
1.25V
EXTERNAL
V REF (2.5V)
V REF
.
–15–
VREF/2
GND
Figure 15(a). Dual Opamp Circuit to Convert a Single Ended
Figure 15(b). Dual Opamp Circuit to Convert a Single Ended
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 16 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.
GND
V REF P-to-P
V REF P-to-P
Unipolar Input into a Differential Input
Bipolar Input into a Differential Input
390
20K
220
390
A
A
220
220
220
220
220
220
10K
10K
V+
V-
V+
V-
V-
V+
V-
V+
27
27
27
27
V IN+
V IN- V
V IN- V
V IN+
AD7450
AD7450
AD7450
EXTERNAL
V REF
EXTERNAL
V REF
V DD
V DD
REF
.
REF
.
.
0.1µF
0.1µF
.
.