Datasheet

AD7356
Rev. A | Page 14 of 20
06505-033
GND
2 × V
REF
p-p
27Ω
27Ω
V+
V–
V+
V–
1.024V
2.048V
0V
1.024V
2.048V
0V
REF
A
/REF
B
V
IN+
AD7356*
V
IN–
220Ω
10µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
220Ω
220Ω
20kΩ
A
440Ω
10kΩ
220Ω
06505-031
+2.048V
GND
–2.048V
AD8138
R
G
1
R
S
*
R
S
*
R
G
2
R
F
2
V
OCM
R
F
1
2.048V
V
IN+
V
IN–
1.024V
0V
2.048V
1.024V
0V
AD7356
C
F
2
C
F
1
10kΩ
10kΩ
10µF
REF
A
/REF
B
*MOUNT AS CLOSE TO THE AD7356 AS POSSIBLE
AND ENSURE THAT HIGH PRECISION R
S
RESISTORS ARE USED.
R
S
– 33Ω; R
G
1 = R
F
1 = R
F
2 = 499Ω; C
F
1 = C
F
2 = 39pF;
R
G
2 = 523Ω
51Ω
Figure 22. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
a Differential Unipolar Signal
VOLTAGE REFERENCE
Figure 20. Using the AD8138 as a Single-Ended-to-Differential Amplifier
The AD7356 allows the choice of a very low temperature drift
internal voltage reference or an external reference. The internal
2.048 V reference of the AD7356 provides excellent perfor-
mance and can be used in almost all applications. When the
internal reference is used, the reference voltage is present on
the REF
A
and REF
B
pins. These pins should be decoupled to
REFGND with 10 F capacitors. The internal reference voltage
can be used elsewhere in the system, provided it is buffered
externally.
If the analog inputs source being used has zero impedance, all
four resistors (R
G
1, R
G
2, R
F
1, and R
F
2) should be the same value
as each other. If the source has a 50 Ω impedance and a 50 Ω
termination, for example, increase the value of R
G
2 by 25 Ω to
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the
same gain. The outputs of the amplifier are perfectly matched
balanced differential outputs of identical amplitude, and are
exactly 180° out of phase.
The REF
A
and REF
B
pins can also be overdriven with an
external voltage reference if desired. The applied reference
voltage can range from 2.048 V + 100 mV to V
DD
. A common
choice would be to use an external 2.5 V reference such as the
ADR441 or ADR431.
Op Amp Pair
An op amp pair can be used to directly couple a differential signal
to one of the analog input pairs of the AD7356. The circuit
configurations shown in Figure 21 and Figure 22 show how
an op amp pair can be used to convert a single-ended signal
into a differential signal for a bipolar and unipolar input signal,
respectively.
ADC TRANSFER FUNCTION
The output coding for the AD7356 is straight binary. The
designed code transitions occur at successive LSB values
(1 LSB, 2 LSBs, and so on). The LSB size is (2 × V
REF
)/4096.
The ideal transfer characteristic of the AD7356 is shown in
Figure 23.
The voltage applied to Point A sets up the common-mode
voltage. In both diagrams, it is connected in some way to the
reference. The AD8022 is a suitable dual op amp that could be
used in this configuration to provide differential drive to the
AD7356.
000 ... 000
000 ... 001
000 ... 010
111 ... 101
111 ... 110
111 ... 111
ADC CODE
ANALOG INPUT
–V
REF
+ 0.5 LSB
–V
REF
+ 1 LSB
+V
REF
– 1.5 LSB
+V
REF
– 1 LSB
06505-014
06505-032
GND
V
REF
p-p
27Ω
27Ω
V+
V–
V+
V–
1.024V
2.048V
0V
1.024V
2.048V
0V
REF
A
/REF
B
V
IN+
AD7356*
V
IN–
220Ω
10µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
220Ω
220Ω
10kΩ
10kΩ
A
220Ω
V
REF
2
Figure 21. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
into a Differential Signal
Figure 23. AD7356 Ideal Transfer Characteristic