AD7356 Analog Devices, AD7356 Datasheet - Page 14

• Manufacturer Part Number: AD7356
• Description: Differential Input, Dual, Simultaneous Sampling, 5 MSPS, 12-Bit, SAR ADC
• Manufacturer: Analog Devices

Specifications of AD7356

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 5MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,2.048 V p-p
• Adc Architecture: SAR
• Pkg Type: SOP

AD7356
If the analog inputs source being used has zero impedance, all
four resistors (R
as each other. If the source has a 50 Ω impedance and a 50 Ω
termination, for example, increase the value of R
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.
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.
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
used in this configuration to provide differential drive to the
AD7356.
V
*MOUNT AS CLOSE TO THE AD7356 AS POSSIBLE
GND
*ADDITIONAL PINS OMITTED FOR CLARITY.
AND ENSURE THAT HIGH PRECISION R
R
R
REF
Figure 21. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
2
S
G
Figure 20. Using the AD8138 as a Single-Ended-to-Differential Amplifier
2 = 523Ω
– 33Ω; R
+2.048V
GND
–2.048V
G
V
1 = R
REF
p-p
F
AD8022
1 = R
G
220Ω
1, R
51Ω
F
A
2 = 499Ω; C
G
into a Differential Signal
2, R
V
R
R
is a suitable dual op amp that could be
OCM
220Ω
220Ω
220Ω
G
G
1
2
F
1, and R
V+
V–
V+
V–
10kΩ
F
AD8138
1 = C
C
R
C
R
F
F
F
F
S
10kΩ
1
1
2
2
10kΩ
RESISTORS ARE USED.
F
27Ω
27Ω
10kΩ
2 = 39pF;
F
2) should be the same value
R
R
S
S
*
*
2.048V
0V
1.024V
2.048V
1.024V
0V
2.048V
1.024V
0V
2.048V
0V
1.024V
G
V
2 by 25 Ω to
V
IN–
IN+
V
V
IN+
IN–
AD7356
AD7356*
REF
REF
A
/REF
A
10µF
/REF
Rev. A | Page 14 of 20
10µF
B
B
VOLTAGE REFERENCE
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
REFGND with 10 μF capacitors. The internal reference voltage
can be used elsewhere in the system, provided it is buffered
externally.
The REF
external voltage reference if desired. The applied reference
voltage can range from 2.048 V + 100 mV to V
choice would be to use an external 2.5 V reference such as the
ADR441
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
The ideal transfer characteristic of the AD7356 is shown in
Figure 23.
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 22. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
GND
111 ... 111
111 ... 110
111 ... 101
000 ... 010
000 ... 001
000 ... 000
A
A
2 × V
or ADR431.
and REF
and REF
–V
220Ω
Figure 23. AD7356 Ideal Transfer Characteristic
REF
REF
p-p
440Ω
+ 0.5 LSB
A
B
B
–V
pins. These pins should be decoupled to
a Differential Unipolar Signal
pins can also be overdriven with an
REF
10kΩ
220Ω
220Ω
220Ω
+ 1 LSB
V+
V–
V+
V–
ANALOG INPUT
20kΩ
27Ω
27Ω
+V
REF
2.048V
1.024V
0V
2.048V
1.024V
0V
– 1.5 LSB
DD
+V
V
V
. A common
REF
IN+
IN–
REF
AD7356*
)/4096.
– 1 LSB
REF
A
/REF
10µF
B