AD7356BRUZ Analog Devices Inc, AD7356BRUZ Datasheet - Page 14

• Manufacturer Part Number: AD7356BRUZ
• Description: 12-Bit Dual Diff Simult 5 MSPS ADC I.C.
• Manufacturer: Analog Devices Inc

Specifications of AD7356BRUZ

• Design Resources: DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7356 (CN0041)
• Number Of Bits: 12
• Sampling Rate (per Second): 3M
• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Converters: 2
• Power Dissipation (max): 59mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7356
DRIVING DIFFERENTIAL INPUTS
Differential operation requires V
simultaneously with two equal signals that are 180° out of phase.
Because not all applications have a signal preconditioned for
differential operation, there is often a need to perform a single-
ended-to-differential conversion.
Differential Amplifier
An ideal method of applying differential drive to the AD7356
is to use a differential amplifier such as the AD8138. This part
can be used as a single-ended-to-differential amplifier or as a
differential-to-differential amplifier. The AD8138 also provides
common-mode level shifting. Figure 20 shows how the AD8138
can be used as a single-ended-to-differential amplifier. The
positive and negative outputs of the AD8138 are connected to
the respective inputs on the ADC via a pair of series resistors
to minimize the effects of switched capacitance on the front
end of the ADC. The architecture of the AD8138 results in
outputs that are very highly balanced over a wide frequency
range without requiring tightly matched external components.
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.
*MOUNT AS CLOSE TO THE AD7356 AS POSSIBLE
AND ENSURE THAT HIGH PRECISION R
R
R
S
G
Figure 20. Using the AD8138 as a Single-Ended-to-Differential Amplifier
2 = 523Ω
– 33Ω; R
+2.048V
GND
–2.048V
G
1 = R
F
1 = R
G
1, R
51Ω
F
2 = 499Ω; C
G
2, R
V
R
R
OCM
G
G
1
2
F
1, and R
F
AD8138
1 = C
C
R
C
R
F
F
F
F
S
1
1
2
2
10kΩ
RESISTORS ARE USED.
F
10kΩ
IN+
2 = 39pF;
F
and V
2) should be the same value
R
R
S
S
*
*
IN−
2.048V
0V
1.024V
2.048V
0V
1.024V
to be driven
G
V
2 by 25 Ω to
V
IN–
IN+
AD7356
REF
A
/REF
10µF
Rev. 0 | Page 14 of 20
B
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
*ADDITIONAL PINS OMITTED FOR CLARITY.
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.
GND
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 22. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
GND
REF
Figure 21. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
111 ... 111
111 ... 110
111 ... 101
000 ... 010
000 ... 001
000 ... 000
2 × V
2 × V
–V
220Ω
Figure 23. AD7356 Ideal Transfer Characteristic
REF
REF
REF
AD8022
p-p
440Ω
p-p
440Ω
+ 0.5 LSB
A
A
–V
a Differential Unipolar Signal
REF
into a Differential Signal
220Ω
220Ω
220Ω
is a suitable dual op amp that could be
220Ω
220Ω
220Ω
20kΩ
+ 1 LSB
V+
V–
V+
V–
V+
V–
V+
V–
ANALOG INPUT
10kΩ
10kΩ
27Ω
27Ω
27Ω
27Ω
+V
REF
2.048V
1.024V
0V
2.048V
1.024V
0V
2.048V
1.024V
0V
2.048V
1.024V
0V
– 1.5 LSB
+V
V
V
REF
REF
IN+
IN–
V
V
IN+
IN–
AD7356*
)/4096.
– 1 LSB
AD7356*
REF
REF
A
/REF
A
10µF
/REF
10µF
B
B