AD7356BRUZ Analog Devices Inc, AD7356BRUZ Datasheet - Page 12

• 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
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7356 is a high speed, dual, 12-bit, single-supply, succes-
sive approximation analog-to-digital converter (ADC). The part
operates from a 2.5 V power supply and features throughput
rates of up to 5 MSPS.
The AD7356 contains two on-chip differential track-and-hold
amplifiers, two successive approximation ADCs, and a serial
interface with two separate data output pins. The part is housed
in a 16-lead TSSOP, offering the user considerable space-saving
advantages over alternative solutions.
The serial clock input accesses data from the part but also
provides the clock source for each successive approximation
ADC. The AD7356 has an on-chip 2.048 V reference. If an
external reference is desired the internal reference can be
overdriven with a reference value ranging from (2.048 V +
100 mV) to V
in the system, then the reference output needs to be buffered
first. The differential analog input range for the AD7356 is
V
The AD7356 features power-down options to allow power
saving between conversions. The power-down feature is
implemented via the standard serial interface, as described
in the Modes of Operation section.
CONVERTER OPERATION
The AD7356 has two successive approximation ADCs, each
based around two capacitive DACs. Figure 14 and Figure 15
show simplified schematics of one of these ADCs in acquisition
and conversion phase. The ADC comprises a control logic, a
SAR, and two capacitive DACs. In Figure 14 (the acquisition
phase), SW3 is closed, SW1 and SW2 are in Position A, the
comparator is held in a balanced condition, and the sampling
capacitor arrays acquire the differential signal on the input.
CM
± V
V
V
IN+
IN–
REF
/2.
B
A
A
B
V
DD
REF
SW1
SW2
. If the internal reference is to be used elsewhere
Figure 14. ADC Acquisition Phase
C
C
S
S
SW3
COMPARATOR
CAPACITIVE
CAPACITIVE
CONTROL
DAC
LOGIC
DAC
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When the ADC starts a conversion (see Figure 15), SW3 opens
and SW1 and SW2 move to Position B, causing the comparator
to become unbalanced. Both inputs are disconnected once the
conversion begins. The control logic and charge redistribution
DACs are used to add and subtract fixed amounts of charge
from the sampling capacitor arrays to bring the comparator
back into a balanced condition. When the comparator is
rebalanced, the conversion is complete. The control logic
generates the ADC output code. The output impedances of
the sources driving the V
otherwise, the two inputs may have different settling times,
resulting in errors.
ANALOG INPUT STRUCTURE
Figure 16 shows the equivalent circuit of the analog input structure
of the AD7356. The four diodes provide ESD protection for the
analog inputs. Care must be taken to ensure that the analog input
signals never exceed the supply rails by more than 300 mV.
This causes these diodes to become forward biased and start
conducting into the substrate. These diodes can conduct up
to 10 mA without causing irreversible damage to the part.
The C1 capacitors in Figure 16 are typically 8 pF and can
primarily be attributed to pin capacitance. The R1 resistors
are lumped components made up of the on resistance of the
switches. The value of these resistors is typically about 30 Ω.
The C2 capacitors are the sampling capacitors of the ADC
with a capacitance of 32 pF typically.
V
V
IN+
IN–
B
A
A
B
V
REF
SW1
SW2
Figure 16. Equivalent Analog Input Circuit,
V
V
IN+
IN–
Conversion Phase–Switches Open,
C1
C1
Figure 15. ADC Conversion Phase
Track Phase—Switches Closed
C
C
S
S
IN+
V
V
DD
DD
and V
D
D
D
D
SW3
COMPARATOR
IN−
pins must be matched;
R1 C2
R1 C2
CAPACITIVE
CAPACITIVE
CONTROL
DAC
LOGIC
DAC