AD7762 Analog Devices, AD7762 Datasheet - Page 19

• Manufacturer Part Number: AD7762
• Description: Parallel Interface, 625 kSPS, 24-Bit Sigma-Delta A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7762

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: (1.6Vref) p-p,4 V p-p,6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

SUPPLY DECOUPLING
Every supply pin must be connected to the appropriate supply
via a ferrite bead and decoupled to the correct ground pin with
a 100 nF, 0603 case size, X7R dielectric capacitor. There are two
exceptions to this:
ADDITIONAL DECOUPLING
There are two other decoupling pins on the AD7762—Pin 8
(DECAPA) and Pin 30 (DECAPB). Pin 8 should be decoupled
with a 100 nF capacitor, and Pin 30 requires a 33 pF capacitor.
REFERENCE VOLTAGE FILTERING
A low noise reference source, such as the ADR431 (2.5 V) or
ADR434 (4.096 V), is suitable for use with the AD7762. The
reference voltage supplied to the AD7762 should be decoupled
and filtered, as shown in Figure 30.
The recommended scheme for the reference voltage supply
is a 100 Ω series resistor connected to a 100 μF tantalum
capacitor, followed by series resistor of 10 Ω, and finally a
10 nF decoupling capacitor very close to the V
+12V
•
•
Pin 12 (AV
the pin and a 10 nF decoupling capacitor.
Pin 27 (AV
capacitor or a direct connection to the supply, but instead
is connected to Pin 14 via a 15 nH inductor.
10μF
C15
+
100nF
C9
DD4
DD2
Figure 30. Reference Connection
) must have a 10 Ω resistor inserted between
) does not require a separate decoupling
2
+VIN
ADR434
GND
4
U3
VOUT
6
100nF
C10
100μF
100Ω
R30
C11
+
REF+
10nF
R17
10Ω
C46
pin.
PIN10
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DIFFERENTIAL AMPLIFIER COMPONENTS
The correct components for use around the on-chip differential
amplifier are detailed in Table 8. Matching the components on
both sides of the differential amplifier is important to minimize
distortion of the signal applied to the amplifier. A tolerance of
0.1% or better is required for these components. Symmetrical
routing of the tracks on both sides of the differential amplifier
also assists in achieving stated performance.
LAYOUT CONSIDERATIONS
While using the correct components is essential to achieve
optimum performance, the correct layout is just as important.
The Design Tools section of the AD7762 product page on
the Analog Devices website contains the gerber files for the
AD7762 evaluation board. These files should be used as a
reference when designing any system using the AD7762.
The location and orientation of some of the components
mentioned in previous sections is critical, and particular
attention must be paid to the components which are located
close to the AD7762. Locating these components further away
from the devices can have a direct impact on the maximum
performance achievable.
The use of ground planes also should be carefully considered.
To ensure that the return currents through the decoupling
capacitors are flowing to the correct ground pin, the ground
side of the capacitors should be as close to the ground pin
associated with that supply. A ground plane should not be relied
on as the sole return path for decoupling capacitors because the
return current path using ground planes is not easily
predictable.
AD7762