AD976AR Analog Devices Inc, AD976AR Datasheet - Page 15

• Manufacturer Part Number: AD976AR
• Description: A/D Converter (A-D) IC
• Manufacturer: Analog Devices Inc

Specifications of AD976AR

• Package/case: 28-SOIC
• Features: 16?Bit, Serial, 100kSPS ADC
• Interface Type: Parallel
• Leaded Process Compatible: No
• Number Of Bits: 16
• Number Of Channels: 1
• Peak Reflow Compatible (260 C): No
• Rohs Status: RoHS non-compliant
• Sampling Rate (per Second): 200k
• Data Interface: Parallel
• Number Of Converters: 1
• Power Dissipation (max): 100mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SOIC (0.300", 7.50mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With: EVAL-AD976CB - BOARD EVAL FOR AD976EVAL-AD976ACB - BOARD EVAL FOR AD976A
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

GROUNDING
The AD976/AD976A has three ground pins; AGND1, AGND2
and DGND. The analog ground pins are the “high quality”
ground reference points and should be connected to the system
analog common. AGND2 is the ground to which most internal
ADC analog signals are referenced. This ground is most
susceptible to current induced voltage drops and thus must be
connected with the least resistance back to the power supply.
AGND1 is the low current analog supply ground and should be
the analog common for the external reference, input op amp
drive circuitry and the input resistor divider circuit. By applying
the inputs referenced to this ground, any ground variations will
be offset and have a minimal effect on the resulting analog input
to the ADC. The digital ground pin, DGND, is the reference
point for all of the digital signals that control the AD976/AD976A.
The AD976/AD976A can be powered with two separate power
supplies or with a single analog supply. When the system digital
supply is noisy or fast switching digital signals are present, it is
recommended to connect the analog supply to both the V
and V
the remaining digital circuitry. With this configuration, AGND1,
AGND2, and DGND should be connected back at the ADC.
When there is significant bus activity on the digital output pins,
the digital and analog supply pins on the ADC should be sepa-
rated. This would eliminate any high speed digital noise from
coupling back to the analog portion of the AD976/AD976A.
In this configuration, the digital ground pin DGND should be
connected to the system digital ground and be separate from the
AGND pins.
REV. C
DIG
pins of the AD976/AD976A and the system supply to
ANA
–15–
BOARD LAYOUT
Designing with high resolution data converters requires careful
attention to board layout. Trace impedance is a significant issue.
A 1.22 mA current through a 0.5
drop of 0.6 mV, which is 2 LSBs at the 16-bit level over the
20 volt full-scale range. Ground circuit impedances should be
reduced as much as possible since any ground potential differ-
ences between the signal source and the ADC appear as an error
voltage in series with the input signal. In addition to ground
drops, inductive and capacitive coupling needs to be considered.
This is especially true when high accuracy analog input signals
share the same board with digital signals. Thus, to minimize
input noise coupling, the input signal leads to V
return leads from AGND should be kept as short as possible. In
addition, power supplies should also be decoupled to filter out
ac noise.
Analog and digital signals should not share a common path.
Each signal should have an appropriate analog or digital return
routed close to it. Using this approach, signal loops enclose a
small area, minimizing the inductive coupling of noise. Wide PC
tracks, large gauge wire and ground planes are highly recom-
mended to provide low impedance signal paths. Separate analog
and digital ground planes are also recommended with a single
interconnection point to minimize ground loops. Analog signals
should be routed as far as possible from high speed digital sig-
nals and should only cross them, if absolutely necessary, at right
angles.
In addition, it is recommended that multilayer PC boards be
used with separate power and ground planes. When designing
the separate sections, careful attention should be paid to the
layout.
trace will develop a voltage
AD976/AD976A
IN
and the signal