AD977/AD977A
–23–REV. D
The ADSP-2181 SPORT0 will now remain synchronized to the
external discontinuous clock for all subsequent conversions.
DR0
SCLK0
PF1
ADSP-2181
DATACLK
DATA
TAG
AD977/
AD977A
OSCILLATOR
RFS0
PF0
SPORT0 CNTRL REG = 0x300F
EXT/INT
CS
R/C
Figure 30. AD977/AD977A to ADSP-2181 Interface
POWER SUPPLIES AND DECOUPLING
The AD977/AD977A has two power supply input pins. V
ANA
and V
DIG
provide the supply voltages to the analog and digital
portions, respectively. V
ANA
is the 5 V supply for the on-chip
analog circuitry, and V
DIG
is the 5 V supply for the on-chip
digital circuitry. The AD977/AD977A is designed to be inde-
pendent of power supply sequencing and thus free from supply
voltage induced latchup.
With high performance linear circuits, changes in the power
supplies can result in undesired circuit performance. Optimally,
well regulated power supplies should be chosen with less than
1% ripple. The ac output impedance of a power supply is a
complex function of frequency and will generally increase with
frequency. Thus, high frequency switching, such as that encoun-
tered with digital circuitry, requires the fast transient currents
that most power supplies cannot adequately provide. Such a
situation results in large voltage spikes on the supplies. To com-
pensate for the finite ac output impedance of most supplies,
charge “reserves” should be stored in bypass capacitors. This
will effectively lower the supplies impedance presented to the
AD977/AD977A V
ANA
and V
DIG
pins and reduce the magnitude
of these spikes. Decoupling capacitors, typically 0.1 µF, should
be placed close to the power supply pins of the AD977/AD977A
to minimize any inductance between the capacitors and the
V
ANA
and V
DIG
pins.
The AD977/AD977A may be operated from a single 5 V
supply. When separate supplies are used, however, it is benefi-
cial to have larger capacitors, 10 µF, placed between the logic
supply (V
DIG
) and digital common (DGND) and between the
analog supply (V
ANA
) and the analog common (AGND2).
Additionally, 10 µF capacitors should be located in the vicinity
of the ADC to further reduce low frequency ripple. In systems
where the device will be subjected to harsh environmental noise,
additional decoupling may be required.
GROUNDING
The AD977/AD977A 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 sus-
ceptible 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 AD977/AD977A.
The AD977/AD977A 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
ANA
and V
DIG
pins of the AD977/AD977A and the system supply to
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
separated. This would eliminate any high speed digital noise
from coupling back to the analog portion of the AD977/
AD977A. In this configuration, the digital ground pin DGND
should be connected to the system digital ground and be
separate from the AGND pins.
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 Ω trace will develop a voltage
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
IN
and the signal
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.
