REV. A

AD974

–19–

The AD974 may be operated from a single +5 V supply.

When separate supplies are used, however, it is beneficial to

have larger (10

µF) capacitors placed between the logic supply

(V

supply (V

Addition-

ally, 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 AD974 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 AD974.

The AD974 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 recom-

mended to connect the analog supply to both the V

pins of the AD974 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 AD974. 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 and 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

differences 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

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 rec-

ommended 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 signals and if absolutely necessary, should only cross

them 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.