AD976/AD976A

–15–

REV. C

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

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.

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

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.