AD627

Rev. D | Page 20 of 24

LAYOUT AND GROUNDING

The use of ground planes is recommended to minimize the

impedance of ground returns (and hence, the size of dc errors).

To isolate low level analog signals from a noisy digital environment,

many data acquisition components have separate analog and

digital ground returns (see Figure 47). Return all ground pins

from mixed-signal components, such as analog-to-digital

converters, through the high quality analog ground plane.

Digital ground lines of mixed-signal components should also

be returned through the analog ground plane. This may seem

to break the rule of separating analog and digital grounds;

however, in general, there is also a requirement to keep the

voltage difference between digital and analog grounds on a

converter as small as possible (typically, <0.3 V). The increased

noise, caused by the digital return currents of the converter

flowing through the analog ground plane, is generally negligible.

To maximize isolation between analog and digital, connect the

ground planes back at the supplies.

If there is only one power supply available, it must be shared by

both digital and analog circuitry. Figure 48 shows how to minimize

interference between the digital and analog circuitry. As in the

previous case, use separate analog and digital ground planes or

use reasonably thick traces as an alternative to a digital ground

plane. Connect the ground planes at the ground pin of the power

supply. Run separate traces (or power planes) from the power

supply to the supply pins of the digital and analog circuits. Ideally,

each device should have its own power supply trace, but they

can be shared by multiple devices if a single trace is not used to

route current to both digital and analog circuitry.

AD627

7

4

4

1

6

14

3

5

3

6

2

AD627

AD7892-2

0.1µF

0.1µF

0.1µF

0.1µF

ANALOG POWER SUPPLY

+5V

–5V

GND

AGND

MICRO-

PROCESSOR

AGND

ADC

DGND

DIGITAL POWER SUPPLY

+5V

GND

12

Figure 47. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies

0.1µF

AD627

7

4

4

1

5

3

6

2

AD7892-2

0.1µF

0.1µF

AGND

MICRO-

PROCESSOR

DGND

ADC

DGND

POWER SUPPLY

5V

GND

12

Figure 48. Optimal Ground Practice in a Single-Supply Environment