AD627
If there is only one power supply available, it must be shared by
both digital and analog circuitry. Figure 41 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 planesꢀ 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.
LAYOUT AND GROUNDING
The use of ground planes is recommended to minimize the
impedance of ground returns (and hence, the size of dc errorsꢀ.
To isolate low level analog signals from a noisy digital environment,
many data acquisition components have separate analog and
digital ground returns (see Figure 47ꢀ. 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 Vꢀ. 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.
ANALOG POWER SUPPLY
DIGITAL POWER SUPPLY
+5V
–5V
GND
GND
+5V
0.1µF 0.1µF
0.1µF
0.1µF
7
4
2
3
1
6
14
AGND
V
V
V
DD
6
4
3
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DGND
12
IN1
DD
AGND
5
MICRO-
PROCESSOR
V
ADC
IN2
AD7892-2
Figure 47. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies
POWER SUPPLY
5V
GND
0.1µF
0.1µF
0.1µF
7
1
4
2
3
V
V
AGND
DGND
12
DGND
MICRO-
DD
DD
6
4
V
AD627
IN
ADC
AD7892-2
5
PROCESSOR
Figure 48. Optimal Ground Practice in a Single-Supply Environment
Rev. D | Page 20 of 24
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