Analog Devices ADuC812 manual Grounding and Board Layout Recommendations, System Grounding Schemes

ADuC812

Grounding and Board Layout Recommendations

As with all high resolution data converters, special attention must be paid to grounding and PC board layout of ADuC812-based designs in order to achieve optimum performance from the ADCs and DAC.

Although the ADuC812 has separate pins for analog and digital ground (AGND and DGND), the user must not tie these to two separate ground planes unless the two ground planes are con- nected together very close to the ADuC812, as illustrated in the simplified example of Figure 45a. In systems where digital and analog ground planes are connected together somewhere else (at the system’s power supply for example), they cannot be con- nected again near the ADuC812 since a ground loop would result. In these cases, tie the ADuC812’s AGND and DGND pins all to the analog ground plane, as illustrated in Figure 45b. In systems with only one ground plane, ensure that the digital and analog components are physically separated onto separate halves of the board such that digital return currents do not flow near analog circuitry and vice versa. The ADuC812 can then be placed between the digital and analog sections, as illustrated in Figure 45c.

In all of these scenarios, and in more complicated real-life appli- cations, keep in mind the flow of current from the supplies and back to ground. Make sure the return paths for all currents are as close as possible to the paths the currents took to reach their destinations. For example, do not power components on the analog side of Figure 45b with DVDD since that would force return currents from DVDD to flow through AGND. Also, try to avoid digital currents flowing under analog circuitry, which could happen if the user placed a noisy digital chip on the left half of the board in Figure 45c. Whenever possible, avoid large discontinuities in the ground plane(s) (such as are formed by a long trace on the same layer), since they force return signals to travel a longer path. And of course, make all connections to the ground plane directly, with little or no trace separating the pin from its via to ground.

If the user plans to connect fast logic signals (rise/fall time < 5 ns) to any of the ADuC812’s digital inputs, add a series resistor to each relevant line to keep rise and fall times longer than 5 ns at the ADuC812 input pins. A value of 100 Ω or 200 Ω is usually sufficient to prevent high-speed signals from coupling capacitively into the ADuC812 and affecting the accuracy of ADC conversions.

A

PLACE ANALOG	PLACE DIGITAL
COMPONENTS HERE	COMPONENTS HERE

AGND DGND

B

PLACE ANALOG	PLACE DIGITAL
COMPONENTS	COMPONENTS
HERE	HERE

AGND DGND

C

PLACE ANALOG	PLACE DIGITAL
COMPONENTS	COMPONENTS
HERE	HERE

GND

Figure 45. System Grounding Schemes

REV. B –43–