Datasheet

ADC08L060

www.ti.com

SNAS167G –MAY 2002–REVISED MARCH 2013

The DR GND connection to the ground plane should not use the same feedthrough used by other ground

connections.

High power digital components should not be located on or near a straight line between the ADC or any linear

component and the power supply area as the resulting common return current path could cause fluctuation in the

analog input “ground” return of the ADC.

Keeping analog and digital return (ground) currents separate from each other will improve system noise

performance. Two methods may be used to do this. Use of traces rather than a solid plane to route power to all

components will accomplish this because return currents follow the path of the outgoing currents. However, the

advantage of the distributed capacitance of a power plane and a ground plane is lost. Analog and digital power

should be routed as far from each other as is practical. The analog power trace should also be routed away from

digital areas of the board.

The use of power and ground planes in adjacent layers will provide distributed capacitance for a low impedance

power distribution system and better system noise performance. The use of separate analog and digital power

planes, both in the same PC board layer, and the use of a single, non-split ground plane will keep analog and

digital currents separated from each other. Of course, locate all analog circuitry and traces over the analog power

plane and the digital circuitry and traces over the digital power plane. To minimize RFI/EMI, give proper attention

to any lines crossing the analog/digital power plane boundary.

Noise performance is also enhanced by driving a single gate with each ADC output pin and locating the gate as

close as possible to the ADC output. Inserting a 47Ω resistor in series with the ADC digital output pins will also

help reduce ADC noise. Be sure to keep the resistors as close to the ADC output pins as possible. Eliminating

ground plane copper beneath the ADC output lines can also help ADC noise performance, but could produce

unacceptable radiation from the board.

Analog and digital circuitry should be kept well away from each other. Especially troublesome is high power

digital components such as processors and large PLDs. Switch mode power supplies, including capacitive DC-

DC converters, can cause noise problems with high speed ADCs. Keep such components well away from ADCs

and low level analog signal areas. Such components should be located as close to the power supply as possible

and should not be in the path of analog signal or power supply currents.

Digital circuits create substantial supply and ground current transients. The noise thus generated could have

significant impact upon system noise performance. The best logic family to use in systems with A/D converters is

one that employs non-saturating transistor designs, or has low noise characteristics, like the 74LS and the

74AC(T)Q families. The worst noise generators are logic families that draw the largest supply current transients

during clock or signal edges, like the 74HC, 74F and 74AC(T) families.

Since digital switching transients are composed largely of high frequency components, total ground plane copper

weight will have little effect upon logic-generated noise. This is because of the skin effect. Total surface area is

more important than is total ground plane volume.

Clock lines should be isolated from ALL other lines, analog AND digital. Even the generally accepted 90°

crossing should be avoided as even a little coupling can cause problems at high frequencies. Best performance

at high frequencies is obtained with a straight signal path.

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