AMC7820YSLASH2K datasheet(26/29 Pages) TI2

26 / 29 page

AMC7820

SBAS231B

www.ti.com

LAYOUT

For optimum performance, care should be taken with the

physical layout of the AMC7820 circuitry. The basic SAR

architecture is sensitive to glitches or sudden changes on the

power supply, reference, ground connections, and digital

inputs that occur just prior to latching the output of the analog

comparator of the ADC. Therefore, during any single conver-

sion for an ‘n-bit’ SAR converter, there are n ‘windows’ in

which large external transient voltages can easily affect the

conversion result. Such glitches might originate from switch-

ing power supplies, nearby digital logic, and high-power

devices. The degree of error in the digital output depends on

the reference voltage, layout, and the exact timing of the

external event.

With this in mind, power to the AMC7820 should be clean

and well bypassed. A 0.1

µF ceramic bypass capacitor should

be placed as close to the device as possible. A 1

µF to 10µF

capacitor may also be needed if the impedance of the

connection between AVDD and the power supply is high.

It is recommended to install a reference bypass capacitor

µF) between the EXT_REF_IN pin and analog ground

when internal reference is used. If an external reference

voltage originates from an op amp, make sure that it can

drive any bypass capacitor that is used without oscillation.

The AMC7820 architecture offers no inherent rejection of

noise or voltage variation coming from an external reference

input. Any noise and ripple from the reference will appear

directly in the analog and digital results.

The AGND and DGND pins should be connected to a clean

ground point. In many cases, this will be the “analog” ground.

Avoid connections that are too near the grounding point of a

microcontroller or digital signal processor. If needed, run a

ground trace directly from the converter to the power-supply

entry or battery-connection point. The ideal layout will include

an analog ground plane dedicated to the AMC7820 and

associated external analog circuitry.

AMC7820Y/2K Datasheet(PDF) 26 Page - Texas Instruments