AMC7820 Datasheet, PDF (26/28 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL

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 0.1µF ceramic bypass capacitor should
be placed as close to the device as possible. A 1µF to 10µF
capacitor may also be needed if the impedance of the
connection between AV_DD and the power supply is high.
It is recommended to install a reference bypass capacitor
(1µF) 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.
26
AMC7820
www.ti.com
SBAS231B