TERMINATING A CONVERSION
external transient voltages can easily affect the conversion
result. Such glitches might originate from switching power
supplies, nearby digital logic, and high-power devices.
There are three methods to terminate the conversion of the
A/D converter in the ADS7823 after the master initiates
conversion:
With this in mind, power to the ADS7823 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 +VDD and the power supply is high.
1) In normal operation sequence (see Figures 3 and 4). The
conversion is terminated after the read-addressing has
been received.
2) A STOP condition will always terminate a conversion. It
will also terminate the HS mode returning the ADS7823 to
the F/S mode.
The ADS7823 architecture offers no inherent rejection of
noise or voltage variation in regards to using an external
reference input. This is of particular concern when the
reference input is tied to the power supply. Any noise and
ripple from the supply will appear directly in the digital results.
While high-frequency noise can be filtered out, voltage varia-
tion due to line frequency (50Hz or 60Hz) can be difficult to
remove.
3) A not-acknowledge by the ADS7823 following a second
command byte will end a conversion.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS7823 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. Therefore, during any single conversion for an
“n-bit” SAR converter, there are n “windows” in which large
The GND pin 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. The ideal layout
will include an analog ground plane dedicated to the con-
verter and associated analog circuitry.
ADS7823
SBAS180B
12
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