Datasheet

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SBAS312B − JANUARY 2005 − REVISED AUGUST 2007
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19
LAYOUT CONSIDERATIONS
POWER SUPPLIES
An applied external digital filter rejects high-frequency
noise. PSRR and CMRR improve at higher frequencies
because the digital filter suppresses high-frequency noise.
However, the suppression of the filter is not infinite, so
high-frequency noise still influences the conversion result.
Inputs to the ADS1205, such as CH x+, CH x−, and CLKIN,
should not be present before the power supply is on.
Violating this condition could cause latch-up. If these
signals are present before the supply is on, series resistors
should be used to limit the input current to a maximum of
10mA. Experimentation may be the best way to determine
the appropriate connection between the ADS1205 and
different power supplies.
GROUNDING
Analog and digital sections of the design must be carefully
and cleanly partitioned. Each section should have its own
ground plane with no overlap between them. Do not join
the ground planes; instead, connect the two with a
moderate signal trace underneath the converter. However,
for different applications with DSPs and switching power
supplies, this process might be different.
For multiple converters, connect the two ground planes as
close as possible to one central location for all of the
converters. In some cases, experimentation may be
required to find the best point to connect the two planes
together.
DECOUPLING
Good decoupling practices must be used for the ADS1205
and for all components in the design. All decoupling
capacitors, specifically the 0.1µF ceramic capacitors,
must be placed as close as possible to the pin being
decoupled. A 1µF and 10µF capacitor, in parallel with the
0.1µF ceramic capacitor, can be used to decouple AV
DD
to AGND as well as BV
DD
to BGND. At least one 0.1µF
ceramic capacitor must be used to decouple every AV
DD
to AGND and BV
DD
to BGND, as well as for the digital
supply on each digital component.
The digital supply sets the I/O voltage for the interface and
can be set within a range of 2.7V to 5.5V.
In cases where both the analog and digital I/O supplies
share the same supply source, an RC filter of 10Ω and
0.1µF can be used to help reduce the noise in the analog
supply.