Datasheet

LAYOUT CONSIDERATIONS
POWER SUPPLIES DECOUPLING
GROUNDING
ADS1204
www.ti.com ........................................................................................................................................... SBAS301C – OCTOBER 2003 – REVISED FEBRUARY 2009
An applied external digital filter rejects high-frequency Good decoupling practices must be used for the
noise. PSRR and CMRR improve at higher ADS1204 and for all components in the design. All
frequencies because the digital filter suppresses decoupling capacitors, specifically the 0.1 µ F ceramic
high-frequency noise. capacitors, must be placed as close as possible to
the pin being decoupled. A 1 µ F and 10 µ F capacitor,
However, the suppression of the filter is not infinite,
in parallel with the 0.1 µ F ceramic capacitor, can be
so high-frequency noise still influences the
used to decouple AV
DD
to AGND as well as BV
DD
to
conversion result. Inputs to the ADS1204, such as
BGND. At least one 0.1 µ F ceramic capacitor must be
CH x+, CH x – , and CLKIN, should not be present
used to decouple every AV
DD
to AGND and BV
DD
to
before the power supply is on. Violating this condition
BGND, as well as for the digital supply on each digital
could cause latch-up. If these signals are present
component.
before the supply is on, series resistors should be
used to limit the input current to a maximum of 10mA. The digital supply sets the I/O voltage for the
Experimentation may be the best way to determine interface and can be set within a range of 2.7V to
the appropriate connection between the ADS1204 5.5V.
and different power supplies.
In cases where both the analog and digital I/O
supplies share the same supply source, an RC filter
of 10 Ω and 0.1 µ F can be used to help reduce the
noise in the analog supply.
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.
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