SBAS367F
–
JUNE 2007
–
REVISED FEBRUARY 2011
APPLICATION INFORMATION
5. Reference Inputs: It is recommended to use a
To
obtain
the
specified
performance
from
the
minimum 10
μ
F tantalum with a 0.1
μ
F ceramic
ADS1274/78, the following layout and component
capacitor directly across the reference inputs,
guidelines should be considered.
VREFP and VREFN. The reference input should
1. Power Supplies: The device requires three
be driven by a low-impedance source. For best
power supplies for operation: DVDD, IOVDD, and
performance, the reference should have less than
AVDD. The allowed range for DVDD is 1.65V to
3
μ
V
RMS
in-band noise. For references with noise
1.95V; (for 32.768MHz
<
f
CLK
≤
37MHz: 2.0V to
higher than this level, external reference filtering
2.2V) the range of IOVDD is 1.65V to 3.6V;
may be necessary.
AVDD is restricted to 4.75V to 5.25V. For all
6. Analog Inputs: The analog input pins must be
supplies,
use
a
10
μ
F
tantalum
capacitor,
driven
differentially
to
achieve
specified
bypassed with a 0.1
μ
F ceramic capacitor, placed
performance.
A
true
differential
driver
or
close to the device pins. Alternatively, a single
transformer (ac applications) can be used for this
10
μ
F ceramic capacitor can be used. The
purpose. Route the analog inputs tracks (AINP,
supplies should be relatively free of noise and
AINN) as a pair from the buffer to the converter
should not be shared with devices that produce
using short, direct tracks and away from digital
voltage spikes (such as relays, LED display
tracks. A 1nF to 10nF capacitor should be used
drivers, etc.). If a switching power-supply source
directly across the analog input pins, AINP and
is used, the voltage ripple should be low (less
AINN. A low-k dielectric (such as COG or film
than 2mV) and the switching frequency outside
type) should be used to maintain low THD.
the passband of the converter.
Capacitors from each analog input to ground can
2. Ground Plane: A single ground plane connecting
be used. They should be no larger than 1/10 the
both AGND and DGND pins can be used. If
size of the difference capacitor (typically 100pF)
separate digital and analog grounds are used,
to preserve the ac common-mode performance.
connect the grounds together at the converter.
7. Component Placement: Place the power supply,
3. Digital
Inputs:
It
is
recommended
to
analog
input,
and
reference
input
bypass
source-terminate the digital inputs to the device
capacitors as close as possible to the device
with 50
Ω
series resistors. The resistors should be
pins. This layout is particularly important for
placed close to the driving end of digital source
small-value ceramic capacitors. Larger (bulk)
(oscillator, logic gates, DSP, etc.) This placement
decoupling capacitors can be located farther from
helps to reduce ringing on the digital lines (ringing
the device than the smaller ceramic capacitors.
may lead to degraded ADC performance).
to
illustrate basic connections
4. Analog/Digital Circuits: Place analog circuitry
and interfaces that can be used with the ADS1274.
(input buffer, reference) and associated tracks
together, keeping them away from digital circuitry
(DSP,
microcontroller,
logic).
Avoid
crossing
digital tracks across analog tracks to reduce
noise coupling and crosstalk.
©
2007
–
2011, Texas Instruments Incorporated
37
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