Datasheet

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SBAS306F − NOVEMBER 2004 − REVISED OCTOBER 2007
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APPLICATION INFORMATION
To obtain the specified performance from the ADS1271,
the following layout and component guidelines should be
considered.
1. Power Supplies: The device requires two power
supplies for operation: DVDD and AVDD. The allowed
range for DVDD is 1.65V to 3.6V, and AVDD is
restricted to 4.75V to 5.25V. Best performance is
achieved when DVDD = 1.8V. For both supplies, use
a 10µF tantalum capacitor, bypassed with a 0.1µF
ceramic capacitor, placed close to the device pins.
Alternatively, a single 10µF ceramic capacitor can be
used. The supplies should be relatively free of noise
and should not be shared with devices that produce
voltage spikes (such as relays, LED display drivers,
etc.). If a switching power supply source is used, the
voltage ripple should be low (< 2mV). The power
supplies may be sequenced in any order.
2. Ground Plane: A single ground plane connecting both
AGND and DGND pins can be used. If separate digital
and analog grounds are used, connect the grounds
together at the converter.
3. Digital Inputs: It is recommended to source terminate
the digital inputs to the device with 50Ω series
resistors. The resistors should be placed close to the
driving end of digital source (oscillator, logic gates,
DSP, etc.) This helps to reduce ringing on the digital
lines, which may lead to degraded ADC performance.
4. Analog/Digital Circuits: Place analog circuitry (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.
5. Reference Inputs: It is recommended to use a
minimum 10µF tantalum with a 0.1µF ceramic
capacitor directly across the reference inputs, VREFP
and VREFN. The reference input should be driven by
a low-impedance source. For best performance, the
reference should have less than 3µV
RMS
broadband
noise. For references with noise higher than this,
external reference filtering may be necessary.
6. Analog Inputs: The analog input pins must be driven
differentially to achieve specified performance. A true
differential driver or transformer (AC applications) can
be used for this purpose. Route the analog inputs
tracks (AINP, AINN) as a pair from the buffer to the
converter using short, direct tracks and away from
digital tracks.
A 1nF to 10nF capacitor should be used directly
across the analog input pins, AINP and AINN. A low-k
dielectric (such as COG or film type) should be used to
maintain low THD. Capacitors from each analog input
to ground should be used. They should be no larger
than 1/10 the size of the difference capacitor (typically
100pF) to preserve the AC common-mode
performance.
7. Component Placement: Place the power supply,
analog input, and reference input bypass capacitors
as close as possible to the device pins. This is
particularly important for the small-value ceramic
capacitors. Surface-mount components are
recommended to avoid the higher inductance of
leaded components.
Figure 68 to Figure 70 illustrate basic connections and
interfaces that can be used with the ADS1271.