Datasheet

ADS1675
SBAS416D –DECEMBER 2008–REVISED AUGUST 2010 www.ti.com
APPLICATION INFORMATION
6. Analog Inputs: The analog input pins must be
To obtain the specified performance from the
driven differentially to achieve specified
ADS1675, the following layout and component
performance. A true differential driver or
guidelines should be considered.
transformer (ac applications) can be used for this
1. Power Supplies: The device requires two power
purpose. Route the analog inputs tracks (AINP,
supplies for operation: DVDD and AVDD. A very
AINN) as a pair from the buffer to the converter
clean and stable AVDD supply is needed to
using short, direct tracks and away from digital
achieve optimal performance from the device. For
tracks. A 750pF capacitor should be used directly
both supplies, use a 10mF tantalum capacitor,
across the analog input pins, AINP and AINN. A
bypassed with a 0.1mF ceramic capacitor, placed
low-k dielectric (such as COG or film type) should
close to the device pins. Alternatively, a single
be used to maintain low THD. Capacitors from
10mF ceramic capacitor can be used. The
each analog input to ground should be used.
supplies should be relatively free of noise and
They should be no larger than 1/10 the size of
should not be shared with devices that produce
the difference capacitor (typically 100pF) to
voltage spikes (such as relays, LED display
preserve the ac common-mode performance.
drivers, etc.). If a switching power-supply source
7. Component Placement: Place the power supply,
is used, the voltage ripple should be low (less
analog input, and reference input bypass
than 2mV). The power supplies may be
capacitors as close as possible to the device
sequenced in any order.
pins. This placement is particularly important for
2. Ground Plane: A single ground plane connecting
the small-value ceramic capacitors.
both AGND and DGND pins can be used. If
Surface-mount components are recommended to
separate digital and analog grounds are used,
avoid the higher inductance of leaded
connect the grounds together at the converter.
components.
3. Digital Inputs: Source terminate the digital inputs
Figure 53 through Figure 55 illustrate the basic
to the device with 50Ω series resistors. The
connections and interfaces that can be used with the
resistors should be placed close to the driving
ADS1675.
end of the digital source (oscillator, logic gates,
DSP, etc.). These resistors help 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: The ADS1675 reference input
has 400Ω across VREFP and VREFN. The
driving amplifier must source current for this static
current, as well as dynamic switching current as a
result of the 32MHz clock. The reference driving
amplifier should be ready to source at least
10.5mA.
28 Submit Documentation Feedback Copyright © 2008–2010, Texas Instruments Incorporated
Product Folder Link(s): ADS1675