ADS1118_13 Datasheet, PDF (26/38 Pages) Texas Instruments – Ultra-Small, Low-Power, SPI™-Compatible, 16-Bit Analog-to-Digital Converter and Temperature Sensor with Internal Reference

ADS1118
SBAS457C – OCTOBER 2010 – REVISED FEBRUARY 2013
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TI recommends employing best design practices when laying out a printed circuit board (PCB) for both analog
and digital components. This recommendation generally means that the layout should separate analog
components [such as ADCs, amplifiers, references, digital-to-analog converters (DACs), and analog MUXs] from
digital components [such as microcontrollers, complex programmable logic devices (CPLDs), field-programmable
gate arrays (FPGAs), radio frequency (RF) transceivers, universal serial bus (USB) transceivers, and switching
regulators]. An example of good component placement is shown in Figure 48. While Figure 48 provides a good
example of component placement, the best placement for each application is unique to the geometries,
components, and PCB fabrication capabilities being employed. That is, there is no single layout that is perfect for
every design and careful consideration must always be used when designing with any analog components.
Ground fill or
Ground plane
Signal
Conditioning
(RC filters
and
amplifiers)
ADS1118
Ground fill or
Ground plane
Ground fill or
Ground plane
Microprocessor
Interface
Tranceiver
Ground fill or
Ground plane
Supply
Generation
Connector
or Antenna
Figure 48. System Component Placement
The use of split analog and digital ground planes is not necessary for improved noise performance (although for
thermal isolation this option is a worthwhile consideration). However, the use of a solid ground plane or ground
fill in PCB areas with no components is essential for optimum performance. If the system being used employs a
split digital and analog ground plane, TI generally recommends that the ground planes be connected together as
close to the ADS1118 as possible. TI also strongly recommends that digital components, especially RF portions,
be kept as far as practically possible from analog circuitry in a given system. Additionally, minimize the distance
that digital control traces run through analog areas and avoid allowing these traces to be near sensitive analog
components. Digital return currents usually flow through a ground path that is as close to the digital path as
possible. If a solid ground connection to a plane is not available, these currents may find paths back to the
source that interfere with analog performance. The implications that layout has on the temperature sensing
functions are much more significant than they are for the ADC functions. Details on layout considerations for the
temperature sensor can be found in the Thermocouple Measurement with Cold Junction Compensation section.
For a detailed layout example, refer to the ADS1118EVM User's Guide (SBAU184).
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