ADS1118_13 Datasheet, PDF (19/38 Pages) Texas Instruments – Ultra-Small, Low-Power, SPI™-Compatible, 16-Bit Analog-to-Digital Converter and Temperature Sensor with Internal Reference

ADS1118
www.ti.com
SBAS457C – OCTOBER 2010 – REVISED FEBRUARY 2013
ALIASING
As with any data converter, if the input signal contains frequencies greater than half the data rate, aliasing
occurs. To prevent aliasing, the input signal must be bandlimited. However, some signals are inherently
bandlimited. For example, the output of a thermocouple (which has a limited rate of change) is inherently
bandlimited. Nevertheless, these signals can still contain noise and interference components. These components
can fold back into the sampling band in the same way as with any other signal.
The ADS1118 digital filter provides some attenuation of high-frequency noise, but the frequency response of this
1st-order sinc filter cannot completely replace an anti-aliasing filter. For a few applications, some external filtering
may be required; in such instances, a simple RC filter is adequate. When designing an input filter circuit, be sure
to take into account the interaction between the filter network and the ADS1118 input impedance. TI
recommends keeping the filter resistance value below 1 kΩ.
RESET AND POWER-UP
When the ADS1118 powers up, a reset is performed. As part of the reset process, the ADS1118 sets all of its
bits in the Config register to the respective default settings. By default, the ADS1118 enters a power-down state
at start-up. The device interface and digital blocks are active, but no data conversions are performed. The initial
power-down state of the ADS1118 is intended to relieve systems with tight power-supply requirements from
encountering a surge during power-up.
OPERATING MODES
The ADS1118 operates in one of two modes: continuous conversion or single-shot mode. The MODE bit in the
Config register selects the respective operating mode.
Continuous Conversion Mode
In continuous conversion mode (MODE bit set to '0'), the ADS1118 continuously performs conversions. When a
conversion completes, the ADS1118 places the result in the Conversion register and immediately begins another
conversion.
Power-Down (Single-Shot Mode)
When the MODE bit in the Config register is set to '1', the ADS1118 enters power-down state and is configured
to operate in single-shot mode. This condition is also the default state that the ADS1118 enters when power is
first applied. In power-down state, the ADS1118 consumes no more than 2 μA of current. During this time, the
device responds to commands, but does not perform any data conversions. The ADS1118 is held in power-down
state until a '1' is written to the SS bit in the Config register. When the SS bit is asserted, the device powers up,
resets the SS bit to '0', and starts a single conversion. When conversion data are ready for retrieval, the device
powers down again. Writing a '1' to the SS bit while a conversion is ongoing has no effect. To exit this mode,
simply write a '0' to the MODE bit in the Config register to start operating in continuous conversion mode.
DUTY CYCLING FOR LOW POWER
The noise performance of a ΔΣ ADC generally improves when lowering the output data rate because more
samples of the internal modulator can be averaged to yield one conversion result. In applications where power
consumption is critical, the improved noise performance at low data rates may not be required. For these
applications, the ADS1118 supports duty cycling that can yield significant power savings by periodically
requesting high data rate readings at an effectively lower data rate. For example, an ADS1118 in power-down
state with a data rate set to 860 SPS can be operated by a microcontroller that instructs a single-shot conversion
every 125 ms (8 SPS). Because a conversion at 860 SPS only requires approximately 1.2 ms, the ADS1118
enters power-down state for the remaining 123.8 ms. In this configuration, the ADS1118 consumes
approximately 1/100th the power that it otherwise would consume in continuous conversion mode. The duty
cycling rate is completely arbitrary and is defined by the master controller. The ADS1118 offers lower data rates
that do not implement duty cycling and also offers improved noise performance if required.
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