Datasheet ADS1015

Magnitude
f
MOD
/ 2 f
MOD
Output
Data Rate
Frequency
External
Antialiasing Filter
Roll-Off
Magnitude
f
MOD
/ 2 f
MOD
Output
Data Rate
Frequency
Digital Filter
Magnitude
f
MOD
/ 2 f
MOD
Output
Data Rate
Frequency
Sensor
Signal
Unwanted
Signals
Unwanted
Signals
Aliasing of
Unwanted Signals
29
ADS1013 , ADS1014 , ADS1015
www.ti.com SBAS473E –MAY 2009–REVISED JANUARY 2018
Product Folder Links: ADS1013 ADS1014 ADS1015
Submit Documentation FeedbackCopyright © 2009–2018, Texas Instruments Incorporated
Application Information (continued)
9.1.5 Analog Input Filtering
Analog input filtering serves two purposes:
1. Limits the effect of aliasing during the sampling process
2. Reduces external noise from being a part of the measurement
Aliasing occurs when frequency components are present in the input signal that are higher than half the sampling
frequency of the ADC (also known as the Nyquist frequency). These frequency components fold back and show
up in the actual frequency band of interest below half the sampling frequency. The filter response of the digital
filter repeats at multiples of the sampling frequency, also known as the modulator frequency (f
MOD
), as shown in
Figure 26. Signals or noise up to a frequency where the filter response repeats are attenuated to a certain
amount by the digital filter depending on the filter architecture. Any frequency components present in the input
signal around the modulator frequency, or multiples thereof, are not attenuated and alias back into the band of
interest, unless attenuated by an external analog filter.
Figure 26. Effect of Aliasing
Many sensor signals are inherently band-limited; for example, the output of a thermocouple has a limited rate of
change. In this case, the sensor signal does not alias back into the pass-band when using a ΔΣ ADC. However,
any noise pick-up along the sensor wiring or the application circuitry can potentially alias into the pass-band.
Power line-cycle frequency and harmonics are one common noise source. External noise can also be generated
from electromagnetic interference (EMI) or radio frequency interference (RFI) sources, such as nearby motors
and cellular phones. Another noise source typically exists on the printed-circuit-board (PCB) itself in the form of
clocks and other digital signals. Analog input filtering helps remove unwanted signals from affecting the
measurement result.
A first-order resistor-capacitor (RC) filter is (in most cases) sufficient to either totally eliminate aliasing, or to
reduce the effect of aliasing to a level within the noise floor of the sensor. Ideally, any signal beyond f
MOD
/ 2 is
attenuated to a level below the noise floor of the ADC. The digital filter of the ADS101x attenuate signals to a
certain degree. In addition, noise components are usually smaller in magnitude than the actual sensor signal.
Therefore, use a first-order RC filter with a cutoff frequency set at the output data rate or 10x higher as a
generally good starting point for a system design.