ADS1015 datasheet(28/46 Pages) TI2

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10.1.5 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, as shown in Figure 10-3, repeats at multiples of the sampling frequency, also known as the

modulator frequency (fMOD). 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.

Magnitude

fMOD / 2

fMOD

Output

Data Rate

Frequency

External

Antialiasing Filter

Roll-Off

Magnitude

fMOD / 2

fMOD

Output

Data Rate

Frequency

Digital Filter

Magnitude

fMOD / 2

fMOD

Output

Data Rate

Frequency

Sensor

Signal

Unwanted

Signals

Unwanted

Signals

Aliasing of

Unwanted Signals

Figure 10-3. 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) 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. Generally, any signal beyond fMOD / 2

is attenuated to a level below the noise floor of the ADC. The digital filter of the ADS111xL attenuates signals to

a certain degree. In addition, noise components are usually smaller in magnitude than the actual sensor signal.

Therefore, using a first-order RC filter with a cutoff frequency set at the output data rate or 10 times higher is

generally a good starting point for a system design.

ADS1114L, ADS1115L

SBASAV5 – DECEMBER 2023

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