10.1.4 Analog Input Filtering

Analog input filtering serves two purposes:

1. Limits the effect of aliasing during the ADC sampling process
2. Attenuates unwanted noise components outside the bandwidth of interest

In most cases, a first-order resistor capacitor (RC) filter is sufficient to completely eliminate aliasing or to reduce the effect of aliasing to a level within the noise floor of the sensor. A good starting point for a system design with the TLA202x is to use a differential RC filter with a cutoff frequency set somewhere between the selected output data rate and 25 kHz. Make the series resistor values as small as possible to reduce voltage drops across the resistors caused by the device input currents to a minimum. However, the resistors should be large enough to limit the current into the analog inputs to less than 10 mA in the event of an overvoltage. Then choose the differential capacitor value to achieve the target filter cutoff frequency. Common-mode filter capacitors to GND can be added as well, but should always be at least ten times smaller than the differential filter capacitor.

Figure 20 shows an example of filtering a differential signal (AIN0, AIN1), and a single-ended signal (AIN3). Equation 3 and Equation 4 show how to calculate the filter cutoff frequencies (f_CO) in the differential and single-ended cases, respectively.