4.5.5.3.1 ADCRS Effects on -3 dB Roll-Off Frequency

The radian values listed in the table above are defined by the ADC²’s hardware. These values are used to calculate the -3 dB roll-off point in terms of frequency. The following equation can be used to determine the -3 dB point.

Where:

Radians @ -3 dB = the value from the Table 4-5 based on the ADCRS value.

T = total sampling time.

The total sampling time is the measured time between samples.

The total sampling time includes the ADC acquisition time, the conversion time, interrupt time, and the time required for post-processing of the ADC result such as serial transmission etc. The number of instructions contained in the ADC routine also influences the total sampling time.

In this example, after acquiring the ADC sample, the ADC result is transmitted over EUSART. Baud rate is kept sufficiently high so the sampling rate is not affected by serial transmission.

The Table 4-6 shows the different roll-off frequencies (Calculated Frequency @ -3 dB Point (Hz)) which is calculated by using, Equation 4-2 where the sampling time T = 625 μs, as the ADC auto conversion trigger is configured to 625 μs in example source code.

For example: when ADCRS =5, Radians @ -3 dB Cut-Off = 0.032, T= 625 μs.

$Frquency@-3dBpoint=\frac{0.032}{2\mathrm{\Pi }\mathrm{x}625}=8.15Hz$

Once the given input AC signal is reached to ~8 Hz, the attenuation in the input AC signal is observed.

A graph illustration using ADCRS = 5 will be demonstrated in the next section.