28.6.2 Analog Noise Canceling Techniques

Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog measurements. If conversion accuracy is critical, the noise level can be reduced by applying the following techniques:

  1. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog ground plane, and keep them well away from high-speed switching digital tracks.

    1.1. The AVCC pin on the device should be connected to the digital VCC supply voltage via an LC network as shown in the figure below.

    1.2. Use the ADC noise canceler function to reduce induced noise from the CPU.

    1.3. If any ADC [3:0] port pins are used as digital outputs, it is essential that these do not switch while a conversion is in progress. However, using the two-wire Interface (ADC4 and ADC5) will only affect the conversion on ADC4 and ADC5 and not the other ADC channels.

Figure 28-9. ADC Power Connections
Note: If the resistivity in the inductor is too high, the AVCC may exceed its range, VCC - 0.3V < AVCC < VCC + 0.3V.