CHAPTER 11 A/D CONVERTER315(4) Countermeasures against noiseTo keep the resolution of 10 bits, prevent noise from being superimposed on the AVREF and ANI0 through ANI11pins. The higher the output impedance of the analog input source, the heavier the influence of noise. To lowernoise, connecting an external capacitor as shown in Figure 11-8 is recommended.Figure 11-8. Processing of Analog Input PinAV REFV DDV SSAV DDAV SSReference voltage inputClamp with diode with a low VF (0.3 V MAX.) if noise higher thanAV REF or lower than AV SS may be generated.C = 100 to 1000 pF(5) ANI0 through ANI11The analog input (ANI0 through ANI11) pins are multiplexed with port pins.To execute A/D conversion with any of ANI0 through ANI11 selected, do not execute an instruction that inputsdata to the port during conversion; otherwise, the resolution may drop.If a digital pulse is applied to pins adjacent to the pin whose input signal is converted into a digital signal, theexpected A/D conversion result may not be obtained because of the influence of coupling noise. Therefore, donot apply a pulse to the adjacent pins.(6) Input impedance of AV REF pinA series resistor string of about 15.5 kΩ (TYP.) is connected between the AV REF and AVSS pins.If the output impedance of the reference voltage source is too high, the series resistor string between the AVREFand AVSS pins are connected in parallel, increasing the error of the reference voltage.
