Appendix C ADC Electrical SpecificationsMC9S12ZVMB Family Reference Manual Rev. 1.3692 NXP SemiconductorsC.1.1.1 Port AD Output Drivers SwitchingPortAD output drivers switching can adversely affect the ADC accuracy whilst converting the analogvoltage on other PortAD pins because the output drivers are supplied from the VDDA/VSSA ADC supplypins. Although internal design measures are implemented to minimize the effect of output driver noise, itis recommended to configure PortAD pins as outputs only for low frequency, low load outputs. The impacton ADC accuracy is load dependent and not specified. The values specified are valid under condition thatno PortAD output drivers switch during conversion.C.1.1.2 Source ResistanceDue to the input pin leakage current as specified in conjunction with the source resistance there will be avoltage drop from the signal source to the ADC input. The maximum source resistance RS specifies resultsin an error (10-bit resolution) of less than 1/2 LSB (2.5 mV) at the maximum leakage current. If device oroperating conditions are less than worst case or leakage induced error is acceptable, larger values of sourceresistance of up to 10Kohm are allowed.C.1.1.3 Source CapacitanceWhen sampling an additional internal capacitor is switched to the input. This can cause a voltage drop dueto charge sharing with the external and the pin capacitance. For a maximum sampling error of the inputvoltage 1LSB (10-bit resolution), then the external filter capacitor, Cf 1024 * (CINS –CINN).C.1.1.4 Current InjectionThe following points must be considered.1. A current is injected into the channel being converted. The channel being stressed has conversionvalues of 0x3FF (in 10-bit mode) for analog inputs greater than V RH and 0x000 for values less thanVRL unless the current is higher than specified as a disruptive condition.2. Current is injected into pins in the neighborhood of the channel being converted. A portion of thiscurrent is picked up by the channel (coupling ratio K), This additional current impacts the accuracyof the conversion depending on the source resistance.The additional input voltage error on the converted channel can be calculated as:VERR = K * R S * IINJwith IINJ being the sum of the currents injected into the two pins adjacent to the converted channel.3. The HVI pins do not include diode structures that inject current when the input voltage goes outsidethe supply-ground range. Thus HVI current injection is limited to below 200uA. However if anHVI impedance converter bypass is enabled, then even currents in this range can corrupt ADCresults from simultaneous conversions on other channels. This can be prevented by disabling thebypass, either by clearing the HVI PTAENLx or PTABYPLx bit.4. Similarly, when the ADC is converting an HVI pin voltage, then the impedance converter bypassmust be disabled to ensure that current injection on PADx pins does not impact the HVI ADCconversion result.
