MPC5644BCAMMJ1 datasheet(76/116 Pages) NXP

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MPC5646C Microcontroller DataSheet, Rev. 7

Electrical Characteristics

NXP Semiconductors

Calling f0 the bandwidth of the source signal (and as a consequence the cut-off frequency of the anti-aliasing filter, fF),

according to the Nyquist theorem the conversion rate fC must be at least 2f0; it means that the constant time of the filter is greater

than or at least equal to twice the conversion period (TC). Again the conversion period TC is longer than the sampling time TS,

which is just a portion of it, even when fixed channel continuous conversion mode is selected (fastest conversion rate at a

specific channel): in conclusion it is evident that the time constant of the filter RFCF is definitively much higher than the

sampling time TS, so the charge level on CS cannot be modified by the analog signal source during the time in which the

sampling switch is closed.

The considerations above lead to impose new constraints on the external circuit, to reduce the accuracy error due to the voltage

drop on CS; from the two charge balance equations above, it is simple to derive Equation 11 between the ideal and real sampled

voltage on CS:

Eqn. 11

From this formula, in the worst case (when VA is maximum, that is for instance 5 V), assuming to accept a maximum error of

half a count, a constraint is evident on CF value:

ADC_0 (10-bit)

Eqn. 12

ADC_1 (12-bit)

Eqn. 13

4.17.1.2

ADC electrical characteristics

Table 41. ADC input leakage current

Symbol C

Parameter

Conditions

Value

Unit

Min

Typ

Max

ILKG CC C Input leakage current TA = −40 °C No current injection on adjacent pin

—

CTA = 25 °C

—

CTA = 105 °C

—

200

PTA = 125 °C

—

400

VA2

------------

CP1 CP2

+CF

CP1 CP2

+CF CS

--------------------------------------------------------

CF 2048 CS

•

CF 8192 CS

•

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