MPC5673K datasheet(83/147 Pages) NXP

83 / 147 page

Electrical characteristics

MPC5675K Microcontroller Data Sheet, Rev. 8.1

NXP Semiconductors

Figure 11. Spectral representation of input signal

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:

Eqn. 12

Table 24. ADC conversion characteristics

No.

Symbol

Parameter

Conditions1

Min Typ Max Unit

1fCK

SR ADC clock frequency (depends on ADC

configuration)

(The duty cycle depends on AD_CK2 frequency)

—3

—

MHz

2fs

SR Sampling frequency

—

959 kHz

3tADC_S D Sample time

60 MHz

383

—

4tADC_S

_PMC

C Sample time of internal PMC channels.

—

717

—

5tADC_E P Evaluation time

60 MHz

600

—

Analog Source Bandwidth (VA)

Sampled Signal Spectrum (fC = conversion Rate)

Anti-Aliasing Filter (fF = RC Filter pole)

2 f0 ≤ fC (Nyquist)

fF = f0 (Anti-aliasing Filtering Condition)

TC ≤ 2 RFCF (Conversion Rate vs. Filter Pole)

Noise

VA2

-----------

CP1 CP2

+CF

CP1 CP2

+CF CS

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

CF 8192 CS

•

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