MPC5607B datasheet(78/110 Pages) FREESCALE

78 / 110 page

MPC5607B Microcontroller Data Sheet, Rev. 6

Electrical characteristics

Freescale Semiconductor

Figure 20. Spectral representation of input signal

Calling f0 the bandwidth of the source signal (and as a consequence the cut-off frequency of the antialiasing 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

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 2048 CS





CF 8192 CS





MPC5607B Datasheet(PDF) 78 Page - Freescale Semiconductor, Inc