PXD20 Datasheet, PDF(95/130 Page) Freescale Semiconductor, Inc

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PXD20 Datasheet, PDF (95/130 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

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Electrical characteristics

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 12 between the ideal and real sampled

voltage on CS:

--V-----A---- = --------C----P---1-----+-----C----P----2----+-----C----F---------

VA2 CP1 + CP2 + CF + CS

Eqn. 12

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

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

CF ï¾ 2048 ï· CS

Eqn. 13

4.16.2 ADC electrical characteristics

Table 44. ADC electrical characteristics

Symbol C

Parameter

Conditions1

Value2

Min

Typ

Max

Unit SpecID

VSSA SR D Voltage on VSSA (ADC

reference) pin with re-

spect to ground (VSS)3

â0.1

0.1

V D15.1

VDDA

VAINx

SR D Voltage on VDDA pin

(ADC reference) with re-

spect to ground (VSS)

SR D Analog input voltage4

VDDE_A â 0. â VDDE_A + 0.1 V D15.2

VSSA â 0.1 â VDDA + 0.1 V D15.3

fADC SR D ADC analog frequency

MHz D15.4

tADC_PU SR D ADC power up delay

1.5

Âµs D15.5

tADC_S CC5 T Sample time6

fADC = 32 MHz,

0.5

ADC_conf_sample_input =

Âµs D15.6

tADC_C CC5 T Conversion time7

fADC = 6 MHz,

ADC_conf_sample_input =

127

fADC = 32 MHz,

ADC_conf_comp = 2

0.625

Âµs D15.7

PXD20 Microcontroller Data Sheet, Rev. 2

Freescale Semiconductor

PreliminaryâSubject to Change Without Notice

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