RPC564L60L5BFSR datasheet(107/137 Pages) STMICROELECTRONICS

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RPC56EL60L5

Electrical characteristics

136

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:

Equation 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:

Equation 12

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

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

8192





Table 24. ADC conversion characteristics

Symbol

Parameter

Conditions(1)

Min

Typ Max

Unit

fCK

ADC Clock frequency (depends on ADC

configuration)

(The duty cycle depends on AD_CK(2)

frequency)

—3

—

MHz

Sampling frequency

—

983.

6(3)

KHz

tsample

D Sample time(4)

60 MHz

383

—

teval

D Evaluation time(5)

60 MHz

600

—

(6)

D ADC input sampling capacitance

—

7.32

CP1

(6)

D ADC input pin capacitance 1

—

5((7))

CP2

(6)

D ADC input pin capacitance 2

—

0.8

RSW1

(6)

D Internal resistance of analog source

VREF range = 4.5 to 5.5 V

—

0.3



VREF range = 3.0 to 3.6 V

—

875

RAD

(6)

D Internal resistance of analog source

—

825

INL

P Integral non linearity

—

–3

—

LSB

DNL

P Differential non linearity(8)

—–1

—

LSB

OFS

T Offset error

—

–6

—

LSB

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