SPC560B50x datasheet(85/116 Pages) STMICROELECTRONICS

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SPC560B40x/50x, SPC560C40x/50x

Package pinouts and signal descriptions

115

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

3.26.3

ADC electrical characteristics

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2048 C





Table 44. ADC input leakage current

Symbol C

Parameter

Conditions

Value

Unit

Min

Typ

Max

ILKG CC

Input leakage current

TA = 40 °C

No current injection on adjacent pin

—1

DTA = 25 °C

—

DTA = 85 °C

—

100

DTA = 105 °C

—

200

PTA = 125 °C

—

400

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