MB96F348CWCPMC-GSE2 datasheet(91/109 Pages) CYPRESS

91 / 109 page

MB96340 Series

Document Number: 002-04579 Rev. *A

Page 91 of 109

14.5.2

Notes on A/D Converter Section

■ About the external impedance of the analog input and the sampling time of the A/D converter (with sample and hold circuit):

If the external impedance is too high to keep sufficient sampling time, the analog voltage charged to the internal sample and hold

capacitor is insufficient, adversely affecting A/D conversion precision

To satisfy the A/D conversion precision standard, the relationship between the external impedance and minimum sampling time must

be considered and then either the resistor value and operating frequency must be adjusted or the external impedance must be

decreased so that the sampling time (Tsamp) is longer than the minimum value. Usually, this value is set to 7where= RC. If the

external input resistance (Rext) connected to the analog input is included, the sampling time is expressed as follows:

Tsamp [min] = 7 (Rext + 2.6k) C for 4.5 AVcc 5.5

Tsamp [min] = 7 (Rext + 12.1k) C for 3.0 AVcc 4.5

If the sampling time cannot be sufficient, connect a capacitor of about 0.1

F to the analog input pin.

■ About the error

The accuracy gets worse as |AVRH - AVRL| becomes smaller.

Analog input circuit model:

Comparator

Sampling switch

Analog input

Reference value:

C = 8.5 pF (Max)

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