PIC16F870SLASHSP datasheet(82/156 Pages) MICROCHIP

82 / 156 page

PIC16F870/871

DS30569A-page 82

Preliminary

FIGURE 10-1: A/D BLOCK DIAGRAM

10.1

A/D Acquisition Requirements

For the A/D converter to meet its specified accuracy,

the charge holding capacitor (CHOLD) must be allowed

to fully charge to the input channel voltage level. The

analog input model is shown in Figure 10-2. The

source impedance (RS) and the internal sampling

switch (RSS) impedance directly affect the time

required to charge the capacitor CHOLD. The sampling

switch (RSS) impedance varies over the device voltage

(VDD), Figure 10-2. The maximum recommended

impedance for analog sources is 10 k

Ω. As the

impedance is decreased, the acquisition time may be

decreased. After the analog input channel is selected

(changed), this acquisition must be done before the

conversion can be started.

calculate

the

minimum

acquisition

time,

Equation 10-1 may be used. This equation assumes

that 1/2 LSb error is used (1024 steps for the A/D). The

1/2 LSb error is the maximum error allowed for the A/D

to meet its specified resolution.

To calculate the minimum acquisition time, TACQ, see

the

PICmicro™

Mid-Range

Reference

Manual

(DS33023).

(Input voltage)

VAIN

VREF+

(Reference

voltage)

VDD

PCFG3:PCFG0

CHS2:CHS0

RE2/AN7(1)

RE1/AN6(1)

RE0/AN5(1)

RA5/AN4

RA3/AN3/VREF+

RA2/AN2/VREF-

RA1/AN1

RA0/AN0

111

110

101

100

011

010

001

000

A/D

Converter

Note 1: Not available on PIC16F870.

VREF-

(Reference

voltage)

VSS

PCFG3:PCFG0

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