PIC16F74-ESLASHP datasheet(88/174 Pages) MICROCHIP

88 / 174 page

PIC16F7X

DS30325B-page 86

 2002 Microchip Technology Inc.

11.1

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 11-2. 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), see

Figure 11-2. The source impedance affects the offset

voltage at the analog input (due to pin leakage current).

The maximum recommended impedance for ana-

log sources is 10 k

Ω. After the analog input channel is

selected (changed), the acquisition period must pass

before the conversion can be started.

To calculate the minimum acquisition time, TACQ, see

the PICmicro™ Mid-Range MCU Family Reference

Manual (DS33023). In general, however, given a max-

imum source impedance of 10 k

Ω and at a temperature

of 100°C, TACQ will be no more than 16

µsec.

FIGURE 11-2:

ANALOG INPUT MODEL

TABLE 11-1:

TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (C))

CPIN

ANx

5 pF

VDD

VT = 0.6V

I leakage

RIC

≤ 1k

Sampling

Switch

RSS

CHOLD

= DAC Capacitance

VSS

Sampling Switch

567 8 9 10 11

Ω)

VDD

= 51.2 pF

± 500 nA

Legend CPIN

I leakage

RIC

CHOLD

= input capacitance

= threshold voltage

= leakage current at the pin due to

= interconnect resistance

= sampling switch

= sample/hold capacitance (from DAC)

various junctions

AD Clock Source (TAD)

Maximum Device Frequency

Operation

ADCS1:ADCS0

Max.

2TOSC

1.25 MHz

8TOSC

5 MHz

32TOSC

20 MHz

RC(1, 2, 3)

(Note 1)

Note 1: The RC source has a typical TAD time of 4

µs but can vary between 2-6 µs.

2: When the device frequencies are greater than 1 MHz, the RC A/D conversion clock source is only

recommended for SLEEP operation.

3: For extended voltage devices (LC), please refer to the Electrical Specifications section.

PIC16F74-E/P Datasheet(PDF) 88 Page - Microchip Technology

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