PIC12F629-I View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC12F629-I

8-Pin FLASH-Based 8-Bit CMOS Microcontrollers

Microchip Technology 

PIC12F629/675

7.2 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 7-3. 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 7-3. The maximum recommended imped-

ance for analog sources is 10 kΩ. As the impedance

is decreased, the acquisition time may be decreased.

EQUATION 7-1: ACQUISITION TIME

TACQ

= Amplifier Settling Time +

Hold Capacitor Charging Time +

Temperature Coefficient

TC

TACQ

= TAMP + TC + TCOFF

= 2µs + TC + [(Temperature -25°C)(0.05µs/°C)]

= CHOLD (RIC + RSS + RS) In(1/2047)

= - 120pF (1kΩ + 7kΩ + 10kΩ) In(0.0004885)

= 16.47µs

= 2µs + 16.47µs + [(50°C -25°C)(0.05µs/°C)

= 19.72µs

After the analog input channel is selected (changed),

this acquisition must be done before the conversion

can be started.

To calculate the minimum acquisition time,

Equation 7-1 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).

Note 1: The reference voltage (VREF) has no effect on the equation, since it cancels itself out.

2: The charge holding capacitor (CHOLD) is not discharged after each conversion.

3: The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin

leakage specification.

FIGURE 7-3:

ANALOG INPUT MODEL

RS ANx

VDD

VT = 0.6V

VA

CPIN

5 pF

VT = 0.6V

Sampling

Switch

RIC ≤ 1K SS RSS

I LEAKAGE

± 500 nA

CHOLD

= DAC capacitance

= 120 pF

VSS

Legend

CPIN

= input capacitance

VT

= threshold voltage

I LEAKAGE = leakage current at the pin due to

various junctions

RIC

= interconnect resistance

SS

= sampling switch

CHOLD = sample/hold capacitance (from DAC)

6V

5V

VDD 4V

3V

2V

5 6 7 8 9 10 11

Sampling Switch

(kΩ)

 2003 Microchip Technology Inc.

DS41190C-page 45

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