PIC12F609T-I/P View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC12F609T-I/P

8-Pin Flash-Based, 8-Bit CMOS Microcontrollers

Microchip Technology 

PIC12F609/615/617/12HV609/615

10.3 A/D Acquisition Requirements

For the ADC 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 10-4. 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 10-4.

The maximum recommended impedance for analog

sources is 10 k. As the source impedance is

decreased, the acquisition time may be decreased.

After the analog input channel is selected (or changed),

an A/D acquisition must be done before the conversion

can be started. To 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

ADC). The 1/2 LSb error is the maximum error allowed

for the ADC to meet its specified resolution.

EQUATION 10-1: ACQUISITION TIME EXAMPLE

Assumptions: Temperature = 50°C and external impedance of 10k 5.0V VDD

TACQ = Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient

= TAMP + TC + TCOFF

= 2µs + TC + Temperature - 25°C0.05µs/°C

The value for TC can be approximated with the following equations:

VAP

P

L

I

E

D





1

–

2----0-1--4---7-

=

VCHOLD



VAPPLIED1

–

e–-R---T--C-C--

=

VCHOLD





;[1] VCHOLD charged to within 1/2 lsb

;[2] VCHOLD charge response to VAPPLIED



VAPPLIED1

–

e –-R---T-C--c- 





=

V

A

P

P

L

I

E

D





1

–

2---0--1-4---7--

;combining [1] and [2]

Solving for TC:

TC = –CHOLDRIC + RSS + RS ln(1/2047)

= –10pF1k + 7k + 10k ln(0.0004885)

= 1.37µs

Therefore:

TACQ = 2µs + 1.37µs + 50°C- 25°C0.05µs/°C

= 4.67µs

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 k. This is required to meet the pin

leakage specification.

DS41302D-page 86

 2010 Microchip Technology Inc.

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