PIC16F1825T-IST 查看數據表（PDF） - Microchip Technology

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PIC16F1825T-IST

14/20-Pin Flash Microcontrollers with nanoWatt XLP Technology

Microchip Technology 

PIC16F/LF1825/1829

16.3 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 16-4. 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), refer

to Figure 16-4. 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 16-1 may be

used. This equation assumes that 1/2 LSb error is used

(1,024 steps for the ADC). The 1/2 LSb error is the

maximum error allowed for the ADC to meet its

specified resolution.

EQUATION 16-1: 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:

VAPPLI

ED





1

–

---2---n----+--1--1------–----1--

=

VCHOLD

VAPP

LIED



1

–

e–-R---T--C-C--

=

VCHOLD





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

;[2] VCHOLD charge response to VAPPLIED

VAPPLI



ED

1

–

e–-R---T-C--c-





=

VA

PP

L

I

E

D





1

–

---2---n----+--1--1------–----1--

;combining [1] and [2]

Note: Where n = number of bits of the ADC.

Solving for TC:

TC = –CHOLDRIC + RSS + RS ln(1/511)

= –10pF1k + 7k + 10k ln(0.001957)

= 1.12µs

Therefore:

TACQ = 2µs + 1.12µs + 50°C- 25°C0.05µs/°C

= 4.42µs

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.

DS41440A-page 162

Preliminary

 2010 Microchip Technology Inc.

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