PIC16F689T-I/SO View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC16F689T-I/SO

20-Pin Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

Microchip Technology 

PIC16F631/677/685/687/689/690

9.3 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 9-4. 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 9-4.

The maximum recommended impedance for analog

sources is 10 kΩ. 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 9-1 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 9-1: ACQUISITION TIME EXAMPLE

Assumptions: Temperature = 50°C and external impedance of 10kΩ 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(1/2047)

= –10pF(1kΩ + 7kΩ + 10kΩ) ln(0.0004885)

= 1.37μs

Therefore:

TACQ = 2μS + 1.37μS + [(50°C- 25°C)(0.05μS/°C)]

= 4.67μS

DS41262E-page 116

© 2008 Microchip Technology Inc.

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