PIC16F913 datasheet(151/272 Pages) MICROCHIP

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Preliminary

DS41250E-page 149

PIC16F917/916/914/913

12.2

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 12-4. 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 12-4. The maximum recom-

mended impedance for analog sources is 10 k

Ω.

As the impedance is decreased, the acquisition time

may be decreased. After the analog input channel is

selected (changed), this acquisition must be done

before the conversion can be started.

calculate

the

minimum

acquisition

time,

Equation 12-1 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 MCU Family Reference

Manual” (DS33023).

EQUATION 12-1:

ACQUISITION TIME

TACQ

Amplifier Settling Time

Hold Capacitor Charging Time

Temperature Coefficient

TAMP

TCOFF

2µs

Temperature - 25°C

() 0.05µs/°C

()

[]

CHOLD RIC

RSS

() ln(1/2047)

–

10pF 1k

Ω 7kΩ 10kΩ

()

–

ln(0.0004885)

1.37

µs

TACQ

2µS

1.37µS

50°C- 25°C

() 0.05µS/°C

()

[]

4.67µS

VAPPLIED 1

–

---------

–

⎝⎠

⎜⎟

⎛⎞

VAPPLIED 1

2047

------------

–

⎝⎠

⎛⎞

VAPPLIED 1

2047

------------

–

⎝⎠

⎛⎞

VCHOLD

VAPPLIED 1

–

----------

–

⎝⎠

⎜⎟

⎛⎞

VCHOLD

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

;[2] VCHOLD charge response to VAPPLIED

;combining [1] and [2]

Where CHOLD is charged to within 1/2 lsb:

Solving for TC:

Therefore:

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

Ω. This is required to meet the pin

leakage specification.

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