PIC16F722-ESLASHML datasheet(106/302 Pages) MICROCHIP

106 / 302 page

PIC16F72X/PIC16LF72X

DS41341E-page 106

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 9-3. The source imped-

ance (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 9-3. The maximum recommended imped-

ance for analog sources is 10 k

Ω. 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 mini-

mum acquisition time, Equation 9-1 may be used. This

equation assumes that 1/2 LSb error is used (256 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

TACQ

Amplifier Settling Time

Hold Capacitor Charging Time

Temperature Coefficient

TAMP

TCOFF

2µs

Temperature - 25°C

() 0.05µs/°C

()

[]

CHOLD RIC

RSS

() ln(1/511)

–

10pF 1k

Ω 7kΩ 10kΩ

()

–

ln(0.001957)

1.12

µs

TACQ

2ΜS

1.12ΜS

50°C- 25°C

() 0.05ΜS/°C

()

[]

4.42ΜS

VAPPLIED 1e

–

---------

–

⎝⎠

⎜⎟

⎛⎞

VAPPLIED 1

() 1

–

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

–

⎝⎠

⎛⎞

VAPPLIED 1

() 1

–

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

–

⎝⎠

⎛⎞

VCHOLD

VAPPLIED 1e

–

----------

–

⎝⎠

⎜⎟

⎛⎞

VCHOLD

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

;[2] VCHOLD charge response to VAPPLIED

;combining [1] and [2]

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

Solving for TC:

Therefore:

Temperature

50°C and external impedance of 10k

Ω 5.0V VDD

Assumptions:

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

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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