PIC16F684 datasheet(74/192 Pages) MICROCHIP

74 / 192 page

PIC16F684

DS41202F-page 72

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 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 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

TACQ

Amplifier Settling Time

Hold Capacitor Charging Time

Temperature Coefficient

TAMP

TCOFF

5µs

Temperature - 25°C

() 0.05µs/°C

()

[]

CHOLD RIC

RSS

() ln(1/2047)

–

10pF 1k

Ω 7kΩ 10kΩ

()

–

ln(0.0004885)

1.37

µs

TACQ

5µS

1.37µS

50°C- 25°C

() 0.05µS/°C

()

[]

7.67µS

VAPPLIED 1e

–

---------

–

⎝⎠

⎜⎟

⎛⎞

VAPPLIED 1

2047

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

–

⎝⎠

⎛⎞

VAPPLIED 1

2047

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

–

⎝⎠

⎛⎞

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 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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