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2002 Microchip Technology Inc.

DS41120B-page 113

PIC16C717/770/771

11.6

A/D Sample Requirements

11.6.1

RECOMMENDED SOURCE

IMPEDANCE

The maximum recommended impedance for ana-

log sources is 2.5 k

. This value is calculated based

on the maximum leakage current of the input pin. The

leakage current is 100 nA max., and the analog input

voltage cannot be varied by more than 1/4 LSb or

250

V due to leakage. This places a requirement on

the input impedance of 250

V/100 nA = 2.5 k.

11.6.2

SAMPLING TIME CALCULATION

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 11-5. 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 11-5. The maximum recommended imped-

ance for analog sources is 2.5 k

. After the analog

input channel is selected (changed) this sampling must

be done before the conversion can be started.

To calculate the minimum sampling time, Equation 11-

2 may be used. This equation assumes that 1/4 LSb

error is used (16384 steps for the A/D). The 1/4 LSb

error is the maximum error allowed for the A/D to meet

its specified resolution.

The CHOLD is assumed to be 25 pF for the 12-bit

A/D.

EXAMPLE 11-2:

A/D SAMPLING TIME

EQUATION

Figure 11-3 shows the calculation of the minimum time

required to charge CHOLD. This calculation is based on

the following system assumptions:

CHOLD = 25 pF

RS = 2.5 k

1/4 LSb error

VDD = 5V

→ RSS = 10 k (worst case)

Temp (system Max.) = 50

°C

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 2.5 k

. This is

required to meet the pin leakage specifi-

cation.

VHOL D

VREF

16384

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

–

VREF

16384

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

–

VREF

() 1 e

–

CHOLD RIC

RSS

()

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

–

VREF 1

16384

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

–

VREF

() 1 e

–

CHOLD RIC

RSS

()

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

–

CHOLD 1kRSS

()

16384

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

–

Solving for TC:

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