41594B datasheet(135/372 Pages) MICROCHIP

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Preliminary

DS41594B-page 135

PIC12LF1840T48A

16.3

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 16-4. 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), refer

Figure 16-4.

The

maximum

recommended

impedance for analog sources is 10 k

. 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 16-1 may be

used. This equation assumes that 1/2 LSb error is used

(1,024 steps for the ADC). The 1/2 LSb error is the

maximum error allowed for the ADC to meet its

specified resolution.

EQUATION 16-1:

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

 7k 10k



–

ln(0.001957)

1.12

µs

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

 5.0V VDD

Assumptions:

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

TACQ

2µs

1.12µs

50°C- 25°C

 0.05µs/°C





4.42µs

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

. This is required to meet the pin

leakage specification.

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