OPTION_REG datasheet(129/222 Pages) MICROCHIP

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DS22331A-page 129

MCP19111

22.4

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 22-4. 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

to Figure 22-4.

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 22-1 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 22-1:

ACQUISITION TIME EXAMPLE

Note 1: The charge holding capacitor (CHOLD) is not discharged after each conversion.

2: The maximum recommended impedance for analog sources is 10 k

. This is required to meet the pin

leakage specification.

ACQ

Amplifier Settling Time

Hold Capacitor Charging Time

Temperature Coefficient

AMP

COFF

2 µs

Temperature - 25°C

 0.05 µs/°C





HOLD

 ln(1/2047)

–

10 pF 1 k

 7 k 10 k



–

ln(0.0004885)

1.37

=µs

APPLIED

–

----------

–







APPLIED

 1

–

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

–







APPLIED

 1

–

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

–







CHOLD

APPLIED

–

----------

–







CHOLD

;[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 10 k

 5.0V V

Assumptions:

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

ACQ

2 µs

1.37µs

50°C- 25°C

 0.05µs/°C





4.67 µs

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