MCP19124 datasheet(127/236 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005619A-page 127

MCP19124/5

19.4

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 19-4. 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 19-4.

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 19-1 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 19-1:

ACQUISITION TIME EXAMPLE

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 VDD

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

Note1: 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.

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