PIC16F720-ESLASHML datasheet(83/244 Pages) MICROCHIP

83 / 244 page

 2010 Microchip Technology Inc.

DS41430A-page 83

PIC16F/LF720/721

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 9-3. 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 9-3.

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 9-1 may be

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

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

maximum error allowed for the ADC to meet its

specified resolution. It is noted that if the device is

operated at or below 2.0V VDD with the FRC clock

selected for the ADC and if the analog input changes

by more than 1 or 2 LSBs from the previous

conversion, then the use of at least 16

s TACQ time is

recommended.

EQUATION 9-1:

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)

–

20pF 1k

 7k 10k



–

ln(0.001957)

2.25

µs

TACQ

2µs

2.25µs

50°C- 25°C

 0.05µs/°C





5.5µs

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.

Note: TCOFF is zero for temperatures below 25 degrees C.

PIC16F720-E/ML 数据表(PDF) 83 Page - Microchip Technology

PIC16F720-E/ML 数据表(HTML) 83 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

数据表下载