PIC17C752-08SLASHP datasheet(171/320 Pages) MICROCHIP

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Preliminary

DS30264A-page 171

PIC17C75X

EXAMPLE 16-1: CALCULATING THE

MINIMUM REQUIRED

ACQUISITION TIME

TACQ = Ampliﬁer Settling Time +

Holding Capacitor Charging Time +

Temperature Coefﬁcient †

† Only required for temperatures

≠ 25°C

TACQ =10

µs + Tcap + [(Temp - 25°C)(0.05 µs/°C)]

TCAP =-CHOLD (RIC + RSS + RS) ln(1/2047)

-200 pF (1 k

Ω + 7 kΩ + 10 kΩ) ln(0.0004885)

-200 pF (18 k

Ω) ln(0.0004885)

-3.6

µs (-7.6241)

27.447

µs

TACQ =10

µs + 27.447 µs + [(50°C - 25°C)(0.05 µs/°C)]

37.447

µs + 1.25 µs

38.697

µs

16.2 Selecting the A/D Conversion Clock

The A/D conversion time per bit is deﬁned as TAD. The

A/D conversion requires a minimum 12TAD per 10-bit

conversion. The source of the A/D conversion clock is

software selected. The four possible options for TAD

are:

•8TOSC

• 32TOSC

• 64TOSC

• Internal RC oscillator

For correct A/D conversions, the A/D conversion clock

(TAD) must be selected to ensure a minimum TAD time

of 1.6

µs.

Table 16-1 and Table 16-2 show the resultant TAD

times derived from the device operating frequencies

and the A/D clock source selected. These times are for

standard voltage range devices.

TABLE 16-1: TAD vs. DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (C))

TABLE 16-2: TAD vs. DEVICE OPERATING FREQUENCIES (EXTENDED VOLTAGE DEVICES (LC))

AD Clock Source (TAD)

Device Frequency

Operation

ADCS1:ADCS0

33 MHz

20 MHz

5 MHz

1.25 MHz

333.33 kHz

8TOSC

242 ns(2)

400 ns(2)

1.6

µs

6.4

µs

32TOSC

970 ns(2)

1.6

µs

6.4

µs

25.6

µs(3)

64TOSC

1.94

µs

3.2

µs

12.8

µs(3)

51.2

µs(3)

192

µs(3)

2 - 6

µs(1, 4)

2 - 6

µs(1, 4)

2 - 6

µs(1, 4)

2 - 6

µs(1, 4)

2 - 6

µs(1)

Legend: Shaded cells are are outside of recommended ranges.

Note 1: The RC source has a typical TAD time of 4

µs.

2: These values violate the minimum required TAD time.

3: For faster conversion times, the selection of another clock source is recommended.

4: When the device frequencies is greater than 1 MHz, the RC A/D conversion clock source is only recom-

mended for sleep operation.

AD Clock Source (TAD)

Device Frequency

Operation

ADCS1:ADCS0

8 MHz

4 MHz

2 MHz

1 MHz

333.33 kHz

8TOSC

1.0

µs(2)

2.0

µs(2)

µs8 µs

µs

32TOSC

4.0

µs8 µs

µs

µs(3)

64TOSC

8.0

µs

µs(3)

192

µs(3)

3 - 9

µs(1, 4)

3 - 9

µs(1, 4)

3 - 9

µs(1, 4)

3 - 9

µs(1)

3 - 9

µs(1)

Legend: Shaded cells are are outside of recommended ranges.

Note 1: The RC source has a typical TAD time of 4

µs.

2: These values violate the minimum required TAD time.

3: For faster conversion times, the selection of another clock source is recommended.

4: When the device frequencies is greater than 1 MHz, the RC A/D conversion clock source is only recom-

mended for sleep operation.

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