PIC18F4423-I/ML【PDF 24/54 页】IC PIC MCU FLASH 8KX16 44QFN

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PIC18F2423/2523/4423/4523

DS39755C-page 30

2009 Microchip Technology Inc.

2.1

A/D Acquisition Requirements

For the A/D Converter 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 2-3.

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). The source impedance affects the offset voltage

at the analog input (due to pin leakage current). The

maximum recommended impedance for analog

sources is 2.5 k

Ω.

After the analog input channel is selected (changed),

the channel must be sampled for at least the minimum

acquisition time before starting a conversion.

To calculate the minimum acquisition time, Equation 2-1

may be used. This equation assumes that 1/2 LSb error

is used (4,096 steps for the A/D). The 1/2 LSb error is the

maximum error allowed for the A/D to meet its specified

resolution.

Example 2-3 shows the calculation of the minimum

required acquisition time, TACQ. This calculation is

based on the application system assumptions shown in

Table 2-1:

EQUATION 2-1:

ACQUISITION TIME

EQUATION 2-2:

A/D MINIMUM CHARGING TIME

EQUATION 2-3:

CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME

Note:

When the conversion is started, the

holding capacitor is disconnected from the

input pin.

TABLE 2-1:

TACQ ASSUMPTIONS

CHOLD

25 pF

2.5 k

Conversion Error

≤

1/2 LSb

VDD

=3V

→ Rss = 4 kΩ

Temperature

°C (system maximum)

TACQ

Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient

=TAMP + TC + TCOFF

VHOLD

(VREF – (VREF/4096)) (1 – e(-TC/CHOLD(RIC + RSS + RS)))

-(CHOLD)(RIC + RSS + RS) ln(1/4096)

TACQ

=TAMP + TC + TCOFF

TAMP

=0.2

μs

TCOFF

(Temp – 25

°C)(0.02 μs/°C)

(85

°C – 25°C)(0.02 μs/°C)

1.2

μs

Temperature coefficient is only required for temperatures > 25

°C. Below 25°C, TCOFF = 0 ms.

-(CHOLD)(RIC + RSS + RS) ln(1/4095)

μs

-(25 pF) (1 k

Ω + 4 kΩ + 2.5 kΩ) ln(0.0004883) μs

1.56

μs

TACQ

=0.2

μs + 1.56 μs + 1.2 μs

2.96

μs

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