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PIC18F2X1X/4X1X

DS39636D-page 218

2009 Microchip Technology Inc.

18.1

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 18-3. 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.

calculate

the

minimum

acquisition

time,

Equation 18-1 may be used. This equation assumes

that 1/2 LSb error is used (1024 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 18-3 shows the calculation of the minimum

required acquisition time TACQ. This calculation is

based

the

following

application

system

assumptions:

CHOLD

25 pF

2.5 k

Conversion Error

≤

1/2 LSb

VDD

=5V

→ Rss = 2 kΩ

Temperature

°C (system max.)

EQUATION 18-1:

ACQUISITION TIME

EQUATION 18-2:

A/D MINIMUM CHARGING TIME

EQUATION 18-3:

CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME

Note:

When the conversion is started, the

holding capacitor is disconnected from the

input pin.

TACQ

Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient

=TAMP + TC + TCOFF

VHOLD

(VREF – (VREF/2048)) (1 – e(-TC/CHOLD(RIC + RSS + RS)))

-(CHOLD)(RIC + RSS + RS) ln(1/2048)

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(1/2047)

-(25 pF) (1 k

Ω + 2 kΩ + 2.5 kΩ) ln(0.0004883)

1.05

μs

TACQ

=0.2

μs + 1 μs + 1.2 μs

2.4

μs

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