PIC18F2523-ISO View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC18F2523-ISO

28/40/44-Pin, Enhanced Flash Microcontrollers with 12-Bit A/D and nanoWatt Technology

Microchip Technology 

PIC18F2423/2523/4423/4523

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 kΩ.

After the analog input channel is selected (changed),

the channel must be sampled for at least the minimum

acquisition time before starting a conversion.

Note:

When the conversion is started, the

holding capacitor is disconnected from the

input pin.

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:

TABLE 2-1: TACQ ASSUMPTIONS

CHOLD

=

25 pF

Rs

=

2.5 kΩ

Conversion Error ≤

1/2 LSb

VDD

=

3V → Rss = 4 kΩ

Temperature

= 85°C (system maximum)

EQUATION 2-1: ACQUISITION TIME

TACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient

= TAMP + TC + TCOFF

EQUATION 2-2: A/D MINIMUM CHARGING TIME

VHOLD =

or

TC

=

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

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

EQUATION 2-3: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME

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.

TC

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

-(25 pF) (1 kΩ + 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

DS39755C-page 30

© 2009 Microchip Technology Inc.

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