PIC16F684 datasheet(70/164 Pages) MICROCHIP

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PIC16F684

DS41202C-page 68

Preliminary

 2004 Microchip Technology Inc.

9.2

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 ana-

log input model is shown in Figure 9-4. 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) imped-

ance varies over the device voltage (VDD), see

Figure 9-4. The maximum recommended impedance

for analog sources is 10 k

Ω. As the impedance is

decreased, the acquisition time may be decreased.

After the analog input channel is selected (changed),

this 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 (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.

To calculate the minimum acquisition time, TACQ, see

the “PICmicro® Mid-Range MCU Family Reference

Manual” (DS33023).

EQUATION 9-1:

ACQUISITION TIME

FIGURE 9-4:

ANALOG INPUT MODEL

Note 1: The reference voltage (VREF) has no effect on the equation, since it cancels itself out.

2: The charge holding capacitor (CHOLD) is not discharged after each conversion.

3: The maximum recommended impedance for analog sources is 10 k

Ω. This is required to meet the pin

leakage specification.

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

= TAMP + TC + TCOFF

= 2

µs + TC + [(Temperature -25°C)(0.05 µs/°C)]

TC = CHOLD (RIC + RSS + RS) In(1/2047)

= -120 pF (1 k

Ω + 7 kΩ + 10 kΩ) In(0.0004885)

= 16.47

µs

TACQ = 2

µs + 16.47 µs + [(50°C-25°C)(0.05 µs/°C)]

= 19.72

µs

CPIN

ANx

5 pF

VDD

VT = 0.6V

I LEAKAGE

RIC

≤ 1k

Sampling

Switch

SS RSS

CHOLD

= DAC capacitance

VSS

Sampling Switch

567 8 9 10 11

Ω)

VDD

= 120 pF

± 500 nA

Legend: CPIN

I LEAKAGE

RIC

CHOLD

= Input Capacitance

= Threshold Voltage

= Leakage Current at the pin due to

= Interconnect Resistance

= Sampling Switch

= Sample/Hold Capacitance (from DAC)

various junctions

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