PIC16LC72AT-20SS View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC16LC72AT-20SS

28-Pin 8-Bit CMOS Microcontrollers

Microchip Technology 

PIC16C62B/72A

9.1 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 9-2. 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 ana-

log input (due to pin leakage current). The maximum

recommended impedance for analog sources is 10

kΩ. After the analog input channel is selected

(changed), this acquisition must pass before the con-

version can be started.

To calculate the minimum acquisition time, TACQ, see

Equation 9-1. This equation calculates the acquisition

time to within 1/2 LSb error (512 steps for the A/D). The

1/2 LSb error is the maximum error allowed for the A/D

to meet its specified accuracy.

Note:

When the conversion is started, the hold-

ing capacitor is disconnected from the

input pin.

In general;

Assuming RS = 10kΩ

Vdd = 3.0V (RSS = 10kΩ)

Temp. = 50°C (122°F)

TACQ ≈ 13.0 µSec

By increasing VDD and reducing RS and Temp., TACQ

can be substantially reduced.

FIGURE 9-2: ANALOG INPUT MODEL

Rs ANx

VA

CPIN

5 pF

VDD

VT = 0.6V

VT = 0.6V

RIC ≤ 1k

Sampling

Switch

SS RSS

I leakage

± 500 nA

CHOLD

= DAC capacitance

= 51.2 pF

VSS

Legend CPIN

= input capacitance

VT

= threshold voltage

I leakage = leakage current at the pin due to

various junctions

RIC

SS

CHOLD

= interconnect resistance

= sampling switch

= sample/hold capacitance (from DAC)

6V

5V

VDD 4V

3V

2V

5 6 7 8 9 10 11

RSS

(kΩ)

EQUATION 9-1: ACQUISITION TIME

TACQ =

Amplifier Settling Time +

Hold Capacitor Charging Time +

Temperature Coefficient

= TAMP + TC + TCOFF

TAMP = 5µS

TC = - (51.2pF)(1kΩ + RSS + RS) In(1/511)

TCOFF = (Temp -25°C)(0.05µS/°C)

DS35008B-page 52

Preliminary

© 1999 Microchip Technology Inc.

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