PIC16F872 Datasheet, PDF (89/160 Pages) Microchip Technology – 28-Pin, 8-Bit CMOS FLASH Microcontroller

PIC16F872
EQUATION 10-1: ACQUISITION TIME
TACQ =
Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
=
=
TC =
=
=
TACQ =
=
TAMP + TC + TCOFF
2µS + TC + [(Temperature -25°C)(0.05µS/°C)]
CHOLD (RIC + RSS + RS) In(1/2047)
- 120pF (1kΩ + 7kΩ + 10kΩ) In(0.0004885)
16.47µS
2µS + 16.47µS + [(50°C -25×C)(0.05µS/×C)
19.72µS
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 kΩ. This is required to meet the pin leak-
age specification.
4: After a conversion has completed, a 2.0TAD delay must complete before acquisition can begin again.
During this time, the holding capacitor is not connected to the selected A/D input channel.
FIGURE 10-2: ANALOG INPUT MODEL
RS ^ANx
VA
CPIN
5 pF
VDD
VT = 0.6V
VT = 0.6V
Sampling
Switch
RIC ≤ 1k ^SS RSS
I LEAKAGE
± 500 nA
CHOLD
= DAC capacitance
= 120 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
Sampling Switch
( kΩ )
© 1999 Microchip Technology Inc.
Preliminary
DS30221A-page 89