PIC16F7X Datasheet, PDF (88/174 Pages) Microchip Technology – 28/40-Pin 8-Bit CMOS FLASH Microcontrollers

PIC16F7X
11.1 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 11-2. 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), see
Figure 11-2. The source impedance affects the offset
voltage at the analog input (due to pin leakage current).
The maximum recommended impedance for ana-
log sources is 10 kΩ. After the analog input channel is
selected (changed), the acquisition period must pass
before the conversion can be started.
To calculate the minimum acquisition time, TACQ, see
the PICmicro™ Mid-Range MCU Family Reference
Manual (DS33023). In general, however, given a max-
imum source impedance of 10 kΩ and at a temperature
of 100°C, TACQ will be no more than 16 µsec.
FIGURE 11-2:
ANALOG INPUT MODEL
VDD
RS ^ANx
VT = 0.6V
VA
CPIN
5 pF
VT = 0.6V
RIC ≤ 1k
Sampling
Switch
SS _RSS
I leakage
± 500 nA
CHOLD
= DAC Capacitance
= 51.2 pF
VSS
Legend CPIN
VT
= input capacitance
= 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)
TABLE 11-1: TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (C))
AD Clock Source (TAD)
Maximum Device Frequency
Operation
ADCS1:ADCS0
Max.
2TOSC
00
1.25 MHz
8TOSC
01
5 MHz
32TOSC
RC^{(1, 2, 3)}
10
20 MHz
11
(Note 1)
Note 1: The RC source has a typical TAD time of 4 µs but can vary between 2-6 µs.
2: When the device frequencies are greater than 1 MHz, the RC A/D conversion clock source is only
recommended for SLEEP operation.
3: For extended voltage devices (LC), please refer to the Electrical Specifications section.
DS30325B-page 86
 2002 Microchip Technology Inc.