Datasheet
PIC16F7X
DS30325B-page 86 2002 Microchip Technology Inc.
11.1 A/D Acquisition Requirements
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (C
HOLD) 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 (R
S) and the internal sampling switch (RSS)
impedance directly affect the time required to charge
the capacitor C
HOLD. The sampling switch (RSS)
impedance varies over the device voltage (V
DD), 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, T
ACQ, 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, T
ACQ will be no more than 16 µsec.
FIGURE 11-2: ANALOG INPUT MODEL
TABLE 11-1: T
AD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (C))
CPIN
VA
R
S
ANx
5 pF
V
DD
VT = 0.6V
V
T = 0.6V
I leakage
R
IC ≤ 1k
Sampling
Switch
SS
R
SS
CHOLD
= DAC Capacitance
V
SS
6V
Sampling Switch
5V
4V
3V
2V
567891011
(k
Ω)
VDD
= 51.2 pF
± 500 nA
Legend CPIN
VT
I leakage
R
IC
SS
C
HOLD
= input capacitance
= threshold voltage
= leakage current at the pin due to
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
various junctions
AD Clock Source (TAD) Maximum Device Frequency
Operation ADCS1:ADCS0 Max.
2T
OSC 00 1.25 MHz
8TOSC 01 5 MHz
32TOSC 10 20 MHz
RC
(1, 2, 3)
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.