PIC16C745/P Microchip Technology, PIC16C745/P Datasheet - Page 92

• Manufacturer Part Number: PIC16C745/P
• Description: 8-Bit CMOS Microcontrollers with USB
• Manufacturer: Microchip Technology

PIC16C745/765
12.1
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (C
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 12-2. The
source impedance (R
switch (R
required to charge the capacitor C
switch (R
(V
offset voltage at the analog input (due to pin leakage
current).
FIGURE 12-2: ANALOG INPUT MODEL
EQUATION 12-1:
DS41124A-page 92
T
DD
ACQ
), Figure 12-2. The source impedance affects the
Legend C
A/D Acquisition Requirements
=
=
SS
SS
) impedance varies over the device voltage
) impedance directly affect the time
Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
T
T
T
T
AMP
AMP
C
COFF
V
I leakage
R
SS
C
T
PIN
IC
HOLD
= - (51.2pF)(1k
+ T
= 5 S
= (Temp -25 C)(0.05 S/ C)
VA
C
ACQUISITION TIME
S
) and the internal sampling
+ T
Rs
= input capacitance
= threshold voltage
= leakage current at the pin due to
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
various junctions
COFF
ANx
C
5 pF
HOLD
PIN
+ R
HOLD
) must be allowed
SS
. The sampling
+ R
Advanced Information
S
) In(1/511)
V
DD
V
V
T
T
= 0.6V
= 0.6V
I leakage
± 500 nA
The maximum recommended impedance for ana-
log sources is 10 k . After the analog input channel is
selected (changed), the acquisition must pass before
the conversion can be started.
To
Equation 12-1 may be used. This equation assumes
that 1/2 LSb error is used (512 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, T
the PICmicro™ Mid-Range MCU Family Reference
Manual (DS33023). In general, however, given a max
of 10k and a worst case temperature of 100°C, T
will be no more than 16µsec.
R
IC
calculate
1k
V
SS R
Sampling
Switch
DD
6V
5V
4V
3V
2V
SS
the
Sampling Switch
5 6 7 8 9 10 11
V
minimum
C
= DAC capacitance
= 51.2 pF
SS
(k )
HOLD
1999 Microchip Technology Inc.
acquisition
ACQ
, see
time,
ACQ