PIC16C773 Microchip Technology, PIC16C773 Datasheet - Page 123

• Manufacturer Part Number: PIC16C773
• Description: 28/40-Pin/ 8-Bit CMOS Microcontrollers w/ 12-Bit A/D
• Manufacturer: Microchip Technology

11.6
11.6.1
The maximum recommended impedance for ana-
log sources is 2.5 k . This value is calculated based
on the maximum leakage current of the input pin. The
leakage current is 100 nA max., and the analog input
voltage cannot be vary by more than 1/4 LSb or
250 mV due to leakage. This places a requirement on
the input impedance of 250 V/100 nA = 2.5 k .
11.6.2
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 11-6: A/D SAMPLING TIME EQUATION
Figure 11-7
required to charge C
the following system assumptions:
1999 Microchip Technology Inc.
C
R
1/4 LSb error
V
Temp (system Max.) = 50 C
Note 1: The reference voltage (V
HOLD
S
DD
= 2.5 k
= 5V
V
A/D Sample Requirements
RECOMMENDED SOURCE IMPEDANCE
SAMPLING TIME CALCULATION
= 25 pF
2: The charge holding capacitor (C
3: The maximum recommended impedance
4: After a conversion has completed, 2 T
HOLD
effect on the equation, since it cancels
itself out.
not discharged after each conversion.
for analog sources is 2.5 k .
required to meet the pin leakage specifi-
cation.
time must be waited before sampling can
begin again. During this time the holding
capacitor is not connected to the selected
A/D input channel.
shows the calculation of the minimum time
=(V
R
SS
REF
= 10 k (worst case)
HOLD
- V
REF
. This calculation is based on
/16384) = (V
HOLD
REF
) must be allowed
Figure
T
) • (1 -e
C
REF
= -C
) has no
11-8. The
Advance Information
HOLD
(-T
HOLD
This is
C
/C (R
(1k + R
) is
AD
IC
+R
SS
+ R
SS
S
+ R
)
) V
source impedance (R
switch (R
required to charge the capacitor C
switch (R
(V
mended impedance for analog sources is 2.5 k .
After the analog input channel is selected (changed)
this sampling must be done before the conversion can
be started.
To
Equation 11-6
that 1/4 LSb error is used (16384 steps for the A/D).
The 1/4 LSb error is the maximum error allowed for the
A/D to meet its specified resolution.
The C
A/D.
DD
REF
S
) In (1/16384)
), see
(1 - 1/16384) = V
calculate
HOLD
SS
SS
) impedance varies over the device voltage
is assumed to be 25 pF for the 12-bit
) impedance directly affect the time
Figure
may be used. This equation assumes
the
REF
11-8. The maximum recom-
• (1 -e
S
) and the internal sampling
minimum
PIC16C77X
(-T
C
/C (R
IC
HOLD
DS30275A-page 123
+R
sampling
SS
. The sampling
+ R
S
)
)
time,