PIC16C770 Microchip Technology Inc., PIC16C770 Datasheet - Page 115

• Manufacturer Part Number: PIC16C770
• Description: 18/20-pin, 8-bit Cmos Microcontrollers With 10/12-bit A/d
• Manufacturer: Microchip Technology Inc.

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 varied by more than 1/4 LSb or
250 V 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 11-5. The source
impedance (R
impedance directly affect the time required to charge
the capacitor C
impedance varies over the device voltage (V
Figure 11-5. The maximum recommended imped-
ance 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 calculate the minimum sampling time, Equation 11-
2 may be used. This equation assumes 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.
2002 Microchip Technology Inc.
HOLD
A/D Sample Requirements
RECOMMENDED SOURCE
IMPEDANCE
SAMPLING TIME CALCULATION
is assumed to be 25 pF for the 12-bit
S
) and the internal sampling switch (R
HOLD
. The sampling switch (R
HOLD
) must be allowed
DD
), see
SS
SS
)
)
EXAMPLE 11-2:
Figure 11-3 shows the calculation of the minimum time
required to charge C
the following system assumptions:
V
V
V
Solving for T
T
C
R
1/4 LSb error
V
Temp (system Max.) = 50 C
Note 1: The reference voltage (V
C
H OL D
R EF
R EF
DD
HOLD
S
PIC16C717/770/771
=
= 2.5 k
–
–
= 5V
1
C
--------------- -
16384
V
=
–
H O LD
R EF
= 25 pF
2: The charge holding capacitor (C
3: The maximum recommended impedance
--------------- -
16384
V
R E F
C
1
effect on the equation, since it cancels
itself out.
not discharged after each conversion.
for analog sources is 2.5 k . This is
required to meet the pin leakage specifi-
cation.
:
=
1k
–
R
--------------- -
16384
V
=
V
+
SS
R E F
R E F
R
V
S S
= 10 k (worst case)
R E F
HOLD
+
A/D SAMPLING TIME
EQUATION
1 e
R
–
S
1 e
. This calculation is based on
–
ln
------------------------------------------------------------------ -
C
H O LD
--------------- -
16384
------------------------------------------------------------------ -
C
H O LD
1
R
I C
DS41120B-page 113
–
T
R
+
C
I C
–
R
T
+
REF
S S
C
R
+
SS
) has no
R
+
HOLD
S
R
S
) is