PIC17C756A MICROCHIP [Microchip Technology], PIC17C756A Datasheet - Page 183

• Manufacturer Part Number: PIC17C756A
• Description: High-Performance 8-bit CMOS EPROM Microcontrollers with 10-bit A/D
• Manufacturer: MICROCHIP [Microchip Technology]

16.1
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 16-3. The
source impedance (R
switch (R
required to charge the capacitor C
switch (R
(V
impedance for analog sources is 10 k . As the
impedance is decreased, the acquisition time may be
decreased. After the analog input channel is selected
(changed) this acquisition must be done before the
conversion can be started.
EQUATION 16-1:
EQUATION 16-2:
EXAMPLE 16-1:
T
T
Temperature coefficient is only required for temperatures > 25 C.
T
T
T
V
or
T
DD
ACQ
2000 Microchip Technology Inc.
ACQ
C
ACQ
ACQ
Note 1: The reference voltage (V
C
HOLD
=
), Figure 16-3. The maximum recommended
=
=
=
=
=
A/D Acquisition Requirements
SS
2: The charge holding capacitor (C
3: The maximum recommended impedance for analog sources is 10 k . This is required to meet the pin
4: After a conversion has completed, a 2.0 T
SS
) impedance varies over the device voltage
leakage specification.
During this time, the holding capacitor is not connected to the selected A/D input channel.
) impedance directly affect the time
=
=
Amplifier Settling Time +
Holding Capacitor Charging Time +
Temperature Coefficient
T
T
2 s + Tc + [(Temp - 25 C)(0.05 s/ C)]
-C
-120 pF (1 k + 7 k + 10 k ) ln(0.0004885)
-120 pF (18 k ) ln(0.0004885)
-2.16 s (-7.6241)
16.47 s
2 s + 16.47 s + [(50×C - 25 C)(0.05 s C)]
18.447 s + 1.25 s
19.72 s
AMP
AMP
HOLD
(V
-(120 pF)(1 k + R
+ T
+ T
REF
(R
ACQUISITION TIME
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
S
C
C
IC
) and the internal sampling
- (V
+ T
+ T
+ R
REF
COFF
COFF
SS
HOLD
/2048)) • (1 - e
+ R
HOLD
S
) must be allowed
) ln(1/2047)
REF
SS
. The sampling
+ R
) has no effect on the equation since it cancels itself out.
S
HOLD
) ln(1/2047)
(-Tc/C
) is not discharged after each conversion.
HOLD
AD
(R
delay must complete before acquisition can begin again.
IC
+ R
To
Equation 16-1 may be used. This equation assumes
that 1/2 LSb error is used (1024 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
Example 16-1 shows the calculation of the minimum
required acquisition time (T
following application system assumptions.
C
Rs
Conversion Error
V
Temperature
V
SS
DD
HOLD
HOLD
+ R
calculate
S
))
)
the
=
=
=
=
=
minimum
PIC17C7XX
ACQ
120 pF
10 k
1/2 LSb
5V
(see graph in Figure 16-3)
50 C (system max.)
0V @ time = 0
). This is based on the
Rss = 7 k
DS30289B-page 183
acquisition
time,