PIC16F688

Manufacturer Part NumberPIC16F688
ManufacturerMicrochip Technology Inc.
PIC16F688 datasheet
 


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Page 75/202:

A/D Acquisition Requirements

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8.3

A/D Acquisition Requirements

For the ADC to meet its specified accuracy, the charge
holding capacitor (C
) must be allowed to fully
HOLD
charge to the input channel voltage level. The Analog
Input model is shown in Figure 8-4. The source
impedance (R
) and the internal sampling switch (R
S
impedance directly affect the time required to charge the
capacitor C
. The sampling switch (R
HOLD
varies over the device voltage (V
), see Figure 8-4.
DD
The maximum recommended impedance for analog
sources is 10 kΩ. As the source impedance is
decreased, the acquisition time may be decreased.
After the analog input channel is selected (or changed),
an A/D acquisition must be done before the conversion
EQUATION 8-1:
ACQUISITION TIME EXAMPLE
Temperature
Assumptions:
T
=
Amplifier Settling Time
ACQ
=
T
+
T
+
AMP
C
[
=
2µs
+
T
+
C
The value for T
can be approximated with the following equations:
C
1
----------- -
V
1
AP PLIE D
2047
T
C
--------- -
RC
V
1 e
AP P LI ED
Tc
-------- -
RC
V
1 e
AP P LIED
Solving for T
:
C
(
T
=
C
R
+
C
IC
HOLD
(
Ω
=
10pF 1k
+
=
1.37
µs
Therefore:
T
=
+
1.37µ
+
ACQ
S
S
=
4.67µ
S
Note 1: The reference voltage (V
REF
2: The charge holding capacitor (C
3: The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin
leakage specification.
© 2007 Microchip Technology Inc.
can be started. To calculate the minimum acquisition
time, Equation 8-1 may be used. This equation
assumes that 1/2 LSb error is used (1024 steps for the
ADC). The 1/2 LSb error is the maximum error allowed
for the ADC to meet its specified resolution.
)
SS
) impedance
SS
=
50°C and external impedance of 10k
+
Hold Capacitor Charging Time
T
COFF
(
) 0.05µs/°C
(
)
]
Temperature - 25°C
=
V
;[1] V
CHOLD
;[2] V
=
V
CHOLD
1
----------- -
;combining [1] and [2]
=
V
1
A P PLIE D
2047
) ln(1/2047)
R
+
R
SS
S
Ω
Ω
)
7k
+
10k
ln(0.0004885)
[
(
) 0.05µ
(
)
]
50°C- 25°C
/°C
S
) has no effect on the equation, since it cancels itself out.
) is not discharged after each conversion.
HOLD
PIC16F688
Ω
5.0V V
DD
+
Temperature Coefficient
charged to within 1/2 lsb
CHOLD
charge response to V
CHOLD
APPLIED
DS41203D-page 73