PIC16F88

Manufacturer Part NumberPIC16F88
ManufacturerMicrochip Technology Inc.
PIC16F88 datasheet
 
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Page 119/228:

A/D Acquisition Requirements

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12.1

A/D Acquisition Requirements

For the A/D converter to meet its specified accuracy, the
charge holding capacitor (C
) must be allowed to
HOLD
fully charge to the input channel voltage level. The ana-
log input model is shown in Figure 12-2. 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 12-2.
DD
The maximum recommended impedance for analog
sources is 10 k . As the impedance is decreased, the
EQUATION 12-1:
ACQUISITION TIME
T
=
Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient
ACQ
=
T
+ T
+ T
AMP
C
COFF
=
2 s + T
+ [(Temperature -25°C)(0.05 s/°C)]
C
=
T
C
(R
+ R
+ R
) In(1/2047)
C
HOLD
IC
SS
S
=
-120 pF (1 k + 7 k + 10 k ) In(0.0004885)
=
16.47 s
T
=
2 s + 16.47 s + [(50°C – 25 C)(0.05 s/ C)
ACQ
=
19.72 s
Note 1: The reference voltage (V
REF
2: The charge holding capacitor (C
3: The maximum recommended impedance for analog sources is 10 k . This is required to meet the pin
leakage specification.
4: After a conversion has completed, a 2.0 T
During this time, the holding capacitor is not connected to the selected A/D input channel.
FIGURE 12-2:
ANALOG INPUT MODEL
ANx
R
S
C
PIN
VA
5 pF
Legend: C
= Input Capacitance
PIN
V
= Threshold Voltage
T
I
= Leakage Current at the pin due to
LEAKAGE
various junctions
R
= Interconnect Resistance
IC
SS
= Sampling Switch
C
= Sample/Hold Capacitance (from DAC)
HOLD
 2005 Microchip Technology Inc.
acquisition time may be decreased. After the analog
input channel is selected (changed), this acquisition
must be done before the conversion can be started.
To
calculate
Equation 12-1 may be used. This equation assumes
that 1/2 LSb error is used (1024 steps for the A/D). The
)
SS
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
) impedance
SS
To calculate the minimum acquisition time, T
the “PICmicro
Manual” (DS33023).
) has no effect on the equation, since it cancels itself out.
) is not discharged after each conversion.
HOLD
delay must complete before acquisition can begin again.
AD
V
DD
Sampling
Switch
V
= 0.6V
T
SS R
R
1K
IC
I
LEAKAGE
V
= 0.6V
T
±500 nA
6V
5V
V
4V
DD
3V
2V
PIC16F87/88
the
minimum
acquisition
time,
, see
ACQ
®
Mid-Range MCU Family Reference
SS
C
HOLD
= DAC Capacitance
= 120 pF
V
SS
5 6 7 8 9 10 11
Sampling Switch
(k )
DS30487C-page 117