
PIC16F87X
DS30292C-page 114
2001 Microchip Technology Inc.
11.1
A/D Acquisition Requirements
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (C
HOLD
) must be allowed
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 11-2. The source
impedance (R
S
) and the internal sampling switch (R
SS
)
impedance directly affect the time required to charge
the capacitor C
HOLD
. The sampling switch (R
SS
)
impedance varies over the device voltage (V
DD
), see
Figure 11-2.
The maximum recommended imped-
ance 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.
To
calculate
the
minimum
Equation 11-1 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.
To calculate the minimum acquisition time, T
ACQ
, see
the
PICmicro
Mid-Range
(DS33023).
acquisition
time,
Reference
Manual
EQUATION 11-1:
ACQUISITION TIME
FIGURE 11-2:
ANALOG INPUT MODEL
T
ACQ
T
C
T
ACQ
=
=
=
=
=
=
=
=
Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
T
AMP
+ T
C
+ T
COFF
2
μ
s + T
C
+ [(Temperature -25
°
C)(0.05
μ
s/
°
C)]
C
HOLD
(R
IC
+ R
SS
+ R
S
) In(1/2047)
- 120pF (1k
+ 7k
+ 10k
) In(0.0004885)
16.47
μ
s
2
μ
s + 16.47
μ
s + [(50
°
C -25
°
C)(0.05
μ
s/
°
C)
19.72
μ
s
Note 1:
The reference voltage (V
REF
) has no effect on the equation, since it cancels itself out.
2:
The charge holding capacitor (C
HOLD
) is not discharged after each conversion.
3:
The maximum recommended impedance for analog sources is 10 k
. This is required to meet the pin leak-
age specification.
4:
After a conversion has completed, a 2.0T
AD
delay must complete before acquisition can begin again.
During this time, the holding capacitor is not connected to the selected A/D input channel.
C
PIN
5 pF
VA
R
S
ANx
V
DD
V
T
= 0.6V
V
T
= 0.6V
I
± 500 nA
R
IC
≤
1k
Sampling
Switch
SS R
SS
C
= DAC capacitance
= 120 pF
V
SS
6V
5V
4V
3V
2V
Sampling Switch
(k
)
5 6 7 8 91011
V
DD
Legend
C
PIN
V
T
I
LEAKAGE
R
IC
SS
C
HOLD
= input capacitance
= threshold voltage
= leakage current at the pin due to
various junctions
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)