PIC16C62B/72A
DS35008C-page 52
Preliminary
1998-2013 Microchip Technology Inc.
9.1
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 9-2. 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
). The
source impedance affects the offset voltage at the ana-
log input (due to pin leakage current).
The maximum
recommended impedance for analog sources is 10
k
. After the analog input channel is selected
(changed), this acquisition must pass before the con-
version can be started.
To calculate the minimum acquisition time, T
ACQ
, see
Equation 9-1. This equation calculates the acquisition
time to within 1/2 LSb error (512 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified accuracy.
In general;
Assuming R
S
= 10k
Vdd
= 3.0V (R
SS
= 10k
Temp. = 50
C (122
F)
T
ACQ
13.0
Sec
By increasing V
DD
and reducing R
S
and Temp., T
ACQ
can be substantially reduced.
FIGURE 9-2:
ANALOG INPUT MODEL
EQUATION 9-1:
ACQUISITION TIME
Note:
When the conversion is started, the hold-
ing capacitor is disconnected from the
input pin.
T
ACQ
=
=
Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
T
AMP
+ T
C
+ T
COFF
T
AMP
= 5
S
T
C
=
- (51.2pF)(1k
+ R
SS
+ R
S
) In(1/511)
T
COFF
=
(Temp -25
C)(0.05
S/
C)
C
PIN
VA
Rs
ANx
5 pF
V
DD
V
T
= 0.6V
V
T
= 0.6V
I leakage
R
IC
1k
Sampling
Switch
SS
R
SS
C
HOLD
= DAC capacitance
V
SS
6V
R
SS
5V
4V
3V
2V
5 6 7 8 9 10 11
(k
)
V
DD
= 51.2 pF
± 500 nA
Legend C
PIN
V
T
I leakage
R
IC
SS
C
HOLD
= input capacitance
= threshold voltage
= leakage current at the pin due to
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
various junctions
Summary of Contents for PIC16C62B/72A
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