PIC16F870/871
EQUATION 10-1:
T
ACQ
=
ACQUISITION TIME
Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
T
AMP
+ T
C
+ T
COFF
2碌S + T
C
+ [(Temperature -25掳C)(0.05碌S/掳C)]
C
HOLD
(R
IC
+ R
SS
+ R
S
) In(1/2047)
- 120pF (1k鈩?+ 7k鈩?+ 10k鈩? In(0.0004885)
16.47碌S
2碌S + 16.47碌S + [(50掳C -25脳C)(0.05碌S/脳C)
19.72碌S
T
C
T
ACQ
=
=
=
=
=
=
=
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鈩? 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.
FIGURE 10-2: ANALOG INPUT MODEL
V
DD
R
S
VA
ANx
C
PIN
5 pF
V
T
= 0.6V
R
IC
鈮?/div>
1k
I
LEAKAGE
卤 500 nA
Sampling
Switch
SS R
SS
C
HOLD
= DAC capacitance
= 120 pF
V
SS
Legend C
PIN
= input capacitance
V
T
= threshold voltage
I
LEAKAGE
= leakage current at the pin due to
various junctions
R
IC
SS
C
HOLD
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
V
T
= 0.6V
6V
5V
V
DD
4V
3V
2V
5 6 7 8 9 10 11
Sampling Switch
( k鈩?)
漏
1999 Microchip Technology Inc.
Preliminary
DS30569A-page 83
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