PIC16F87X
EQUATION 11-1: ACQUISITION TIME
TACQ
=
Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
=
=
=
=
=
=
=
TAMP + TC + TCOFF
2µS + TC + [(Temperature -25°C)(0.05µS/°C)]
CHOLD (RIC + RSS + RS) In(1/2047)
- 120pF (1kΩ + 7kΩ + 10kΩ) In(0.0004885)
16.47µS
2µS + 16.47µS + [(50°C -25×C)(0.05µS/×C)
19.72µS
TC
TACQ
Note 1: The reference voltage (VREF) has no effect on the equation, since it cancels itself out.
2: The charge holding capacitor (CHOLD) is not discharged after each conversion.
3: The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin leak-
age specification.
4: After a conversion has completed, a 2.0TAD 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 11-2: ANALOG INPUT MODEL
VDD
Sampling
Switch
VT = 0.6V
ANx
SS
RIC ≤ 1k
RSS
RS
CHOLD
CPIN
5 pF
= DAC capacitance
= 120 pF
VA
I LEAKAGE
VT = 0.6V
± 500 nA
VSS
Legend CPIN
VT
= input capacitance
= threshold voltage
6V
5V
I LEAKAGE = leakage current at the pin due to
VDD 4V
3V
various junctions
2V
RIC
SS
= interconnect resistance
= sampling switch
CHOLD
= sample/hold capacitance (from DAC)
5 6 7 8 9 1011
Sampling Switch
( kΩ )
1999 Microchip Technology Inc.
DS30292B-page 115
MICROCHIP [ MICROCHIP ]
