PIC16F631/677/685/687/689/690
8.7
Analog Input Connection
Considerations
Note 1: When reading a PORT register, all pins
configured as analog inputs will read as a
‘0’. Pins configured as digital inputs will
convert as an analog input, according to
the input specification.
A simplified circuit for an analog input is shown in
Figure 8-6. Since the analog input pins share their con-
nection with a digital input, they have reverse biased
ESD protection diodes to VDD and VSS. The analog
input, therefore, must be between VSS and VDD. If the
input voltage deviates from this range by more than
0.6V in either direction, one of the diodes is forward
biased and a latch-up may occur.
2: Analog levels on any pin defined as a
digital input, may cause the input buffer to
consume more current than is specified.
A maximum source impedance of 10 kΩ is recommended
for the analog sources. Also, any external component
connected to an analog input pin, such as a capacitor or
a Zener diode, should have very little leakage current to
minimize inaccuracies introduced.
FIGURE 8-6:
ANALOG INPUT MODEL
VDD
VT ≈ 0.6V
RIC
Rs < 10K
To ADC Input
AIN
ILEAKAGE
±500 nA
CPIN
5 pF
VA
VT ≈ 0.6V
Vss
Legend: CPIN
= Input Capacitance
ILEAKAGE = Leakage Current at the pin due to various junctions
RIC
RS
VA
= Interconnect Resistance
= Source Impedance
= Analog Voltage
VT
= Threshold Voltage
DS41262D-page 100
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
