RFPIC12F675 Datasheet, PDF (43/136 Pages) Microchip Technology – FLASH-Based Microcontroller with ASK/FSK Transmitter

rfPIC12F675
7.2 A/D Acquisition Requirements
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (CHOLD) must be allowed
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 7-3. The source
impedance (RS) and the internal sampling switch (RSS)
impedance directly affect the time required to charge
the capacitor CHOLD. The sampling switch (RSS)
impedance varies over the device voltage (VDD), see
Figure 7-3. The maximum recommended imped-
ance for analog sources is 10 k. As the impedance
EQUATION 7-1: ACQUISITION TIME
TACQ
= Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
TC
TACQ
= TAMP + TC + TCOFF
= 2s + TC + [(Temperature -25°C)(0.05s/°C)]
= CHOLD (RIC + RSS + RS) In(1/2047)
= - 120pF (1k + 7k + 10k) In(0.0004885)
= 16.47s
= 2s + 16.47s + [(50°C -25C)(0.05s/C)
= 19.72s
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 acquisition time,
Equation 7-1 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, TACQ, see
the PIC Mid-Range Reference Manual (DS33023).
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 k. This is required to meet the pin 
leakage specification.
FIGURE 7-3:
ANALOG INPUT MODEL
RS ^ANx
VDD
VT = 0.6V
VA
CPIN
5 pF
VT = 0.6V
Sampling
Switch
RIC  1K ^SS RSS
I LEAKAGE
± 500 nA
CHOLD
= DAC capacitance
= 120 pF
VSS
Legend
CPIN
= input capacitance
VT
= threshold voltage
I LEAKAGE = leakage current at the pin due to
various junctions
RIC
= interconnect resistance
SS
= sampling switch
CHOLD = sample/hold capacitance (from DAC)
6V
5V
VDD 4V
3V
2V
5 6 7 8 9 10 11
Sampling Switch
(k)
 2003-2013 Microchip Technology Inc.
Preliminary
DS70091B-page 43