PIC16F87_05 Datasheet, PDF (119/228 Pages) Microchip Technology – 18/20/28-Pin Enhanced Flash Microcontrollers with nanoWatt Technology

PIC16F87/88
12.1 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 ana-
log input model is shown in Figure 12-2. 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 12-2.
The maximum recommended impedance for analog
sources is 10 kΩ. As the impedance 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 12-1 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 “PICmicro^® Mid-Range MCU Family Reference
Manual” (DS33023).
EQUATION 12-1: ACQUISITION TIME
TACQ = Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient
TC
TACQ
= TAMP + TC + TCOFF
= 2 µs + TC + [(Temperature -25°C)(0.05 µs/°C)]
= CHOLD (RIC + RSS + RS) In(1/2047)
= -120 pF (1 kΩ + 7 kΩ + 10 kΩ) In(0.0004885)
= 16.47 µs
= 2 µs + 16.47 µs + [(50°C – 25°C)(0.05 µs/°C)
= 19.72 µs
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
leakage specification.
4: After a conversion has completed, a 2.0 TAD 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 12-2:
ANALOG INPUT MODEL
RS ^ANx
VDD
VT = 0.6V
Sampling
Switch
RIC ≤ 1K SS RSS
VA
CPIN
5 pF
VT = 0.6V
ILEAKAGE
±500 nA
CHOLD
= DAC Capacitance
= 120 pF
VSS
Legend: CPIN
VT
ILEAKAGE
RIC
SS
CHOLD
= Input Capacitance
= Threshold Voltage
= Leakage Current at the pin due to
various junctions
= Interconnect Resistance
= Sampling Switch
= Sample/Hold Capacitance (from DAC)
6V
5V
VDD 4V
3V
2V
5 6 7 8 9 10 11
Sampling Switch
(kΩ)
 2005 Microchip Technology Inc.
DS30487C-page 117