PIC16F946 Datasheet, PDF (157/274 Pages) Microchip Technology – 64-Pin Flash-Based, 8-Bit CMOS Microcontrollers with LCD Driver and nanoWatt Technology

PIC16F946
12.2 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 12-4. 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 12-4. The maximum recom-
mended 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
= TAMP + TC + TCOFF
= 2µs + TC + [(Temperature - 25°C)(0.05µs/°C)]
Where CHOLD is charged to within 1/2 lsb:
V
A
P
P
LI
E
D
⎛
⎝
1
–
2----0-1--4---7-^⎠⎞
=
VCHOLD
⎛
VAPPLIED⎜1
–
e ^{–-R---T-C--C--}⎟⎞
=
VCHOLD
⎝
⎠
;[1] VCHOLD charged to within 1/2 lsb
;[2] VCHOLD charge response to VAPPLIED
⎛
VAPPLIED⎜1
–
e ^{–-R---T-C--c-} ⎟⎞
⎝
⎠
=
V
A
P
P
L
I
E
D
⎛
⎝
1
–
2---0--1-4---7--^⎠⎞
;combining [1] and [2]
Solving for TC:
TC = –CHOLD(RIC + RSS + RS ) ln(1/2047)
= –10pF(1kΩ + 7kΩ + 10kΩ) ln(0.0004885)
= 1.37µs
Therefore:
TACQ = 2µS + 1.37µS + [(50°C- 25°C)(0.05µS/°C)]
= 4.67µS
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.
© 2005 Microchip Technology Inc.
Preliminary
DS41265A-page 155