PIC16HV785 Microchip Technology Inc., PIC16HV785 Datasheet - Page 88

• Manufacturer Part Number: PIC16HV785
• Description: 20-pin Flash-based 8-bit Cmos Microcontroller
• Manufacturer: Microchip Technology Inc.

PIC16F785/HV785
12.2
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (C
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 12-4. The
source impedance (R
switch (R
required to charge the capacitor C
switch (R
(V
mended impedance for analog sources is 10 k . 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.
EQUATION 12-1:
DS41249D-page 86
DD
Note 1: The reference voltage (V
), see Figure 12-4. The maximum recom-
Assumptions:
The value for Tc can be approximated with the following equations:
Solving for Tc:
Therefore:
2: The charge holding capacitor (C
3: The maximum recommended impedance for analog sources is 10 k . This is required to meet the pin
A/D Acquisition Requirements
SS
SS
) impedance varies over the device voltage
V
Tacq
leakage specification.
) impedance directly affect the time
V
V
T
AP P LIED
T
A PP LIED
A PP LIE D
ACQ
c
=
=
=
=
=
=
=
=
Temperature
–
5µs
–
7.67
ACQUISITION TIME EXAMPLE
1.37
S
10pF 1k
⎛
⎝
C
Amplifier Settling Time
T
5µs
) and the internal sampling
⎛
⎜
⎝
⎛
⎜
⎝
1
AMP
HOLD
1 e
1 e
+
µs
µs
–
–
+
–
1.37µs
----------- -
2047
+
Tc
HOLD
Ric
1
–
--------- -
–
-------- -
RC
RC
Tc
T
Tc
+
HOLD
C
+
⎞
⎠
⎞
⎟
⎠
⎞
⎟
⎠
+
+
+
) must be allowed
REF
7k
=
Temperature - 25°C 0.05µs/°C
T
Rss
=
=
=
. The sampling
COFF
50°C- 25°C 0.05µs/°C
) has no effect on the equation, since it cancels itself out.
50°C and external impedance of 10k
V
V
V
+
+
C HOLD
AP P LI ED
CHOLD
HOLD
10k
Rs
+
) is not discharged after each conversion.
ln(1/2047)
Preliminary
Hold Capacitor Charging Time
ln(0.0004885)
⎛
⎝
1
–
----------- -
2047
1
To
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, T
the “PICmicro
Manual” (DS33023).
⎞
⎠
calculate
;[2] Vchold charge response to Vapplied
;Combining [1] and [2]
;[1] Vchold charged to within 1/2 lsb
®
Mid-Range MCU Family Reference
the
5.0V V
+
Temperature Coefficient
minimum
© 2006 Microchip Technology Inc.
DD
acquisition
ACQ
, see
time,