PIC16F72X/PIC16LF72X
impedance is decreased, the acquisition time may be
decreased. After the analog input channel is selected
(or changed), an A/D acquisition must be done before
the conversion can be started. To calculate the mini-
mum acquisition time, Equation 9-1 may be used. This
equation assumes that 1/2 LSb error is used (256 steps
for the ADC). The 1/2 LSb error is the maximum error
allowed for the ADC to meet its specified resolution.
9.3
A/D Acquisition Requirements
For the ADC 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 9-3. The source imped-
ance (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), refer
to Figure 9-3. The maximum recommended imped-
ance for analog sources is 10 kΩ. As the source
EQUATION 9-1:
ACQUISITION TIME EXAMPLE
Temperature = 50°C and external impedance of 10kΩ 5.0V VDD
Assumptions:
TACQ = Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient
= TAMP + TC + TCOFF
= 2µs + TC + [(Temperature - 25°C)(0.05µs/°C)]
The value for TC can be approximated with the following equations:
1
⎛
⎞
;[1] VCHOLD charged to within 1/2 lsb
VAPPLIED 1 – -------------------------- = VCHOLD
(2n + 1) – 1
⎝
⎠
–TC
---------
⎛
⎞
VAPPLIED 1 – e RC = VCHOLD
⎜
⎝
⎟
⎠
;[2] VCHOLD charge response to VAPPLIED
;combining [1] and [2]
–Tc
--------
⎛
⎞
1
VAPPLIED 1 – eRC = VAPPLIED 1 – --------------------------
⎛
⎞
⎠
⎜
⎝
⎟
⎠
(2n + 1) – 1
⎝
Note: Where n = number of bits of the ADC.
Solving for TC:
TC = –CHOLD(RIC + RSS + RS) ln(1/511)
= –10pF(1kΩ + 7kΩ + 10kΩ) ln(0.001957)
= 1.12µs
Therefore:
TACQ = 2ΜS + 1.12ΜS + [(50°C- 25°C)(0.05ΜS/°C)]
= 4.42ΜS
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.
DS41341E-page 106
© 2009 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
