PIC16F/LF1946/47
source 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 minimum acquisition time, Equation 15-1 may be
used. This equation assumes that 1/2 LSb error is used
(1,024 steps for the ADC). The 1/2 LSb error is the
maximum error allowed for the ADC to meet its
specified resolution.
15.3 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 15-4. 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), refer
to Figure 15-4. The maximum recommended
impedance for analog sources is 10 k. As the
EQUATION 15-1: ACQUISITION TIME EXAMPLE
Temperature = 50°C and external impedance of 10k 5.0V VDD
Assumptions:
TACQ = Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient
= TAMP + TC + TCOFF
= 2µs + TC + Temperature - 25°C0.05µs/°C
The value for TC can be approximated with the following equations:
1
;[1] VCHOLD charged to within 1/2 lsb
VAPPLIED1 – -------------------------- = VCHOLD
2n + 1 – 1
–TC
---------
RC
VAPPLIED 1 – e
= VCHOLD
;[2] VCHOLD charge response to VAPPLIED
;combining [1] and [2]
–Tc
--------
RC
1
= VAPPLIED1 – --------------------------
2n + 1 – 1
VAPPLIED 1 – e
Note: Where n = number of bits of the ADC.
Solving for TC:
TC = –CHOLDRIC + RSS + RS ln(1/511)
= –10pF1k + 7k + 10k ln(0.001957)
= 1.12µs
Therefore:
TACQ = 2µs + 1.12µs + 50°C- 25°C0.05µs/°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 k. This is required to meet the pin
leakage specification.
2010 Microchip Technology Inc.
Preliminary
DS41414A-page 163
MICROCHIP [ MICROCHIP ]
