PIC18F87K90 Datasheet, PDF (381/566 Pages) Microchip Technology – 64/80-Pin, High-Performance Microcontrollers with LCD Driver and nanoWatt XLP Technology

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23.3 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 23-5. 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).
The source impedance affects the offset voltage at the
analog input (due to pin leakage current). The maxi-
mum recommended impedance for analog sources
is 2.5 k. After the analog input channel is selected or
changed, the channel must be sampled for at least the
minimum acquisition time before starting a conversion.
Note:
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
To calculate the minimum acquisition time,
Equation 23-1 can be used. This equation assumes
that 1/2 LSb error is used (1,024 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
Equation 23-3 shows the calculation of the minimum
required acquisition time, TACQ. This calculation is
based on the following application system
assumptions:
• CHOLD
=
• Rs
=
• Conversion Error 
• VDD
=
• Temperature
=
25 pF
2.5 k
1/2 LSb
3V  Rss = 2 k
85C (system max.)
EQUATION 23-1: ACQUISITION TIME
TACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
= TAMP + TC + TCOFF
EQUATION 23-2: A/D MINIMUM CHARGING TIME
VHOLD =
or
TC
=
(VREF – (VREF/2048)) • (1 – e^{(-TC/CHOLD(RIC + RSS + RS))})
-(CHOLD)(RIC + RSS + RS) ln(1/2048)
EQUATION 23-3: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
TACQ = TAMP + TC + TCOFF
TAMP = 0.2 s
TCOFF =
(Temp – 25C)(0.02 s/C)
(85C – 25C)(0.02 s/C)
1.2 s
Temperature coefficient is only required for temperatures > 25C. Below 25C, TCOFF = 0 ms.
TC
= -(CHOLD)(RIC + RSS + RS) ln(1/2048) s
-(25 pF) (1 k + 2 k + 2.5 k) ln(0.0004883) s
1.05 s
TACQ = 0.2 s + 1.05 s + 1.2 s
2.45 s
 2010 Microchip Technology Inc.
Preliminary
DS39957B-page 381