Datasheet

Table Of Contents
dsPIC30F6011A/6012A/6013A/6014A
DS70143E-page 138 © 2011 Microchip Technology Inc.
FIGURE 19-3: CONVERTING 1 CHANNEL AT 200 KSPS, AUTO-SAMPLE START, 1 TAD
SAMPLING TIME
19.8 ADC Acquisition Requirements
The analog input model of the 12-bit ADC is shown in
Figure 19-4. The total sampling time for the ADC is a
function of the internal amplifier settling time and the
holding capacitor charge time.
For the ADC to meet its specified accuracy, the charge
holding capacitor (C
HOLD) must be allowed to fully
charge to the voltage level on the analog input pin. The
source impedance (R
S), the interconnect impedance
(RIC) and the internal sampling switch (RSS)
impedance combine to directly affect the time required
to charge the capacitor C
HOLD. The combined
impedance of the analog sources must therefore be
small enough to fully charge the holding capacitor
within the chosen sample time. To minimize the effects
of pin leakage currents on the accuracy of the ADC, the
maximum recommended source impedance, R
S, is 2.5
kΩ. After the analog input channel is selected
(changed), this sampling function must be completed
prior to starting the conversion. The internal holding
capacitor will be in a discharged state prior to each
sample operation.
FIGURE 19-4: 12-BIT ADC ANALOG INPUT MODEL
TCONV
= 14 TAD
TSAMP
= 1 TAD
TSAMP
= 1 TAD
ADCLK
SAMP
DONE
ADCBUF0
ADCBUF1
Instruction Execution BSET ADCON1, ASAM
TCONV
= 14 TAD
CPIN
VA
Rs
ANx
V
T = 0.6V
V
T = 0.6V
I leakage
R
IC 250Ω
Sampling
Switch
R
SS
CHOLD
= DAC capacitance
V
SS
VDD
= 18 pF
± 500 nA
Legend: CPIN
VT
I leakage
R
IC
RSS
CHOLD
= input capacitance
= threshold voltage
= leakage current at the pin due to
= interconnect resistance
= sampling switch resistance
= sample/hold capacitance (from DAC)
various junctions
Note: CPIN value depends on device package and is not tested. Effect of CPIN negligible if Rs 2.5 kΩ.
RSS 3 kΩ