Datasheet

© 2006 Microchip Technology Inc. DS70117F-page 141
dsPIC30F6011/6012/6013/6014
19.8 A/D Acquisition Requirements
The analog input model of the 12-bit A/D converter
is shown inFigure 19-4. The total sampling time for the
A/D is a function of the internal amplifier settling time
and the holding capacitor charge time.
For the A/D converter 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 (RS), the interconnect
impedance (R
IC), and the internal sampling switch
(R
SS) impedance combine to directly affect the time
required to charge the capacitor CHOLD. 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 A/D con-
verter, the maximum recommended source imped-
ance, 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 A/D CONVERTER ANALOG INPUT MODEL
CPIN
VA
Rs
ANx
V
T = 0.6V
VT = 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