Datasheet

Table Of Contents
PIC10F220/222
DS41270E-page 32 © 2007 Microchip Technology Inc.
7.9 A/D Acquisition Requirements
For the ADC to meet its specified accuracy, the charge
holding capacitor (C
HOLD) must be allowed to fully
charge to the input channel voltage level. The Analog
Input model is shown in Figure 7-1. The source
impedance (R
S) 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 (V
DD), see Figure 7-1.
The maximum recommended impedance for analog
sources is 10 kΩ. As the 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 7-1 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.
EQUATION 7-1: ACQUISITION TIME EXAMPLE
FIGURE 7-1: ANALOG INPUT MODULE
Note 1: The charge holding capacitor (CHOLD) is not discharged after each conversion.
2: The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin
leakage specification.
Assumptions:
Temperature = 50°C and external impedance of 10 k
Ω
5.0V VDD
Tacq = Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient
=T
AMP + TC + TCOFF
=2
μ
s + TC + [(Temperature - 25°C)(0.05
μ
s/°C)]
Solving for Tc:
Tc = C
HOLD (RIC + RSS + RS) In(1/512)
= -25pF (l k
Ω
+ 7 k
Ω
+ 10 k
Ω
) In(0.00196)
=2.81
μ
s
Therefore:
Tacq = 2
μ
s + 2.81
μ
s + [(50°C-25°C)(0.0 5
μ
s/°C)]
=6.06
μ
s
CPIN
VA
Rs
ANx
5 pF
V
DD
VT = 0.6V
V
T = 0.6V
I
LEAKAGE
RIC 1k
Sampling
Switch
SS
Rss
C
HOLD = 25 pF
V
SS/VREF-
6V
Sampling Switch
5V
4V
3V
2V
567891011
(kΩ)
V
DD
± 500 nA
RSS
Legend: CPIN = Input Capacitance
V
T = Threshold Voltage
I
LEAKAGE = Leakage current at the pin due
to various junctions
R
IC = Interconnect Resistance
SS = Sampling Switch
C
HOLD = Sample/Hold Capacitance