AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 16

• Manufacturer Part Number: AN2438
• Description: ADC Definitions and Specifications
• Manufacturer: Freescale Semiconductor / Motorola

AN2438/D
Acquisition Time (Sample and Hold) and Error Calculations
16
A number of steps are required during an analog-to-digital conversion. When
an analog-to-digital conversion begins, one channel select switch is closed,
allowing the sample and hold capacitor (C
switch is then opened. The charge applied to the sample and hold capacitor is
then converted into a digital representation by the successive approximation
register. A conversion complete flag will be set and an optional interrupt can be
generated.
Proper sampling is dependent on the following factors:
Freescale Semiconductor, Inc.
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Analog Source Impedance — This is the resistive (or real, in the case
of high frequencies) portion of the network driving the analog input
voltage V
Analog Source Capacitance (C
on the analog input, which (if large enough) may help the analog source
network charge the ATD input in the case of high R
ATD Input Resistance (R
ATD circuit in the path between the external ATD input and the ATD
sample and hold circuit. This resistance varies with temperature,
voltage, and process variation but a worst case number is necessary to
compute worst case sample error.
ATD Input Capacitance (C
ATD sample and hold circuit. This capacitance varies with temperature,
voltage, and process variation but a worst case number is necessary to
compute worst case sample error.
ADC Definitions and Specifications
V
AIN
—
+
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Figure 6. Resistor and Capacitor Placement
AIN
R
AS
.
C
AS
INPUT PIN
INPUT PIN
INPUT PIN
INPUT PIN
AIN
AIN
) — This is the internal resistance of the
) — This is the internal capacitance of the
R
R
R
R
AIN1
AIN2
AIN3
AINn
.
. .
AS
AIN
) — This is the filtering capacitance
CHANNEL
CHANNEL
CHANNEL
CHANNEL
SELECT 0
SELECT 1
SELECT 2
SELECT n
) to charge. The channel select
C
AIN
AS
.
ATD SAR
ENGINE
MOTOROLA