AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 17

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

MOTOROLA
In cases where there is no external filtering capacitance (C
error is determined by the number of time constants of charging and the change
in input voltage relative to the resolution of the ATD:
The maximum sampling error (assuming maximum change on the input
voltage) will be:
In the case where an external filtering capacitance is applied, the sampling
error can be reduced based on the size of the source capacitor (C
to the analog input capacitance (C
(R
In the case of a 0.1 µF C
regardless of R
f
only by R
This is a worst case sampling error which does not account for R
the combination of C
that high values of R
sufficient time to recharge C
achieve accuracy specified under the worst case conditions of maximum
SAMP
AS
•
•
•
•
Freescale Semiconductor, Inc.
), C
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sampling error in LSB (E
, R
ATD Conversion Clock Frequency (f
of the clock input to the ATD. It clocks the counter for the successive
approximation register and times the sample and hold and conversion
sequencing. This clock is normally driven from the MCU’s bus clock and
may employ a clock prescaler. This frequency will determine the width
of the sample window and the time for the conversion. In these
examples, we will use 14 ATDCLK cycles for the total conversion.
Input Sample Frequency (f
input channel is sampled.
Delta-Input Sample Voltage (∆V
between the current input voltage (intended for conversion) and the
previously sampled voltage (which may be from a different channel).
Delta-Analog Input Voltage (∆V
the current input voltage and the input voltage during the last conversion
on a given channel.
E
# of time constants (τ) = (14 / f
AS
E
AS
E
S
AS
S
+ (∆V
ADC Definitions and Specifications
S
= (3.6/3.6) * e–
will charge C
must re-charge C
= 2
= 2
(not R
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N
N
AS
SAMP
* {(∆V
* (∆V
. However, in the case of repeated conversions at a rate of
AIN
AS
AS
/ (V
), and reduces the overall sampling error to:
e
SAMP
AIN
−(1 / (f
(>10 kΩ) are possible if a large C
and C
AS
Acquisition Time (Sample and Hold) and Error Calculations
AIN
REFH
(14/((7 k + 10 k) * 50 p * 2 M))
, a worst case sampling error of 0.5 LSB is achieved
/ (V
to a value of:
/ (V
AS
ATDCLK
AS
AIN
S
REFH
– V
is provided between samples. In order to
) = 2
. This recharge is continuous and controlled
REFH
during the sample window. It does illustrate
REFL
AIN
SAMP
– V
* (R
N
). Ignoring the analog source impedance
– V
* (∆V
)) * Min[(C
AS
ATDCLK
REFL
AIN
) — This is the frequency that a given
SAMP
REFL
+ R
) — This is the difference between
SAMP
)) * e
AIN
ATDCLK
)) * (C
) — This is the difference
) / ((R
) * C
−(1 / (f
/ (V
AIN
AIN
* 1024 = 0.271 LSB
AIN
AS
REFH
/ (C
) — This is the frequency
SAMP
)
]}
+ R
/ (C
AIN
AS
– V
AIN
AIN
AS
* R
+ C
is used and
), the sampling
REFL
) * C
AS
+ C
AS
AS
* C
)) * e
)),
AIN
AS
AS
AS
recharging
) relative
))
AN2438/D
)
)
−τ
17