AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 14

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

AN2438/D
Input Leakage Offset Error (E
14
There is an additional source of input error that is often overlooked but can be
extremely significant. Leakage on the Analog input pin or on the PC board can
cause a voltage drop across the resistive portion of the Analog Input Source.
Many circuits (especially battery voltage and temperature detection circuits)
use high value resistive voltage dividers to create the analog reference.
Usually, a capacitor is placed on the input so the Analog AC Source Impedance
(Z
However, the Analog DC Source Resistance (R
(perhaps 100 kΩ).
Leakage on the analog inputs (I
Typical numbers vary from 5 nA (older process at room temperature) to 500 nA
(newer process at hot temperature). This leakage on all processes increases
exponentially with temperature. Added to this is PC board resistance (R
which can be as low as 1 MΩ but is usually around 10 MΩ. Given this, the Input
Leakage Offset Error is:
This error can be on the order of 10’s of LSB if the Analog DC Source
Resistance is high (example, 1 µA*100k/2.5 mV = 40 LSB). There is no way of
perfectly trimming this error especially since it is temperature dependent. The
user can set up a parallel input with the same Analog Source Impedance and
similar, but fixed, V
is variable from pad to pad this method will not be perfect.
AIN
Freescale Semiconductor, Inc.
•
•
) is reduced so the ADC will be able to acquire the signal properly.
For More Information On This Product,
of noise includes line noise and DC-bias shifts due to regulator load
changes, display drivers, etc.
High Frequency Events (Random Frequency) — Random Frequency
noise that is at more than ¼ the ADC conversion cycle frequency can be
averaged out in the same manner as low frequency nose (four samples
for every 1LSB of noise for ¼LSB error). White noise and other
background noise are of this type.
High Frequency Event (Single Frequency) — Any single-frequency
noise at higher than ¼ the ADC conversion cycle frequency will beat with
the ADC frequency, causing aliasing to occur at the beat frequency.
Removing this aliasing error requires that four samples (for every 1LSB
of noise) be taken at uniform intervals of at least twice the beat
frequency. Higher levels of over-sampling may be necessary if the beat
frequency is not known precisely. This type of noise, which includes
asynchronous clock switching or higher frequency communication
devices such as CAN, does not fit the uniform-distribution requirement
necessary for accurate noise reduction.
E
INO
INO
ADC Definitions and Specifications
)
= (I
Go to: www.freescale.com
IN
+ V
IN
to get a rough idea of the error, but since the input leakage
IN
/R
B
)*R
AIN
IN
) is usually specified at a maximum of 1 µA.
/ 1LSB
AIN
) is still potentially very high
MOTOROLA
B
),