AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 12

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

AN2438/D
Noise Error Mechanism
12
Regardless of the type of noise (Power Supply Noise or Input to Reference
Differential Noise), the error mechanism is the same. While the DC-average of
most noise is zero (primarily due to the power supply which sources the
reference or input), the short-term average is non-zero as shown in
The amount of error depends on the magnitude and decay rate of the noise
relative to the width of the sample or compare window.
Figure 5
purposes. In reality, phase error due to parasitic impedances in the noise-
coupling path, analog source impedance, and the gain and slew rate of the
comparator will tend to shift the sensitivity either towards the beginning of the
sample/compare window (in the cases of phase shift and comparator gain) or
end of the sample window (in the case of analog source impedance). The
general mechanism is the same in all cases.
Freescale Semiconductor, Inc.
•
•
•
For More Information On This Product,
Input Bypassing — Placement of a capacitor from the input to the
reference increases the Differential Coupling Ratio, reducing the
amount of differential noise. The size of the capacitor should be as large
as possible, allowing for the required dV/dt of the input given the analog
source impedance and the function of the signal.
Star Routing — The references and analog power supplies should be
star-routed so that all loads see an equal impedance to the source, and
there is little coupling from one load to another that is not common to all
loads.
Input Shielding — Placing shields (preferably a double shield using first
one of the references and then V
signals (including supplies) can increase differential coupling and
reduce input coupling. This also reduces EMC issues.
is a simplification of the true mechanism meant for illustrative
ADC Definitions and Specifications
Input Noise
Sample/Compare Window
Go to: www.freescale.com
Figure 5. Noise Error Graph
Noise Error
SSAD
) between the inputs and all other
Long-term DC-average
Running DC-average
MOTOROLA
Figure
5.