AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 11

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

Input to Reference Differential Noise Error
MOTOROLA
Input to Reference Differential Noise Error, or Input Noise Error (ENIN), is the
error due to noise, or short-term deviation from the DC-average, on the input
(V
Error is caused by:
Input to Reference Differential Noise is primarily externally controllable through
good PC board layout and decoupling. Internally, shielding and isolation from
switching signals and clock feed-through reduction techniques are usually
employed to reduce internal sources. External methods of reducing Input Noise
include:
IN
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) or either reference (V
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as possible. Most importantly, no Output Drivers on the microcontroller
should be activated during the ADC conversion. The preferred mode for
ADC conversions is in WAIT mode.
Reference Coupling Ratio — This defines the amount of noise that is
injected onto one reference (i.e., V
other reference (i.e., V
references are to each other. If the coupling ratio is low, then between
half and all of this noise will be differential to the input signal, depending
on which reference the input is more closely coupled to and which
reference the input voltage is closer to. The external (board-level)
network of capacitors and parasitic impedances connected between the
references and noise sources usually defines this ratio.
Differential Coupling Ratio — This defines the amount of noise that is
injected onto either the input or either reference that will be coupled onto
the other. The higher the coupling ratio, the less differential noise will be
seen between the input and the reference. This ratio primarily depends
on the decoupling or filtering method that is used on the input.
Input Coupling Ratio — This defines the amount of noise that is
generated from a given noise source that is injected onto the input or
reference. This depends on the layout and parasitics on the external
board as well as the internal circuit.
EMC — Conducted or Radiated noise can be picked up by the input or
reference signals even in otherwise good printed circuit board layouts.
This pick-up is usually dependent on the impedance of the input or
reference source. It commonly affects the input more than the reference
since the reference is usually much lower impedance.
Reference Bypassing — The high and low references (V
V
V
capacitor near source, low resistive impedance and no choke). This
increases the Reference Coupling Ratio, reducing differential noise.
REFL
DDAD
ADC Definitions and Specifications
) should be bypassed to each other similar to the method used on
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and V
SSAD
(high-frequency capacitor near pins, low frequency
REFH
REFL
or V
). The higher the coupling ratio, the closer the
REFL
Input to Reference Differential Noise Error
) relative to the other. Input Noise
REFH
) that will be coupled onto the
REFH
AN2438/D
and
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