AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 10

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

AN2438/D
Power Supply Noise Error
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Power Supply Noise Error (ENS) is the difference in conversion caused by
power supply noise (on the V
Power Supply Noise Errors are caused by:
Power Supply Noise Error is ultimately inherent in any ADC design. It is
reduced in the design phase by the use of differential, cascaded circuits
(increases PSRR), reducing parasitics (increase PSDR), and matching
parasitics on the input and reference paths (increases DPSDR). However,
these techniques are not perfect. The end user can apply the following
techniques to reduce Power Supply Noise Error:
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Comparator Power Supply Rejection Ratio (PSRR) — This ratio
defines the ability of the comparator to reject power supply noise
(relative to the inputs) when making a differential comparison between
the inputs. PSRR is commonly measured in decibels (db). A comparator
with a PSRR of 60 db will interpret 1-V power supply noise the same as
1 mV of differential input voltage.
Power Supply Decoupling Ratio (PSDR) — This ratio defines how
much of the power supply noise is coupled onto the inputs (either
reference or input). A decoupling ratio of 24 db indicates that 16 mV of
power supply noise will cause 1 mV of input or reference noise.
Differential Power Supply Decoupling Ratio (DPSDR) — This ratio
defines how much of the power supply noise is coupled onto the
differential between the inputs. The most common way of reducing
Power Supply Noise Error is by decoupling the reference and input
equally (since it is impossible to reduce coupling altogether). A
differential decoupling ratio of 30 db means that 32 mV of power supply
noise will be interpreted as 1 mV of differential input noise.
Analog Power Supply Bypassing — The ADC power supply must be
bypassed to its ground as close to the MCU pins as possible. The
preferred method is with a high-frequency capacitor (0.01 µF or 0.1 µF)
near the MCU and a larger capacitor near the power supply’s source.
Resistive impedance must be minimum, and chokes should never be
used.
Digital Power Supply Bypassing — The input channel for the ADC
often is selected on a multi-purpose I/O pin. In this case, there is
considerable coupling to the digital power supply. Additionally, most
ADC’s integrated into microcontrollers are built on the same silicon
substrate as the digital circuits, which means ground noise will be
coupled. For these reasons, the same bypassing techniques are
recommended for the digital power supplies as the analog supplies.
Elimination of Noise Sources — The best way to eliminate Power
Supply Noise Error is to eliminate Power Supply Noise. Critical ADC
conversions must be conducted when the rest of the system is as quiet
ADC Definitions and Specifications
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DDAD
; ADC power or V
SSAD
; ADC ground pins).
MOTOROLA