AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 10

• Manufacturer Part Number: AN1058
• Description: Reducing A/D Errors in Microcontroller Applications
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
Application Note
Typically, a large contributor to malfunction is the printed circuit board
(PCB) layout. Since the PCB can influence many of the circuit
parametrics (reactance, voltage, etc.), the PCB layout can help or hinder
ADC performance. Yet, the PCB layout is not typically done by the
circuit’s designer. More importantly, laying out the PCB artwork, up to
and including the width and placement of traces, is often performed by
people without a detailed knowledge of correct electrical circuit design
practices. Many PCB designers are only concerned with ensuring that
they have connected all the points connected in the schematic. Although
this has its economic advantages, this can be a dangerous proposition
with regard to ADC performance.
Figure 3
shows an example of such a PCB layout which, although it
manages to distribute the power to all of the devices, provides several
potential sources of ADC/MCU performance problems.
First, the MCU/ADC is placed farthest from the power terminal, meaning
that the MCU return currents will be mixed with the digital circuit currents
between the MCU and the power terminal. Although the MCU may not
produce large return currents in the power return, high-speed digital
circuits typically do. The inductance of PCB traces at high frequencies
can be significant enough to produce large noise spikes when measured
between the ground pin of the MCU and the ground terminal of the
board.
Second, the opamp, which buffers the signals to the ADC inputs on the
MCU, is physically located close to the MCU but is electrically located in
a very poor place. As with the MCU, the opamp power supply return will
be corrupted with high-frequency spikes. However, the voltage drops
measured between the opamp and the MCU will be even worse than
those measured between the MCU and the power terminal. When
deciding which parts are to be jointly located on the PCB, the electrical
impact of conductor distance and tolerance to any induced noise must
be considered.
Third, the bypass capacitors, as shown, are ineffectual in reducing high-
frequency noises on V
. To perform the decoupling function properly,
DD
bypass capacitors should be attached as close as possible to the IC
power pins they are intended to bypass. In addition, PCB trace
AN1058
10
MOTOROLA
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