MCP602-IP Microchip Technology, MCP602-IP Datasheet - Page 2

• Manufacturer Part Number: MCP602-IP
• Description: 2.7V to 5.5V Single Supply CMOS Op Amp
• Manufacturer: Microchip Technology

AN688
ONE MAJOR STEP TOWARDS
DISASTER
The size of this circuit seems manageable. So small that
one may be tempted to use an auto router layout tool. If
this type of tool is used, it should be used carefully. If the
tool is capable of implementing restrictions into the lay-
out implementation, the layout design may have a fight-
ing chance. If restrictions are not implemented by the
auto routing tool, the best approach is to not use it at all.
GENERAL LAYOUT GUIDELINES
Device Placement
Device placement is critical. In general, there are some
noise sensitive devices in this layout and other devices
that are major problem creators. Here is a quick way to
identify the good, from the bad, from the ugly.
1.
2.
The board layout strategy should map the diagram
shown in Figure 2. Notice the relationship of digital ver-
sus analog and high speed versus slower speeds to the
board connector.
FIGURE 2: The placement of an active component on
a PCB is critical in precision 12-bit+ circuits.
FIGURE 3: Layout of the top and bottom layers of the circuit in Figure 1. Note that this layout does not have a ground or
power plane.
DS00688B-page 2
a) High frequency components
high
low
+5V Connect
Dual
Amp
Op
should be placed near the
connector
Separate the circuit devices into two categories:
high speed (>40MHz) and low speed.
Separate the above categories into three
sub-categories: pure digital, pure analog, and
mixed signal.
.
Reference
2.5V
Top Layer
b) Separate the digital and
analog portions of the circuit.
Pressure
Sensor
Connection
12-Bit
A/D Converter
Analog
Digital
Digital
Buffer
A/D
Dual
Amp
Op
In Figure 2b. the digital and analog circuit is shown as
being separate from the digital devices, which are closest
to the connector or power supply.
The pure analog devices are furthest away for the digital
devices to insure that switching noise is not coupled into
the analog signal path.
The treatment of the A/D Converter in layout varies from
technology to technology. For instance, if the A/D Con-
verter uses a Successive Approximation Register (SAR)
design approach, the entire device should be connected
to the analog power and ground planes. A common error
is to have the converter straddle the analog and digital
planes. This strategy may work, but as the accuracy
specifications of the A/D Converter improve the digital
ground and power plane noise begins to cause prob-
lems. For high resolution SAR converters, a digital buffer
should be used to isolate the converter from bus activity
on the digital side of the circuit.
In contrast, if the A/D Converter is designed using a
delta-sigma technology, it should straddle the analog
and digital planes. This is due to the fact that the
Delta-Sigma Converter is primarily a digital IC.
Ground and Power Supply Strategy
Once the general vicinity of the devices are deter-
mined, the ground planes and power planes should be
defined. The strategy of the implementation of these
planes are a bit tricky.
First of all, assuming that a ground plane is not needed
is a dangerous assumption in any circuit with analog
and/or mixed signal devices. Ground noise problems
are more difficult to deal with than power supply noise
problems because analog signals are most typically
referenced to ground. For instance, in the circuit shown
in Figure 1, the A/D Converter’s inverting input pin
(MCP3201) is connected to ground. Additionally, the
negative side of the pressure sensor is also connected
to ground.
A layout for the circuit in Figure 1 is shown in Figure 3.
This layout implementation does not have ground or
power planes on the board.
Reference
2.5V
Bottom Layer
1999 Microchip Technology Inc.
Pressure
Sensor
Connection
Ground Connect
12-Bit
A/D Converter