MCP602-IP Microchip Technology, MCP602-IP Datasheet - Page 3

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

With this circuit layout, the controller is dedicated to inter-
facing with the converter and sending the converter’s
results to the LCD display. The digital output of the con-
verter over time is shown in Figure 4. This data was col-
lected with no excitation being applied to the sensor.
FIGURE 4: This is a histogram of 4096 samples from
the output of the A/D Converter from a PCB that does
not have a ground or power plane as shown in the PCB
layout in Figure 3. The by-pass capacitors are installed.
When determining the grounding strategy of a board,
the task at hand should actually be to determine if the
circuit can work adequately with just one ground plane
or does it need multiple planes.
Figure 5 shows the same layout shown in Figure 3,
plus a ground plane. It should be noted that the ground
plane has a few breaks due to signal traces. These
breaks should be kept to a minimum. Current return
paths should not be “pinched” as a consequence of
these traces restricting the easy flow of current from the
device to the power connector. The histogram for the
A/D Converter output is shown in Figure 6. Compared
to Figure 4, the output codes are much tighter. The
same active devices were used for both tests. The pas-
sive devices were different causing a slight offset differ-
ence. The noise shown with the A/D Converter digital
code is assignable to the op amp noise and the
absence of an anti-aliasing filter.
If the circuit has a “minimum” amount of digital circuitry
on board, a single ground plane and a single power
plane may be appropriate. The qualifier “minimum” is
defined by the board designer. The danger of connect-
ing the digital and analog ground planes together is that
the analog circuitry can pick-up the noise on the supply
pins and couple it into the signal path. In either case,
the analog and digital grounds and power supplies
should be connected together at one or more points in
the circuit to insure that the power supply, input and out-
put ratings of all of the devices are not violated.
The inclusion of a power plane in a 12-bit system is not
as critical as the required ground plane. Although a
power plane can solve many problems, power noise
can be reduced by making the power traces two or
three times wider than other traces on the board and by
using by-pass capacitors effectively.
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1999 Microchip Technology Inc.
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Output Code of 12-bit A/D Converter
Digital Code VS. Occurrences
FIGURE 5: Layout of the top and bottom layers of the
circuit in Figure 1. Note that this layout DOES have a
ground.
FIGURE 6: This is a histogram of 4096 samples from
the output of the A/D Converter on the PCB that has a
ground plane as shown in the PCB layout in Figure 5.
Note that the power traces are made considerably wider
than the signal traces in order to reduce power supply
trace inductance. This circuit has all by-pass capacitors
installed.
+5V Connect
Dual
Amp
Op
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Output Code of 12-bit A/D Converter
Reference
2.5V
Digital Code VS. Occurrences
Bottom Layer
Top Layer
Pressure
Sensor
Connection
12-Bit
Ground Connect
A/D Converter
AN688
DS00688B-page 3