MCP602-IP Microchip Technology, MCP602-IP Datasheet - Page 4

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

AN688
Signal Traces
Generally speaking, the signal traces on the board
(both digital and analog) should be a short as possible.
This basic guideline will minimize the opportunities for
extraneous signals to couple into the signal path.
One area to be particularly cautious of is the input ter-
minals of analog devices. These input terminals nor-
mally have a higher impedance than the output or
power supply pins. As an example, the voltage refer-
ence input pin to the analog to digital converter is most
sensitive while a conversion is occurring. With the type
of 12-bit converter shown in Figure 1, the input termi-
nals (IN+ and IN ) are also sensitive to injected noise.
Another potential for noise injection into the signal path
is the input terminals of an operational amplifier. These
terminals have typically 10
These high impedance input terminals are sensitive to
injected currents. This can occur if the trace from a high
impedance input is next to a trace that has fast chang-
ing voltages, such as a digital or clock signal. When a
high impedance trace is in close proximity to a trace
with these types of voltage changes, charge is capaci-
tivly coupled into the high impedance trace.
FIGURE 7: A capacitor can be constructed on a PCB
by placing two traces in close proximity. With this PCB
capacitor, signals can be coupled between the traces.
As shown in Figure 7, the value of the capacitance
between two traces is primarily dependent on the dis-
tance (d) between the traces and the distance that the
two traces are in parallel (L). From this model, the
amount of current generated into the high impedance
trace is equal to:
where
DS00688B-page 4
C =
w = thickness of PCB trace
L = length of PCB trace
d = distance between the two PCB traces
e
e
o
r
= dielectric constant of air = 8.85 X 10
= dielectric constant of substrate coating relative to air
I equals the current that appears on the high
impedance trace
C equals the value of capacitance between the two
PCB traces
switching, and
change took to get from one level to the next.
I = C V/ t
V equals the change in voltage of the trace that is
t equals the amount of time that the voltage
w x L x e
d
o
x e
r
(typ 0.003mm)
pF
w
9
PCB Trace
to 10
L
13
-12
d
F/m
input impedance.
Cross-Section
PCB
DID I SAY BY-PASS?
A good rule concerning by-pass capacitors is to always
include them in the circuit. If they are not included, the
power supply noise may very well eliminate any chance
for 12-bit precision.
By-pass capacitors belong in two locations on the
board: one at the power supply (10 F to 100 F or both)
and one for every active device (digital and analog).
The value of the device’s by-pass capacitor is depen-
dent on the device in question. If the bandwidth of the
device is less than or equal to ~1MHz, a 1 F will reduce
injected noise dramatically. If the bandwidth of the
device is above ~10MHz, a 0.1 F capacitor is probably
appropriate. In between these two frequencies, both or
either one could be used. Refer to the manufacturer’s
guidelines for specifics.
Every active device on the board requires a by-pass
capacitor. The by-pass capacitor must be placed as
close as possible to the power supply pin of the device
as shown in Figure 5. If two by-pass capacitors are
used for one device, the smaller one should be closest
to the device pin. Finally, the lead length of the by-pass
capacitor should be as short as possible.
To illustrate the benefits of by-pass capacitors, data is
collected from the layout shown in Figure 5, minus the
by-pass capacitors. This data is shown in Figure 8.
FIGURE 8: This 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 3.
With this circuit implementation, all by-pass capacitors
have been removed.
1400
1200
1000
800
600
400
200
0
Output Code of 12-bit A/D Converter
Digital Code VS. Occurrences
1999 Microchip Technology Inc.