Microchip MCP6V03 Bedienungsanleitung

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2009-2012 Microchip Technology Inc. DS01258B-page 1
AN1258
INTRODUCTION
This application note covers Printed Circuit Board
(PCB) effects encountered in high (DC) precision op
amp circuits. It provides techniques for improving the
performance, giving more flexibility in solving a given
design problem. It demonstrates one important factor
necessary to convert a good schematic into a working
precision design.
This material is for engineers who design slow
precision circuits, including those with op amps. It is
aimed at those engineers with little experience in this
kind of design, but can also help experienced
engineers that are looking for alternate solutions to a
design problem.
The information in this application note can be applied
to all precision (DC) analog designs, with some thought
and diligence. The focus is on common op amp circuits
so that the reader can quickly convert this material into
improvements in their own op amp designs.
Additional material at the end of the application note
includes references to the literature and the schematic
of a PCB used in the design example.
Key Words and Phrases
• Op Amp
• Temperature
• Thermal Gradient
• Thermocouple Junction
• Thermoelectric Voltage
• IC Sockets
• Contact Potential
• PCB Surface Contamination
Related Application Notes
The following application notes, together with this one,
form a series about precision op amp design topics.
They cover both theory and practical methods to
improve a design’s performance.
• AN1177 on DC Errors [2]
• AN1228 on Random Noise [3]
THERMOCOUPLE JUNCTION
BEHAVIOR
While thermocouples are a common temperature
sensor [5], it is not commonly known that every PCB
design includes many unintended thermocouple
junctions that modify the signal voltages. This section
covers the physics behind this effect and gives
practical illustrations.
Seebeck Effect
When two dissimilar conductors (or semiconductors)
are joined together, and their junction is heated, a
voltage results between them (Seebeck or
thermoelectric voltage); this is known as the Seebeck
effect. This voltage is roughly proportional to absolute
temperature. There are many references that discuss
this effect in detail, including the “Temperature
Products” section of reference [8]; see especially
pages Z-13, Z-14 and Z-23 through Z-32.
Figure 1 shows the Seebeck voltage as a function of
temperature for the standard type K thermocouple.
Notice that the response is not strictly linear, but can be
linearized over small temperature ranges (e.g., ±10°C).
FIGURE 1: Type K Thermocouple’s
Response.
Most thermocouple junctions behave in a similar
manner. The following are examples of thermocouple
junctions on a PCB:
• Components soldered to a copper pad
• Wires mechanically attached to the PCB
• Jumpers
• Solder joints
• PCB vias
Author: Kumen Blake
Microchip Technology Inc.
10
15
20
25
30
35
40
45
50
55
60
o
couple Voltage (mV)
ITS-90
Type K Thermocouple
-10
-5
0
5
10
-300
-200
-100
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Therm
o
Thermocouple Temperature (°C)
Op Amp Precision Design: PCB Layout Techniques
AN1258
DS01258B-page 2 2009-2012 Microchip Technology Inc.
The linearized relationship between temperature and
thermoelectric voltage, for small temperature ranges, is
given in Equation 1. The Seebeck coefficients for the
junctions found on PCBs are typically, but not always,
below ±100 µV/°C.
EQUATION 1: SEEBECK VOLTAGE
Illustrations Using a Resistor
Three different temperature profiles will be shown that
illustrate how thermocouple junctions behave on PCB
designs. Obviously, many other components will also
produce thermoelectric voltages (e.g., PCB edge
connectors).
Figure 2 shows a surface mount resistor with two metal
(copper) traces on a PCB. The resistor is built with end
caps for soldering to the PCB and a very thin
conducting film that produces the desired resistance.
Thus, there are three conductor types shown in this
figure, with four junctions.
FIGURE 2: Resistor and Metal Traces
on PCB.
For illustrative purposes, we’ll use the arbitrary values
shown in Table 1. Notice that junctions 1 and 4 are the
same, but the values are shown with opposite
polarities; this is one way to account for the direction
current flows through these junctions (the same applies
to junctions 2 and 3).
TABLE 1: ASSUMED THERMOCOUPLE
JUNCTION PARAMETERS
CONSTANT TEMPERATURE
In this illustration, temperature is constant across the
PCB. This means that the junctions are at the same
temperature. Let’s also assume that this temperature is
+125°C and that the voltage on the left trace is 0V. The
results are shown in Figure 3. Notice that V
TH is the
voltage change from one conductor to the next.
FIGURE 3: Constant Temperature
Results.
TEMPERATURE CHANGE IN THE NORMAL
DIRECTION
In this illustration, temperature changes vertically in
Figure 2 (normal to the resistor’s axial direction), but
does not change in the axial direction (horizontally).
The metal areas maintain almost constant voltages in
the normal direction, so this case is basically the same
as the previous one.
VTH kJTJTREF
– 
Where:
VTH = Change in Seebeck voltage (V)
kJ= Seebeck coefficient (V/°C)
TJ= Junction Temperature (°C)
TREF = Reference Temperature (°C)
VTH = Seebeck voltage (V)
VREF = Seebeck voltage at TREF (V)
VTH VREF VTH
+=
Copper ResistorResistor
Junction #1
Junction #2
Junction #4
Junction #3
Film End CapsTraces
Junction No. VREF
(mV)
kJ
(µV/°C)
1 10 40
2 -4 -10
3 4 10
4 -10 -40
Note 1: VREF and kJ have polarities that assume a left-
to-right horizontal direction.
2: TREF = 25 °C.
Note: When temperature is constant along the
direction of current flow, the net change in
thermoelectric voltage between two
conductors of the same material is zero.
+125.00°C
+125.00°C
+125.00°C
+125.00°C
0 mV
14 mV 9 mV 14 mV
0 mV
Location VREF
(mV)
VTH
(mV)
VTH
(mV)
Junction #1 10 4 14
Junction #2 -4 -1 -5
Junction #3 4 1 5
Junction #4 -10 -4 -14
2009-2012 Microchip Technology Inc. DS01258B-page 3
AN1258
TEMPERATURE CHANGE IN THE AXIAL
DIRECTION
In this illustration, temperature changes horizontally in
Figure 2 (along the resistor’s axial direction), but does
not change in the normal direction (vertically). Let’s
assume 0V on the left copper trace, +125°C at
Junction #1, a temperature gradient of 10°C/in
(0.394°C/mm) from left to right (0 in the vertical
direction) and a 1206 SMD resistor.
The resistor is 0.12 inches long (3.05 mm) and
0.06 inches wide (1.52 mm). Assume the end caps are
about 0.01 inches long (0.25 mm) and the metal film is
about 0.10 inches long (2.54 mm). The results are
shown in Figure 4.
FIGURE 4: Axial Gradient Results.
Thus, the temperature gradient of 10°C/in (1.2°C
increase from left to right) caused a total of -38 µV to
appear across this resistor. Notice that adding the
same temperature change to all junction temperatures
will not change this result.
PREVENTING LARGE
THERMOELECTRIC VOLTAGES
This section includes several general techniques that
prevent the appearance of large temperature gradients
at critical components.
Reduced Heat Generation
When a PCB’s thermal gradient is mainly caused by
components attached to it, then find components that
dissipate less power. This can be easy to do (e.g.,
change resistors) or hard (change a PICmicro®
microcontroller).
Increasing the load resistance, and other resistor
values, also reduces the dissipated power. Choose
lower power supply voltages, where possible, to further
reduce the dissipated power.
Redirect the Heat Flow
Changing the direction that heat flows on a PCB, or in
its immediate environment, can significantly reduce
temperature gradients. The goal is to create nearly
constant temperatures in critical areas.
ALTERNATE HEAT PATHS
Adding heat sinks to parts that dissipate a lot of power
will redirect the heat to the surrounding air. One form of
heat sink that is often overlooked is either ground
planes or power planes in the PCB; they have the
advantage of making temperature gradients on a PCB
lower because of their large (horizontal) thermal
conductivity.
Adding a fan to a design will also redirect heat to the
surrounding air, which reduces the temperature drop
on the PCB. This approach, however, is usually
avoided to minimize other design issues (random
temperature fluctuations, acoustic noise, power, cost,
etc.). It is important to minimize air (convection)
currents near critical components. Enclose either the
parts with significant temperature rise, or the critical
parts. Conformal coating may also help.
ISOLATION FROM HEAT GENERATORS
It is possible to thermally isolate critical areas on the
PCB. Regions with little or no metal act like a good
thermal insulator. Signals that need to cross these
regions can be sent through series resistors, which will
also act as poorly conducting thermal elements.
Place heat sources as far away from critical points as
possible. Since many heat sources are in the external
environment, it can be important to place these critical
points far away from the edges of the PCB.
Components that dissipate a lot of power should be
kept far away from critical areas of the PCB.
Note: Shifting all of the junction temperatures by
the same amount does not change the
temperature gradient. This means that the
voltage drop between any two points in the
circuit using the same conductive material
is the same (assuming we’re within the
linear region of response).
+125.0°C
+125.1°C
+126.1°C
+126.2°C
0 mV
14.000 mV
8.999 mV
14.010 mV
-0.038 mV
Location VREF
(mV)
VTH
(mV)
VTH
(mV)
Junction #1 10 4.000 14.000
Junction #2 -4 -1.001 -5.001
Junction #3 4 1.011 5.011
Junction #4 -10 -4.048 -14.048

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Marke: Microchip
Kategorie: Nicht kategorisiert
Modell: MCP6V03

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