AN13.19

AN13.19
Resistance Error Correction
Author:
Wayne Little
Microchip Technology Inc.
INTRODUCTION
This application note describes the Resistance Error
Correction feature available on many Microchip
temperature sensor devices.
OVERVIEW
The information presented will show system designers
that the Resistance Error Correction (REC) feature
removes the need to compensate for series resistance
in the thermal diode connection. Figure 1 shows a
typical system and the remote diode-connected
transistor could be a central processing unit (CPU)
thermal diode or a discrete transistor located where the
temperature must be measured.
Remote
Sensor
System Controller
with SMBus
Interface
DP
Remote
Diode-Connected
Transistor
DN
SMBus
Interface
FIGURE 1:
SMBus
Block Diagram of Typical Temperature Sensing System.
 2005-2014 Microchip Technology Inc.
DS00001852A-page 1
AN13.19
POSITIVE TEMPERATURE OFFSET
RESULTING FROM SERIES
RESISTANCE
Review of Temperature Sensing Method
A typical temperature sensor forces two fixed currents
(IF1 and IF2) into the thermal diode to measure
temperature, as shown in Figure 2 below. The forward
bias voltage (VF) of the diode is measured as each of
the two fixed currents is sourced into the diode. In
Figure 2, the value of VF measured at the DP/DN pins
inside the chip is equivalent to the value of VBE at the
remote diode-connected transistor.
IF2
IF1
Remote
Diode-Connected
Transistor
DP
Voltage to
Temperature
Conversion
VF
VBE
DN
FIGURE 2:
Two Current Sources.
The difference between the two values of VF
(VF2 – VF1 = VBE) is used to determine the
temperature, as shown in Equation 1.
EQUATION 1:
I F2
kT
V F2 – VF1 =  --------- ln  -------
 I F1
q
Where:
k = Boltzmann’s constant
T = Absolute temperature in Kelvin
q = Electron charge
η = Diode ideality factor
DS00001852A-page 2
 2005-2014 Microchip Technology Inc.
AN13.19
Figure 3 shows that the relationship of VF2 – VF1 to
temperature is linear. In this plot, the Ideality Factor (ƞ) is
assumed to be 1.000 and the IF2/IF1 ratio is 17. The value
of VF2 – VF1 will change to 244 µV when the temperature
changes from 25°C to 26°C or from 100°C to 101°C.
Positive Temperature Offset Resulting
from Series Resistance
In the real world, series resistance will be present in the
path from the DP pin to the actual junction of the diode
and back to the DN pin of the temperature sensor.
Sources of series resistance include package leads,
Printed Circuit Board (PCB) traces, other forms of
interconnect and the physical structure of the remote
diode itself. In Figure 4, all these sources of series
resistance are combined and shown as RS.
95
(VF2 – VF1) = 0.2441 X Temperature
90
VF2 – VF1 (mV)
85
80
75
70
65
60
273
293
FIGURE 3:
313
333
Temperature (K)
353
373
VF2 – VF1 vs. Temperature.
IF2
IF1
RS
Remote
Diode-Connected
Transistor
DP
Voltage to
Temperature
Conversion
VBE
VF
DN
FIGURE 4:
Block Diagram of Temperature Monitoring Circuit.
When series resistance is present in the system, the VF
value measured at the DP/DN pins inside the chip is no
longer equivalent to the value of VBE. The VF value with
series resistance is shown in Equation 2.
This RS term will always induce a positive temperature
measurement offset error. The reported temperature
will be higher than the actual amount by the value
obtained using Equation 3.
EQUATION 2:
EQUATION 3:
V F = V BE +  IF  RS 
q  I F2 – I F1 RS
T OFFSET = -------- -------------------------------k
I F2
ln  -------
 I F1
This means that a system could be operating at an
acceptable temperature but the sensor would report
that it is beyond critical temperature because of the
series resistance.
 2005-2014 Microchip Technology Inc.
DS00001852A-page 3
AN13.19
Example with Series Resistance
EQUATION 4:
 IF2 – IF1 R S =  170  A – 10  A   3  = 480  V
For an IF ratio of 17, a 1° change in temperature
equates to a 244 mV change in the VF2 – VF1 term.
5.00E-02
4.50E-02
4.00E-02
3.50E-02
3.00E-02
20
40
60
80
Temperature (°C)
100
FIGURE 6:
Trace Resistance vs.
Temperature (250 µm Traces, 0.5 oz Copper
Plating).
EQUATION 5:
480  V
------------------ = 1.96
244  V
This series resistance term results in approximately a
2° error to the “real” temperature.
The remote thermal diode is often connected to the
temperature sensor using PCB traces. Figure 5 shows
typical values of series resistance for PCB traces at
room temperature.
As the temperature of the PCB traces increases from
20°C to 60°C, the series resistance changes by
approximately 32%. These small error terms should not
be overlooked when designing systems with ±1°C
accuracy components. The desired way to handle
series resistance is to design with a Microchip
temperature sensor that incorporates automatic
resistance error correction.
Resistance Error Correction
0.1000
7UDFH5HVLVWDQFHȍFP
5.50E-02
5HVLVWDQFHȍFP
This example provides insight into the effects of series
resistance on the detected temperature. Assume a
value of IF1 = 10 µA and IF2 = 170 µA. A typical value
of series resistance from a CPU data sheet is 3Ω.
Microchip temperature sensor devices that include
resistance error correction implement in the analog
front end of the chip. Resistance Error Correction is an
automatic feature that eliminates the need to
characterize and compensate for the series resistance.
The REC feature corrects for as much as 100 of
series resistance.
0.0800
0.0600
0.5 oz
0.0400
0.0200
1 oz
CONCLUSION
0.0000
0
0.2
0.4
0.6
Trace Width (mm)
0.8
1
FIGURE 5:
Trace Resistance vs. Trace
Width (at T = 25°C).
In some systems, it may be practical to compensate for
the error caused by series resistance by subtracting a
constant offset value. However, this would require
calculating a new offset value for each system because
the total amount of series resistance added by the PCB
traces will change depending on the physical properties of
the board and on the dimensions of the traces. Moreover,
changing the offset value often requires changing
firmware that would otherwise not require a change. In
addition, Figure 6 shows that the series resistance of PCB
traces also changes over temperature.
DS00001852A-page 4
In conclusion, using a Microchip temperature sensor
with REC capabilities automatically eliminates
temperature errors induced by the series resistance
that is present in all systems.
Microchip supplies a family of temperature sensors for
a variety of applications. Several enhanced features,
such as beta compensation and ideality configuration,
are available. In addition, some devices are designed
to work specifically with CPU thermal diodes. Please
consult
your
Microchip
representative
or
www.microchip.com for additional information.
REFERENCES
• Microchip Application Note 10.14 – “Using
Temperature Sensing Diodes with Remote
Thermal Sensors” (DS00001839)
 2005-2014 Microchip Technology Inc.
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ISBN: 978-1-63276-718-9
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DS00001852A-page 5
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