
P
P
- 35 - 
 Andigilog, Inc. 2006 
www.andigilog.com
October 2006 - 70A05007 
aSC7611
Applications Information
Remote Diodes 
The aSC7611 is designed to work with a variety of 
remote sensors in the form of the substrate thermal diode 
of a CPU or graphics controller or a diode-connected 
transistor. Actual diodes are not suited for these 
measurements. 
There is some variation in the performance of these 
diodes, described in terms of its departure from the ideal 
diode equation. This factor is called diode non-
ideality,
.
nf
The equation relating diode temperature to a change in 
thermal diode voltage with two driving currents is: 
Δ
V
BE
=
  (nf )KT
q
ln(N)
where: 
nf
 = diode non-ideality factor, (nominal 1.009). 
K
 = Boltzman’s constant, (1.38 x 10
-23
). 
T
 = diode junction temperature in Kelvins. 
q
 = electron charge (1.6 x 10
 Coulombs). 
N
 = ratio of the two driving currents (16). 
The aSC7611 is designed and trimmed for an expected 
nf
  value of 1.009, based on the typical value for the Intel 
Pentium III and AMD Athlon. There is also a 
tolerance on the value provided. The values for other 
CPUs and the 2N3904 may have different nominal values 
and tolerances. Consult the CPU or GPU manufacturer’s 
data sheet for the 
nf
 factor. Table 20 gives a 
representative sample of what one may expect in the 
range of non-ideality. The trend with CPUs is for a lower 
value with a larger spread. 
When thermal diode has a non-ideality factor other than 
1.009 the difference in temperature reading at a particular 
temperature may be interpreted with the following 
equation: 
T
actual
=
 T
reported
1.009
n
actual
where: 
T
reported
T
n
Temperatures are in Kelvins or °C + 273.15. 
= reported temperature in temperature register. 
actual
= actual remote diode temperature. 
= selected diode’s non-ideality factor,
 nf
 . 
actual
This equation assumes that the series resistance of the 
remote diode is the same for each. This resistance is 
given in the data sheet for the CPU and may vary from 
2.5
 to 4.5
. 
Although the temperature error caused by non-ideality 
difference is directly proportional to the difference from 
1.008, but a small difference in non-ideality results in a 
relatively large difference in temperature reading. For 
example, if there were a ±1% tolerance in the non-Ideality 
of a diode it would result in a ±2.7 degree difference (at 
0°C) in the result (0.01 x 273.15). 
This difference varies with temperature such that a fixed 
offset value may only be used over a very narrow range. 
Typical correction method required when measuring a 
wide range of temperature values is to scale the 
temperature reading in the host firmware. 
Part 
nf
 Min 
nf
 Nom 
nf
 Max
Series
Res 
Pentium III 
(CPUID 68h) 
Pentium 4, 
130nM 
Pentium 4,  90nM 
Pentium 4,  65nM 
Intel Pentium M 
AMD Athlon 
Model 6 
AMD Duron 
Models 7 and 8 
AMD Athlon 
Models 8 and 10 
2N3904 
1.0057 
1.008 
1.0125 
1.001 
1.002 
1.003 
3.64 
1.011 
1.009 
1.0022 
3.33 
4.52 
3.06 
1.0015 
1.0029 
1.002 
1.008 
1.016 
1.002 
1.008 
1.016 
1.0000 
1.0037 
1.0090 
1.003 
1.0046 
1.005 
Table 20 Representative CPU Thermal Diode 
and Transistor Non-Ideality Factors 
CPU or ASIC Substrate Remote Diodes 
A substrate diode is a parasitic PNP transistor that has its 
collector tied to ground through the substrate and the 
base (Remote -) and emitter (Remote +) brought out to 
pins. Connection to these pins is shown in Figure 11. The 
non-ideality figures in Table 20 include the effects of any 
package resistance and represent the value seen from 
the CPU socket. The temperature indicated will need to 
be compensated for the departure from a non-ideality of 
1.008. 
Remote + 
aSC7611
Remote - 
CPU 
Substrate 
Figure 11 CPU Remote Diode Connection