Datasheet

PCB

62.0

LM95234

SNIS136D –AUGUST 2006–REVISED MARCH 2013

www.ti.com

Calculating Total System Accuracy

The voltage seen by the LM95234 also includes the I

voltage drop of the series resistance. The non-ideality

factor, η, is the only other parameter not accounted for and depends on the diode that is used for measurement.

Since ΔV

is proportional to both η and T, the variations in η cannot be distinguished from variations in

temperature. Since the non-ideality factor is not controlled by the temperature sensor, it will directly add to the

inaccuracy of the sensor. For the for Intel processor on 65 nm process, Intel specifies a +4.06%/−0.897%

variation in η from part to part when the processor diode is measured by a circuit that assumes diode equation,

Equation 4, as true. As an example, assume a temperature sensor has an accuracy specification of ±1.0°C at a

temperature of 80°C (353 Kelvin) and the processor diode has a non-ideality variation of +1.19%/−0.27%. The

resulting system accuracy of the processor temperature being sensed will be:

ACC

= + 1.0°C + (+4.06% of 353 K) = +15.3 °C (6)

and

ACC

= - 1.0°C + (−0.89% of 353 K) = −4.1 °C (7)

TrueTherm technology uses the transistor equation, Equation 4, resulting in a non-ideality spread that truly

reflects the process variation which is very small. The transistor equation non-ideality spread is ±0.39% for the

Intel processor on 90 nm process. The resulting accuracy when using TruTherm BJT beta compensation

technology improves to:

ACC

= ±0.75°C + (±0.39% of 353 K) = ± 2.16 °C (8)

The next error term to be discussed is that due to the series resistance of the thermal diode and printed circuit

board traces. The thermal diode series resistance is specified on most processor data sheets. For Intel

processors in 65 nm process, this is specified at 4.52Ω typical. The LM95234 accommodates the typical series

resistance of Intel Processor on 65 nm process. The error that is not accounted for is the spread of the

processor's series resistance, that is 2.79Ω to 6.24Ω or ±1.73Ω. The equation to calculate the temperature error

due to series resistance (T

) for the LM95234 is simply:

(9)

Solving Equation 9 for R

PCB

equal to ±1.73Ω results in the additional error due to the spread in the series

resistance of ±1.07°C. The spread in error cannot be canceled out, as it would require measuring each individual

thermal diode device. This is quite difficult and impractical in a large volume production environment.

Equation 9 can also be used to calculate the additional error caused by series resistance on the printed circuit

board. Since the variation of the PCB series resistance is minimal, the bulk of the error term is always positive

and can simply be cancelled out by subtracting it from the output readings of the LM95234.

Product Folder Links: LM95234