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LM86: How to calibrate LM86 to match a brand new remote sensing thermal diode?

zhipei zhang
Prodigy 185 points
Part Number: LM86
Other Parts Discussed in Thread: TMP451, TMP461

Hi,

I'm using LM86CIM/NOPB to measure chip die temperature via chip integrated thermal diode. in the datasheet of LM86, There are two registers named after REMOTE TEMPERATURE OFFSET REGISTERS(RTOHB and RTOLB).  What's the exact meaning of them? There are no detailed descriptions in the datasheet. My understanding is that LM86 is configured with some by-default diodes and should be re-tuned in case of any new remote sensing diode been connected.  Do you have any application note about how to tune LM86 to use new diode?

Thanks!

Zhipei

  • 0
    TI__Expert 6820 points

    Hi Zhipei,

    The REMOTE TEMPERATURE OFFSET REGISTERS(RTOHB and RTOLB) are described in the "Functional Description" section of the datasheet and table 11 and 12.

    "Two offset registers (RTOLB and RTOHB) can be used to compensate for nonideality error. The remote temperature reading reported is adjusted by subtracting from or adding to the actual temperature reading the value placed in the offset registers."

    Please see "Compensating for Diode Nonideality section of the datasheet" for calibration information.  

    To summarize,  if the BJT's N-factor that you are using is not 1.008, the offset registers can be used as a a first order calibration by adding or subtracting to the value the LM86 will report . We suggest using 2N3904 or 2N3906 because these BJT's N-factor is close to 1.008. 

    Thanks,

  • 0 in reply to David Vaseliou
    Prodigy 185 points

    Hi David,

    Thanks. I can find the contents in the datasheet now. but I still have below question.

    - RTOHB and RTOLB are about temperature value. How to correlate Non-ideality factor with values these two register: do we need to measure the factor separately and calculate the temperature accuracy impacted by the delta from 1.008. if so, any BKM from TI we can refer to?

    regards,

    Zhipei 

  • 0 in reply to David Vaseliou
    Prodigy 185 points

    Hi,

    if the remote sense diode is base-emitter in PNP transistor array instead of NPN type like recommended 2N3904. Can we still use LM86 to measure the temperature? if yes, any advice to come out the offset register value?

    from the datasheet, it seems that LM86 is to measure temperature basing on measured Vbe, selected Ibe/Is,  and non-ideality factor. As for Ibe/Is, it should be fixed value in LM86. could you tell what's the value of Is? also what's the Ibe/Is ratio in the measurement process.

    regards,

    Zhipei 

  • 0 in reply to zhipei zhang
    TI__Expert 6820 points

    Hi Zhipei,

    Yes you can use PNP transistor instead of an NPN transistor.  Please see Typical Application figure in the datasheet for transistor configuration or attached figure for diode configuration.  If you are using a PNP transistor with an N-factor of 1.008 you should not see any temperature error and should not need to use offset registers. 

    If the N-factor is not 1.008 use the offset register to factor out the error caused by the different N-factor.  

    Most of our newer devices, such as the TMP461 and TMP451, have a N-factor correction register which is what we typically use to correct for this error.

    Since this is an older device and does not have this register, try using the below method:

    1. Force known temperatures in the temperature range of interest to the remote junction

    2. Sample and average the temperature reading from Remote Temperature Sensor at each temperature point

    3. If there is any error between the known temperature and the temperature reading from the device use the offset registers to minimize the error from the reading

    The LM86 applies a ratio of currents to the transistor to calculate temperature.  In equation 4 of the datasheet this is the variable "N".  The algorithm is proprietary, but the Diode Source Currents can be found in the "Temperature-to-Digital Converter Characteristics" table.  

    Thanks,

  • 0 in reply to David Vaseliou
    Prodigy 185 points

    Hi David,

    Thanks! it's clear for me now.

    regards,

    Zhipei