REF02: TEMP pin

Hello,

I have a question about the TEMP pin of REF02.

The explanation of the TEMP pin has the description in page15 of the REF02 data sheet.

There is a description that recommends the OpAmp of a low bias like AD8641 to be used for here.

Is this bias current allowed up to how many pA?

Best Regards,

Mr.K

  • 0
    •  Analog Employees 
    on Nov 20, 2014 7:18 PM

    Hi Mr.K,

    For a single supply configuration, Vs = +5V. The input bias current of the AD8641 at 25°C is 0.25pA(typical value) to 1pA(maximum value). However, if the ambient temperature is ranging between -40°C to 125°C the maximum input bias current of AD8641 can go up to 180 pA. Kindly refer to the table below.

    Below is the input bias current range for a dual supply configuration, Vs=+/-13V.

    In addition to that, the reason why the TEMP pin of the REF02 needs to be buffered with an op amp with a low input bias current is to minimize the current being drawn from the TEMP pin. It helps to prevent the voltage output of the REF02 to fall out of regulation.

    Regards,

    Jay

  • Hello,

    Thank you for your reply.

    In data sheet of the REF02(Rev.I  page3), the current of the TEPM pin is limited to 50nA.

    However, it has changed into the description that uses the OpAmp with low bias current in the data sheet Rev.K

    The bias cuurent of the AD8641 is 260pA(max).

    Should I use the OpAmp which the bias current is lower than that of 260pA?
    Could you tell me the value of the allowed bias current?

    Best Regards,

    Mr.K

  • 0
    •  Analog Employees 
    on Dec 2, 2014 11:33 PM

    Hi Mr. K,

    I just want to clarify that bias current is the amount of current that the input circuitry of op amp requires for its proper operation. If the bias current of AD8641 is 260pA(max), then it is also the maximum current that the AD8641 can be drawn from the temp pin of REF02. Since the 260pA/0.26nA bias current is lower than the limited current of 50nA, the temperature monitoring circuit shown in figure 36 of REF02 datasheet should work.

    Regards,

    Jay

  • Hello,

    Thank you for your reply.

    The low bias OpAmp is expensive.

    I want to use a cheap OpAmp.

    For instance, can the OpAmp of Ib=500nA be used?

    In the datasheet of REF02(Rev.K), there is a description that recommends the OpAmp of a low bias current.

    Could you tell me a maximum bias current?

    Best Regards,

    Mr.K

  • Mr. K,

      We don't make cheap op amps.  We make Lexus, not Civics.

    Looking at the schematic, the temp pin comes from the heart of the bandgap reference,

    so the more load on that pin, the more error.  No, we do not characterize this.

    The TEMP output is meant to be an uncalibrated relative indicator.  There is part

    to part variation.  It simply tells you the part is hotter or colder than room.

    If you want to measure temperature accurately, we have many good temp sensors,

    such as ADT7320 or TMP05.

    You can use any op amp you want, but the higher the bias current, the more the error on temp,

    and the more you affect the reference output.

    I would use two parts;  one for an accurate reference voltage, and one for accurate temperature.

    Harry