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INA146 internal resistor tolerances

Calum Mackinnon
Genius 4155 points

Hi,

 

I have a customer who is asking for the % tolerances on the 5 internal resisotrs? Page 8 of http://focus.ti.com/lit/ds/symlink/ina146.pdf

 

Cheers

Calum

  • 0
    Prodigy 30 points

    Tolerances of the on-chip resistors are 25 to 30%. Their relative (ratio) accuracy is very high, not their absolute value.

  • 0 in reply to Herbert Knopf
    Genius 4155 points

    Thanks - Any idea the relative % tolerance difference can be?

    This is for an Aerospace apllication so they are doing a detailed analysis.

    Cheers

    Calum

  • 0 in reply to Calum Mackinnon
    Prodigy 30 points

    Well, I've had a closer look to the data sheet and found that actually there is one resistor, R5, that is more accurate than usual, see page 4 of the datasheet:

    Resistor at A1 Output (that is R5 in the internal schematics) is 10k with +/-1% tolerance and 100ppm temperature drift coefficient. Bear in mind that these are typical values, not guaranteed limits though! On page 8 it says:

    The output of A1 is connected to the noninverting input of A2 through a 10k

    resistor which is trimmed to ±1% absolute accuracy. The A2 input is available for applications such as a filter or a precision current source.

    Now for the other resistors and their ratio tolerance. On page we find:

    Source impedances greater than 800 ohms are not recommended, even if they are perfectly matched. Internal resistors are laser trimmed for accurate ratios, not to absolute values. Adding equal resistors greater than 800 ohms can cause a mismatch in the total resistor ratios, degrading CMR.

    This, together with the worst case specifications, gives a first hint.

    Gain accuracy is guaranteed to be at least 0.1% with 100ppm temperature drift (page 3). This suggests that the ratios of R1/R2 and R3/R4 must have an absolute accuracy of better than 0.124% each.

    From the CMRR specs we can further determine how well the ratios of R1/R2 and R3/R4 match relative to each other. CMRR is specified to be at least 70dB at 25°C, indicating that the divider ratios match within 0.035% or better.

    CMRR can drop to 64dB over the full industrial temperature range (-40 to +85°C), which indicates that the divider ratios match within 0.07% at these worst case conditions.

    Hope this helps,

    Regards, Herbert