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INA330: Part Fails When Rset is Too different from Rtherm

May
Genius 9880 points
Part Number: INA330

Hi,

Customer is having an odd issue with the INA330,details below.

"I have a thermistor with a nominal value of around 17K (22K at 25 deg. C). The thermistor is implanted inside of a mouse nose, so when the mouse breaths, the temperature fluctuates. This fluctuation is amplified by the circuit. I have chosen an Rg of 220K. V1=V2=1.65V and Vadj = 1.65V nominally. When originally prototyping with this part, we were using an Rset of 14590 by mistake, rather than something around 17K. This was causing a part failure after a few seconds of operation. After three chips, we noticed this mistake and switched to a ~17K resistor for Rset, which seems to have solved the problem, but this will likely cause issues in the future for us, especially when we move to a final PCB. Do you have any insights into why this part failure was happening? PNG of the schematic is attached. Planned revision to the design includes a digital pot for Rset and a DAC for Vadj instead of a voltage divider. Would these help/hurt this particular problem or do you recommend an alternative?

"

Thank you in advance.

Regards,
Maynard

  • 0
    TI__Genius 10850 points

    Hi Maynard,

    Thanks for your post, I can help you. To recap your circuit set-up / issue:

    • 3.3V supply
    • Vadj = Vexcite = V1 = V2 = 1.65V
    • Rg = 220k-ohm
    • Rtherm = 17k-ohm
    • When Rset = 17k-ohm results are "good". When Rset = 14590-ohm results fail.

    The output voltage for this circuit is shown on page 7 of the datasheet:

    When Rset = 17k-ohm, Vo = 1.65V. When Rset = 14590-ohm, Vo = -1.877V.

    The output of this amplifier can swing 10mV from either rail, so on a 3.3V supply, that's 0.01V to 3.29V. When Rset is equal to 14590-ohm, the output wants to swing to -1.877V which is outside the linear range of the amplifier. 

    When Rset is too different from Rtherm (like the 15% you have shown here), the error contribution is too high, leading to failure. This E2E post shows how to calculate error due to different tolerance resistors. 

    In regards to your question about using a DAC for Vadj instead of a voltage divider, that is okay. Page 8 of the datasheet says the following: "The set-point temperature is adjusted with VADJ. Thus, the voltage at VO is the sum of (IO)(RG) + VADJ. VADJ can be manually adjusted or set with a Digital-to-Analog (D/A) converter."