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INA331: REF pin

Aaron Spaete
Expert 4330 points
Part Number: INA331
Other Parts Discussed in Thread: LM7705

Team,

My customer has the following question:

I believe you guys took over burr brown INA331. I need help with it. I see in datasheet page 9 the gain is set by R1 and R2 to get desired gain (in my case 34.8k for R2 and 4.99k for R1 to get a gain of 40). I see it in the pic on page 9 that REF pin where R1 is connected is left open. What happen if I have a ground connected there? 

 

I’m seeing failure around this circuit so I’m wondering if grounding this pin is ok because they had this designed with this pin grounded and I’d like to know if it’s ok.

 

I wrote equation for three amplifiers and came out as a gain of 40 with REF pin grounded and with 34.8k and 4.99k. SO, I think we are ok there unless your application engineer suggests that is not advisable to use with REF pin grounded.

Please forward this to your engineer anyway. I thought I share it with what I found out after crunching the equations..


Regards,

Aaron

  • 0
    Guru 140200 points

    Hi Aaron,

    the REF pin is not intended to be left free. Here, a suited reference voltage with very low source impedance must be applied. This can be 0V (signal ground) for a bipolar supply voltage. But for a single supply voltage 0V is very probably no good reference voltage.

    How do you power the INA331? Single supply or bipolar supply voltage? And what is your input signal? What is the common mode input voltage?

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 4330 points
    It’s on a single 5V supply. Input signal range 0-120mV coming from GMR (resistive sensing device). Command mode should be very low since it’s coming from GMR device powered by less than 2.5V (abs max).
  • 0 in reply to Aaron Spaete
    Guru 140200 points

    Hi Aaron,

    the INA331 cannot go fully down to 0V at the output, when being powered by a +5V single supply voltage. The following simulation shows a 1kHz sine with 50mV amplitude sitting on a 50mV DC offset at the input of your INA331 circuit:

    If the DC offset voltage of 50mVp sine is 80mV, then the output no longer remains in saturation:

    A remedy would be to use a bipolar supply voltage or at least a negative supply voltage of 0.23V, like it's provided by the LM7705:

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 11135 points
    Aaron

    We haven't heard back from you so we assume this answered your question. If not post another reply below.

    Thanks
    Dennis