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LMV951: LMV951 charge amp

Koji Ikeda
Expert 5335 points
Part Number: LMV951
Other Parts Discussed in Thread: TLV316,

Hello support team,

My customer would like to use LMV951 as charge amp and achieve same gain/phase performance with Vcc 1.3V as TLV316. However, the performance looks different from TLV316. Please attached file, their PSPICE simulation result.

*Why such a difference can be seen between LMV951 and TLV316?

*To achieve same performance as TLV316, how should they change the external design on LMV951?

TLV316 and LMV951 difference.xlsx

Thanks,

Koji Ikeda

  • 0
    Guru 140200 points

    Hi Koji,

    the LMV951 has a huge input bias current of 80nA compared to the 10pA of the TLV316. So, the LMV951 will not work with the 200M feedback resistance.  

    I would step up the supply voltage to 1.8V and use the TLV316.

    Kai

  • Former Member
    0 Former Member

    Hello Ikeda-san,

    I have a few comments for you.  Please see below and let me know what you think.

    1.  I noticed that for the LMV951, your gain is extremely low.  When the gain is -200dB at 1Hz, this can often mean that the amplifier is railed to 0V.  Can you check the DC operating point of the amplifier especially the output?

    2.  The LMV951 is a 6-pin part.  However, the model shown here only has 5 pins.  I imagine that you took the PSPICE model from ti.com and put it into TINA.  Is that correct?  The missing pin is pin 5 and that is the shutdown pin.  I suspect that if the shutdown pin is not connected, your amplifier may be in the shutdown mode.  This may explain strange gain results.  I could be wrong, but I think this is worth looking into.

    3.  As Kai has stated, the bias current for the TLV316 is much lower than for the LMV951.  Let's take the datasheet's maximum input bias current for the LMV951.  Under low voltage conditions where all the bias current flows through the feedback resistor, we get an output error of 80nA * 200MΩ = 16V!  This is far larger than the supply itself!  In comparison, we can take the typical datasheet input bias current of the TLV316 and estimate the typical output error: 10pA * 200MΩ = 2mV.  This is much more acceptable.  I would recommend selecting a part with a lower input bias current.  Otherwise, you will have to significantly downsize the feedback impedance.

    Regards,

    Daniel

  • Former Member
    0 Former Member

    Hello Ikeda-san,

    I have not heard back from you in a couple of days.  Is anything unclear?

    Please let me know if you have any further questions and I will be happy to help.

    Regards,

    Daniel

  • 0 in reply to Former Member
    TI__Expert 5335 points

    Daniel-san,

    Thank you for your support. They will try to change external components value and confirm the performance. If they have another question, I will post again.

    Thanks,

    Koji Ikeda

  • Former Member
    0 Former Member in reply to Koji Ikeda

    Hello Ikeda-san,

    Very well.  If they have any further questions, I will be happy to help.

    Regards,

    Daniel