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TLV9361: Unexpected Gain Reduction in Circuit Using TLV9361 and LP2982 LDO with Series Batteries

yunsik kim
Prodigy 140 points
Part Number: TLV9361
Other Parts Discussed in Thread: LMV1012, , OPA375, LP2982

Tool/software:

Hi, Ti.

I constructed an electret microphone amplification circuit using LMV1012 and TLV9361.
When I designed a circuit with a single 3.7V battery and an OPA375 amplifier, it worked well.
However, to double the output voltage swing range, I replaced the OPA375 with a TLV9361 and used an LP2982 5V LDO to set the bias voltage for the LMV1012,
designing a circuit that uses two 3.7V batteries. But it doesn't work properly.
The input signal is fed through J4 from a signal generator, and the two batteries are connected in series through J3 and J6.
According to the design, the gain of the LMV1012-25 should be 24dB, and the circuit should add an additional 40dB, resulting in a total of approximately 64dB.
However, the actual gain was about 30dB lower than expected.
I am not sure if the issue is that the LDO is supplying voltage to the op amp input or if it's a problem with the circuit design.
The schematic is below.

Thank you.

  • 0
    TI__Guru**** 158636 points

    Hello Yunsik,

    LMV1012 (not my group) has a voltage gain spec. Connecting it to an op amp inverting input (a virtual short) with a capacitor will AC short out the LMV1012 and the op amp amplifies the shorted current into an output voltage. I expect this is not what you wanted and not what the LMV1012 expected. 

    Try adding a resistor in series with TC4. 1k ohms for 40dB gain in TLV9361, but lower cutoff goes to 70Hz.  AC load is now 1k ||2.2k =687 ohms not 2.2k , so LMV1012 gain may drop. So consider increasing R1, so new resistor can be larger too. 

  • 0 in reply to Ron Michallick
    Prodigy 140 points

    Hi Mr.Ron,

    The schematic below is from a circuit I designed previously that operates with a 3.7V battery.

    In that circuit, it worked properly even without a resistor at the inverting input of the op amp.

    I'm curious about what's different in the above circuit that requires an additional resistor.

    Thank you

  • 0 in reply to yunsik kim
    TI__Guru**** 158636 points

    Yunsik,

    For the op amp , I don't see any meaningful change. I transferred this post to the engineer supporting LMV1012. He should know if this affect the LMV1012 or not.

  • 0 in reply to yunsik kim
    TI__Guru** 105175 points

    Hi Yunsik,

    It seems to me that the circuit is pretty much the same, you just replaced OPA375 with TLV9361 and increased the power supply range. However, the passive components associated with the op amp are the same, thus the same gain should be expected. Correct?

    If you compare the signal output from LMV1012 between the two circuits, is there any difference?

    Perhaps OPA375 tolerates the lack of input series resistor while TLV9361 does not.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 140 points

    Hi Ivan,

    I confirmed that the virtual short of the TLV9361 is not functioning properly.

    The op amp inverting input should show 3.7V, but it actually showed about 1V, and this was the same even with a series resistor added to TC4.

    The LMV1012 is working properly, and it amplifies the signal by 30 dB.

    Best Regards.

  • 0 in reply to yunsik kim
    TI__Guru**** 158636 points

    Yunsik,

    Can you tell me the voltages on all TLV9361 pins?

  • 0 in reply to Ron Michallick
    Prodigy 140 points

    Hi Ron,

    Below are the waveform photos from each pin.

    I applied an AC 1mV to the input of the LMV1012.

    The photos show the minimum and maximum values of the waveform.

    Looking at the non-inverting input of the TLV9361, it is oscillating at 7.4 Vpp, which seems to be the problem.

    Ground pin was 0V and power supply pin was 7.37V.

    Non-inverting input pin

    inverting input pin

    output pin

    Best regards.

  • 0 in reply to yunsik kim
    TI__Guru**** 158636 points

    Yunsik,

    So odd to see huge oscillation on Non-inverting input, which is pin 1 in the schematics. I see nothing on this pin that can output such a signal.

    If the DBV package was used with board setup for DCK pinout, that could make such a signal.

     

  • 0 in reply to Ron Michallick
    Prodigy 140 points

    Upon checking, I realized that the board designed for the DCK pinout was using the DBV package.

    The issue has been resolved.

    Thank you.

    Best regards.