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LMV932-N: Op amp input biasing question

William Crawford
Prodigy 10 points
Part Number: LMV932-N

Hi,

 We use the attached circuit in one of our products. In some cases the wiper of a potentiometer connected between +5V and ground is connected to IN1, but in most cases no potentiometer is connected. I have found that when no pot. is connected input IN1 tends to drift below +5V on some devices, evidenced by OUT1 voltage dropping, so I believe the original designer did not bias U1A properly.

 My understanding is that the common mode input range for the LMV932MAX/NOPB includes VCC (+5V). A short term fix is to connect IN1 to +5V, which seems to work, but I would like to know if there are any long-term negative effects in doing this.

 Ideally I think it would be best to bias IN1 at about +4V using a resistor divider, but I don't have pads on the board for another resistor. Can you tell me if using a lower value resistor for R1 would work, and how to calculate the best value?

 Thanks in advance,

Bill

  • 0
    TI__Mastermind 28486 points

    Hey William,

    Is the input dropping when pin3 on U1A is left completely open? This may happen if your input bias current pulls the open pin down. Putting a large resistor value will cause a voltage shift downward (about 35mV typical). This can be reduced if you lower the resistor value. You could use your input bias current to your advantage to force a lower voltage on the input if you put a very large resistor to V+ on the input (~30MEG). 

    Connecting the 1M resistor to V+ will keep the input bias current from pulling the pin down, as you have some known voltage being established on the input of your amp. There is not a long-term negative effect, as you are still in the valid common-mode voltage range. 

    Best,
    Jerry

  • 0 in reply to Jerry Madalvanos
    TI__Mastermind 28486 points

    Hey William,

    One additional note.

    The way you calculate the value of the resistor you need to drop the desired amount of voltage is you divide you desired voltage drop by your input bias current. Note that this will change over temp, your max bias current over temp is 50nA, so you can calculate with this value, but this value will typically be lower. (Typ input bias current is 14nA at room, which for a 1V drop means you need a 70MEG resistor)

    Best,
    Jerry