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LM7321: Minimum Input at IN+ pin

Cedrick
Expert 8270 points
Part Number: LM7321

Hi, 

Good day. I hope you are doing fine.

Our customer is using the LM7321 and they have inquiry about the minimum voltage at IN+ pin. Kindly refer to the inquiry below. 

"I know for a power source there is a minimum 2.5+ VDC, but for the signal that it is amplifying which I believe is IN+ on the pin configuration from the datasheet, is there a minimum voltage that should be here? Right now I have .03 VDC here with a gain of 11, but it seems to be only giving me an output voltage of .203 which is around a 6.7 gain"

I hope you could help us with this matter. Thank you for your kind support. 


Regards,

Cedrick

  • 0
    TI__Genius 15410 points

    Hi Cedrick,

    This device is a rail to rail input device meaning that the common mode voltage extends from the negative supply to the positive supply. The common mode voltage for a non-inverting input is equal to the input signal amplitude. The common mode voltage for an inverting amplifier is equal to the voltage at IN+, which is typically a bias voltage or GND. 

    Please know that the maximum offset voltage of the device is 5mV. So in a gain off 11V/V you can see an error of 55mV at the output. There are other error sources to consider such as input bias current.

    Can you please post a schematic so we can take a look at what might be causing the issue?

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    TI__Guru**** 151276 points

    Cedrick,

    Read output voltage for at least two different input voltages. Gain of 11 is dVOUT/dVIN because VOUT = VIN * G + B and we don't really know B.

    Be sure to place you negative meter lead at the ground side of the ground resistor like this. Now you will get VOUT = (VIN +Vos) * 11

    If the VIN comes from a current sense resistor, check your layout kelvin connection very carefully.

  • 0
    Prodigy 10 points

    Part Number: LM7321

    Hello, I know for a power source there is a minimum 2.5+ VDC, but for the signal that it is amplifying which I believe is IN+ on the pin configuration from the datasheet, is there a minimum voltage that should be here?Right now I have .03 VDC here with a gain of 11, but it seems to be only giving me an output voltage of .203 which is around a 6.7 gain.

    I spoke with Cedrick Infantado from Texas Instruments Customer Support who mentioned that the maximum offset voltage of the device is 5mV, so with a gain of 11 it could be an offset of 55 mV, but I am seeing around 130 mV offset on the output. I can provide the schematic for the opamp if it helps. 

  • 0
    TI__Mastermind 28796 points

    Hello Shane,

    Could you please provide your schematic? That would help me understand your question.

    Best,
    Jerry