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LMP7701: LMP7701/04

Shlomi Yehezkia
Mastermind 6150 points

Part Number: LMP7701
Other Parts Discussed in Thread: LMP7704,

Hi Team,

My customer would like to use a LMP7704 device as an inverting amplifier (with gain=1), this amplifier used as a bias voltage for low noise amplifier.

The  source of VG_LNA is output of DAC  DAC121S101QCMK/NOPB, the output range of DAC is 0÷3v so the output of amplifier is -3÷0v.

The schematic of amplifier is attached.

While VCC3V3 is 3.3v and VSS3V is -3v.

The question is : at this scenario the positive supply voltage of amplifier (pin 4) could be connected to GND or for best noise performance this pin should be connected to positive supply voltage?

Thanks,

Shlomi

LMP7701.docx

 

  • 0
    TI__Guru 74561 points

    The output of LMP7704 can swing only within 0.2V from either rail (see below AOL conditions) so if your customer needs the output to swing from 0V to -3V they MUST use minimum supplies of 0.2V (pin 4) and -3.2V (pin 11).  Thus, for 0 to -3V supplies a linear output voltage range is only from -2.8V to -0.2V

  • 0 in reply to Marek Lis
    Mastermind 6150 points

    Hi Marek,

    I think you not understood my question.

    The question was: if at output of OPAMP the voltage swing -3 to 0v so is enough supply voltage -3.3v to negative pin of OPAMP or I must also supply positive voltage to OPAMP?

     

    According to the TINA simulation, single negative supply is enough, but my customer claims supply also positive voltage, the reason is: it will be improve a noise at output.

     

    Thanks,

    Shlomi

  • 0 in reply to Shlomi Yehezkia
    TI__Guru 74561 points

    Shlomi,

    I’ve understood your question perfectly fine but you keep changing the negative supply voltage.  Initially, you had VSS at -3V (see below) but now you claim it’s at -3.3V;  anyway, the main problem is NOT with the noise issue but rather with violation of linear output voltage range.  Vout can NOT swing all the way to either  rail but rather only within 200mV to the supplies used.

    Therefore, on 0V to -3.3V  supplies, the output linear range is from -0.2V to -3.1V but for 0V to -3V supplies the Output range is from -0.2V to -2.8V, etc. (always 0.2V less than the supplies).  This is the reason why they need at least 0.2V positive supply in order to be able to swing all the way to 0V (200mV below the positive supply voltage).

  • 0 in reply to Marek Lis
    Mastermind 6150 points

    Perfect !!!

    So the noise performance not depended on the supply voltage range?

  • 0 in reply to Shlomi Yehezkia
    TI__Guru 74561 points

    As long as your total supply voltage is within 2.7V < Vs <12V and you do not violate the input common-mode voltage or linear output voltage range (see AOL conditions below), the input noise performance will not be a function of the supply voltage (at least to first order).