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LMP7721: Dual OpAmp Configuration Issue

krishna prasad
Intellectual 415 points
Part Number: LMP7721


We are Using the Below Configuration as Transimpedance Amplifer with Following Parameters

Please refer to schematics Attached:- Circuit Portion 1

1)IS1OUT- Output from Nanosensor Which is in the range of 0nA to 10uA 

2) U6 Output converts the IS1Out to Voltage and compared with Vref=5V.

3)This circuit being NIV Transimpedance amplifier initially output is 5V at IS1out=0nA and Decreases with a resolution of 1mV based on raise of IS1out. Range is 0 to 5uA.

4)Load for U6 is Circuit Portion 2

Problem Seen 

1)If Vref is above 3.3V in the circuit attached for  temperatures in the range of 25 to 45C the Output reaches 0 and recovers back when temperature is reduced to 25C .

2)Attached is typical data log for Reference. 

Can we get a feedback on what would be the cause of the Problem seen??

Note: 2 Attachments Schematics in word doc file and data log in Excel fileTIA Chamber Test Data.xlsxTI_Schematics_June26-2019.docx

  • 0
    TI__Guru 68541 points

    Krishna,

    The linear range of LMP7721 input common-mode voltage range is from 300mV below negative rail to 1V below positve rail (see below) BUT your application violates this specification - your input is within 0.5V below positve rail.

    Since your AVDD is at 5.5V, the maximum voltage you may apply at the non-inverting input terminal is Vref =< AVDD-1V = 5.5V-1V = 4.5V and not 5V - see below.

  • 0 in reply to Marek Lis
    Intellectual 415 points

    Marek Lis

    Understood the CM Diff Voltage should not be >1V & Causing a issue but why is the Output stable up to 35C & when Temp goes beyond this it goes to 0 nA. 

    Is there a specific reason technical for this?? for a deeper understanding.

  • 0 in reply to krishna prasad
    TI__Guru 68541 points

    It's the input common-mode voltage of LMP7721 that must be at least 1V below positve rail (Vcm<4.5V on Vcc of 5.5V).  The output may seem to be working with the input within 0.5V below Vcc but this does not mean it is linear - this means that the input offset, drift, noise, etc. does not meet datasheet limits.