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LM2902: Output of LM2902 is clipped

Liz Jiang
Prodigy 70 points
Part Number: LM2902
Other Parts Discussed in Thread: OPA4990, OPA4170, TLV9304, TLV4170, ADS7028, LM7705, TLV9061, TLV9302

Tool/software:

Dear,

         We wrote to check the issue as below, U3 and U5 are powered by +24VDC, for the U3, we add a + 1.25V to the + port, and the Vout seems right,  but as for U5, we connected the +port to the GND, and the output of the U5 is not right when the input is very low. Is it the reason for this issue is the OPA(U3/U5) which we selected is not rail to rail? If yes, Would U like to recommend us a high precision, rail to rail, 4 channels, low speed application OPA? Thanks.

 

 

  • 0
    Guru 318490 points

    The LM2902 can output voltages near GND only if the load is not too high.

    Quad R2R-output opamps with similar paramters are the TLV9304, TLV4170, OPA4170, or OPA4990.

  • 0
    TI__Mastermind 26475 points

    Hello, 

    I agree with Clemens answers. To add on you can see the output swing to the rail under light load (10k) vs a heavier load (2k). This spec is the non-linear operating swing to the rail. 

    The linear swing to the rail is defined by the Aol (open loop gain spec). Below you can see that for a 15V single sided supply the linear swing to the negative rail (GND) is 1V above the negative rail or GND in this example. The positive linear swing to the rail is 11V or 4V below the positive rail. 

    Let us know if we can be of further assistance. 

    For more detail on input and output op amp limitations please see our Texas Instruments Precision Lab Training series below:

    https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Prodigy 70 points

    Hi Clemens and Chris,

              Thank u for your help. For my application the load is a TI ADC ADS7028IRTE#, and it's a light load. Does it means if I use TLV9304, the output of it will be improved when the value near GND? For our application the higher the accuracy, the better. Thanks.

    Best Regards,

    Liz

  • 0 in reply to Liz Jiang
    Guru 318490 points

    The ADC input can be modelled as an R-C circuit (see section 8.3.1 of the ADS7028 datasheet). The TLV9304 will be able to drive it, but to ensure that the output can go to 0 V even if there is an offset voltage, and to move the output into the linear range, consider using a negative supply for the TLV9304 (if you don't have one, add the LM7705).

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points

    Hi Clemens,

            I add a negative power to  TLV9302， and compare the result to add a positive 1.25V to the +port of TLV9302, it seems same. We can see in the below, the wave of the Vout and Vout1 are the same. In our device library, there are TLV9061 and positive power supply. Can I use these devices to build my circuit. So the 0 input becomes a 1.25V input. The the full range will be 1.25~3.23V, is there any risk. or will the output tolerance of this solution no better than the + - powered Rail to rail OPA?

    Thanks,

    Liz

  • 0 in reply to Liz Jiang
    TI__Guru* 77541 points

    Hi Liz,

    Here is my simulation. Please attach your schematic in the future, which it will save some time. 

    TLV9032 DA 11222024.TSC

    Let us know if you have other issues. 

    Best,

    Raymond

  • 0 in reply to Liz Jiang
    Guru 318490 points

    Yes; shifting the ADC range away from GND would be the easiest solution.