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TLV9151: Slew rate TLV9151 problem

Michal Karas1
Prodigy 130 points
Part Number: TLV9151
Other Parts Discussed in Thread: TLV9161, TLV9051, OPA323

Tool/software:

Hello,

I have a problem with the op amp TL9151SIDBVR. It operates in a differential to single ended circuit. The problem is that for frequencies above 50kHz the sinusoidal signal starts to look like a triangle. This looks like a slew rate problem, but how can this be when its slew rate is 20V/us. What could be the reason for this?

Schematic 

Layout of  U8 diff to single ended stage

Below Measurements

50 kHz

75kHz

100Khz 

Simulation Tina for 100Khz works fine

  • 0
    TI__Mastermind 18907 points

    Hey Michal, 

    Welcome to e2e Slight smile

    Interesting! I have a starting recommendation. The reference is produced by one of the opamps (U12), which is driving 100nF capacitive load. Unfortunately, the TLV9151 cannot support such a large capacitive load. With respect to Figure 6-29, I would reduce C68 to be ~1nF and at max 2.5nF. 

     

    Could you please share your TINA sim? The file may be attached using the following process. 

    All the best,
    Carolina

  • 0 in reply to Caro Walter
    Prodigy 130 points

    TLV9152_Simulation.TSC

    Yes I forgot to write that C67 and C68 are not assembled. 

  • +1 in reply to Michal Karas1
    TI__Mastermind 18907 points

    Hey Michal, 

    Thanks for the simulation profile and clarification of C67 and C68. 
    I agree with the original statement that the issue may be slew rate. The problem is described in this application note, Ramping on Slew Rate. The TLV9151 has slew boost architecture and the full slew rate is not achieved without a large enough input differential, in this case it requires atleast 1V VID (IN+ - IN-). 

    I recommend the evaluation of the TLV9161 as it has better performance in SR for this low of VID: 

    All the best,
    Carolina

  • 0
    Prodigy 20 points

    Hello,

    I confirm the issue with the slew rate, when the TLV9151 is powered from single 3V3 source. Take look on a step response:

    Please note that the slope is steeper at the beginning, but it flattens out when the output reaches higher levels. The slew rate in the last part is only about 13mV/us!

    When power supply is changed to a higher voltage - 6V:

    The slope is steeper by a higher range of the output amplitude.

    In the datasheet the slew rate is given only for 16V supply:

    Despite name "rail-to-rail output", TLV9151 can not sustain promised slew rate in the full output swing.

    It is a shame, that the datasheet does not specify slew rate in the other conditions, or a relation between the slew rate and a supply voltage or output swing.

    It looks that the ramping on a small differential signal is not a problem in that case - the problem is due to the output stage of the amplifier.

  • 0 in reply to Aleksander Kaweczynski
    TI__Mastermind 23170 points

    Hi Aleksander,

    Aleksander Kaweczynski said:
    Despite name "rail-to-rail output", TLV9151 can not sustain promised slew rate in the full output swing.

    Actually "rail-to-rail output means that the output is able to swing the output very close to both the positive and negative supply rails. It has nothing to do with the slew rate.  

    Aleksander Kaweczynski said:
    It is a shame, that the datasheet does not specify slew rate in the other conditions, or a relation between the slew rate and a supply voltage or output swing.

    I can see the frustration. Lot's of op amp datasheets with slew boost don't share much information on how it behaves outside of the test conditions shown. However, with this being a high voltage part, we determined most of the use cases where people will need a 3.3V supply voltage would just use a low voltage device. 

    Also why did you choose the TLV9151 over a low voltage device like the TLV9051 or the OPA323 which have very high slew rates (15V/us and 30V/us respectfully)? 

    Best Regards,

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 20 points
    Robert Clifton56 said:
    Actually "rail-to-rail output means that the output is able to swing the output very close to both the positive and negative supply rails. It has nothing to do with the slew rate.  

    Hi Robert,

    Technically you are correct. But the more explicit note in the datasheet, something like "better keep your output signal 1,5V away from supply rails to get best performance, better than 5V/us" will be appreciated.

    It is "trap for young players" - a secret that you should use opamp with supply voltage range as close to your power supply as possible.

    Robert Clifton56 said:
    Also why did you choose the TLV9151

    I have just made a cross-check for my colleague Michal. But I truly understand his choice - the selected device meets the design requirements, and there was no obvious "red flags". Also TINA does not shown any possible problems. But, as I understand, the simulation model does not represent internal structure, but only the general behavior, made by for ex. voltage dependent sources.

  • 0 in reply to Aleksander Kaweczynski
    TI__Mastermind 23170 points

    Hi Aleksander,

    Aleksander Kaweczynski said:
    But the more explicit note in the datasheet, something like "better keep your output signal 1,5V away from supply rails to get best performance, better than 5V/us" will be appreciated.

    I agree, this would be helpful to have stated somewhere. I can reach out to the team responsible for the datasheet and let them know your thoughts. 

    Aleksander Kaweczynski said:
    Also TINA does not shown any possible problems. But, as I understand, the simulation model does not represent internal structure, but only the general behavior, made by for ex. voltage dependent sources.

    We do our best to make the model as accurate as possible. There's of course limitations. 

    Best Regards,

    Robert Clifton