• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 48 subscribers
  • Views 258 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BUF802: DC to 100MHz Frequency Response

Chi-Yuan Huang
Prodigy 20 points
Part Number: BUF802
Other Parts Discussed in Thread: OPA140, OPA814, OPA817, OPA659

Tool/software:

Dear TI Teams,

I tried to use the file provided by TI BUF802 page to simulate an 1M oscilloscope front end.

Although it is possible to get a flat gain across the range with default setup, the unity gain response is no longer available.

By matching the feedback resistor R7 into 20k yield the unity gain response at low frequency, but that still cannot achieve unity gain response with OPA140

Do you have any buffer solution that can provide unity gain from DC to 100MHz?


  • 0
    TI__Genius 11505 points

    Hi,

    R7 in this circuit is meant to be adjusted to get a flat frequency response. In the actual application this value will vary depending on many factors including the tolerance of the components in the circuit. Figure 9-5 in the datasheet highlights a response that can be achieved by tuning this value in the circuit. Technically you do not need this composite loop circuit, and it could be the BUF802 alone, however DC precision will suffer. Are you looking for a simpler solution using a regular voltage feedback amplifier capable of being configured as a buffer? 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 20 points

    Hello Ignacio,

    Yes, I know the purpose of the R7 is to get a flat response across full frequency range, but my question is, there's no way to get 0db from DC to 100MHz because the gain of BUF802 is not 1 at high frequency.

    It would be nice if you can teach me how to get the frequency response just like Fig. 9-5 in BUF802 datasheet. 
    Or if you have any suggestion on simpler solution to do that, I would love to knowBlush

    BR, 

    Chi-Yuan

  • +1 in reply to Chi-Yuan Huang
    TI__Genius 11505 points

    Hi Chi-Yuan,

    Yes, you are correct the BUF802 is not exactly 0dB. Figure 9-5 does highlight a relatively flat response; however, the data is normalized which is why it sits right on the 0dB point. If you require a response that is truly 0dB from DC to 100MHz, you could look at a buffer configuration using something like the OPA814. The BUF802 is a very unique device with various performance metrics that a standard voltage feedback amplifier cannot meet, however if the goal is a buffer configuration something like the OPA814 could be considered.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Prodigy 10 points

    Hi Chi-Yuan,

    If your application is to use BUF802 for a 1MOhm Oscilloscope Front End, the composite loop will help you satisfy the 300V DC Compliance that usually Oscilloscopes demand. The Gain Error that comes with BUF802(G is close to 0.99) can be digitally rectified after the ADC Sampling happens. BUF802 has very good flatness for higher frequencies and by tuning R7, you can change the gain of the composite loop to match with BUF802's gain, which results in a smooth crossover and a flat response from DC - 1GHz. Since other OP AMPS doesn't use any composite loop, the max voltage compliance will not be met. 

    But, if your application is not for an oscilloscope front end, and you are looking for any op amp that can give you unity gain response from DC-100MHz, you can go with OPA814 as suggested by Ignacio. You can also take a look at OPA817 which supports large signals till 100MHz, or you can also go with OPA659. OPA659 and OPA817 have higher bandwidth than OPA814 at the cost of higher Quiescent Current. 

    Regards,

    Saket

  • 0
    Prodigy 20 points

    Hi Ignacio and Saket,

    Thanks for your kind reply, I think in my case it is hard to calibrate the front-end digitally to get 0dB gain.

    Anyway, it seems that OPA659, OPA814 and OPA817 can meet my requirements.

    Thanks for your support.

    BR,

    Chi-Yuan