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WEBENCH® Tools/PGA281: Output amplitude decreasing while increasing the input frequency

Eashwar Kumar
Prodigy 40 points

Part Number: PGA281

Tool/software: WEBENCH® Design Tools

we are using PGA281 in our design as shown in the image attached.

the requirement is to find the upper and lower peak of a signal that varies from +/-1Vp-p to +/-10Vp-p at frequencies upto 2MHz.

we are currently giving 2Vp-p sine wave at frequency of 100KHz to 2MHz,when ever the frequency crosses the 130KHz the amplitude at the output of PGA281 is decreases.

this amplitude decreasing continues as frequency increases.

whether the PGA281 will work for 2MHz input signal????

if not what is the frequency  limit???

please suggest any hardware changes to support max possible frequency.

Thanks 

Eashwar Kumar K

  • 0
    TI__Guru* 93105 points

    Hello Eashwar,

    Like all amplifiers the PGA281 has an upper bandwidth limit. Since it is a PGA, that bandwidth changes with the gain setting. The datasheet on Pg. 9, Figure 16, shows a graph for SMALL-SIGNAL GAIN vs FREQUENCY. Note that this is labeled as small-signal gain so likely, the test signal had a low level such as a few tens-of-millivolts. A 2 Vp-p signal would actually be a large signal and the bandwidth may be somewhat less than that achieved with a small-signal.

    You can see from graph that the bandwidth for the higher gain settings roll off after about 20 to 100 kHz. As the gain is reduced the bandwidth extends out a bit further, but still not much more than 1 MHz. before it rolls off. The low gains and gains less than 1 V/V (0 dB) actually have significant gain peaking, but their peaks begin to roll off after 1 MHz. For useful, accurate gain setting vs frequency it appears that the upper limit is somewhere between 20 and 100 kHz depending on the gain.

    So if the gain is rolling off after 130 kHz that likely coincides with the gain setting you are using. Roll off is normal when the bandwidth limit is reached and should be expected. The PGA281 has little gain remaining at 2 MHz; thus, it shouldn't be used for frequencies that high.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points
    Hello Thomas,

    Thanks for the quick response.
    could you please suggest a PGA that can attenuate a +/-10Vp-p 2MHz signal to +/-2.5Vp-p 2MHz signal.

    Thanks
    Eashwar
  • 0 in reply to Eashwar Kumar
    TI__Guru* 93105 points

    Hello Eashwar,

    TI doesn't produce a PGA that accomplishes what you have specified. Is the application requirement for a fixed gain of 0.25 V/V, or must the gain be switchable to other values?

    Regards, Thomas

    Precision Amplifiers Applications Engineering