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About ac analysis by LTSPICE of AD8606

Category: Hardware
Product Number: AD8606

I checked the frequency characteristics of the AD8606 with LTSPICE.

The result of ac analysis was -3.87dB at around 20MHz, but the result of tran analysis was about 50mV/1V=-26dB at frequency = 20MHz.

Could you tell me what causes this difference?

thank you.

Parents Reply
  • Thank you.

    If you zoom into the transient waveform you will see the problem.  Rather than a nice sinewave, it is quite triangular - a sure indication that you are running into the slew rate limit of the amplifier.  Checking the datasheet, it is 5V/us (typ).  For a sinewave, the max slew rate is A*2*pi*f.  Rearranging this for signal amplitude, at 20 MHz, max A = 5000000/125663706 = 40mV.  If you drop the source below this level, you start to see attenuation closer to the expected result from the AC small signal analysis (~-4dB at 20MHz).  The ac analysis is 'small signal analysis' so will not take into account slew rate, only the bandwidth of the amplifier.  However, you will see 'large signal' effects like slew rate limitation in the transient simulation.  This is actually quite a good example of the difference between amplifier bandwidth and slew rate.

    I hope that helps.

    Run the following simulation and check the error log for the apparent attenuation due to slew rate for different values of input signal amplitudes: