# Analysis on the output waveform from LTspice

Discussion in 'The Projects Forum' started by xw0927, Mar 8, 2014.

1. ### xw0927 Thread Starter Member

Dec 19, 2010
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0
According to the data sheet of LT6202, it can be amplify frequency up to 100MHz, but when I input 4Mhz,the output become distorted, so any explanation about it?

2. ### crutschow Expert

Mar 14, 2008
21,440
6,141
You need to read a little further in the data sheet. 100MHz (actually 90MHz typical) is the small signal gain-bandwidth product. There is another factor that affects the large signal high frequency response and that is output slew rate, which is 24V/μs typical, 17V/μs min. for the LT6202. The maximum slew rate of a sinewave is 2$\pi$fVp where f is the frequency and Vp is the peak sinewave voltage. This means the typical maximum output amplitude the op amp can generate @ 4MHz is 0.95V peak. For a gain of 2 this means the maximum input can be no more than 0.475V.

If you want a higher voltage at that frequency you need to find an op amp with a higher slew rate. This typically means a higher power op amp.

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3. ### xw0927 Thread Starter Member

Dec 19, 2010
114
0
HI crustshow, I have found a better opamp with high slew rate LT1818, and the gain is increasing as shown in attachment, and between can I use this to compensate sinc imperfect of DAC? as shown in other attachment.

• ###### LT1818.PNG
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Last edited: Mar 9, 2014
4. ### crutschow Expert

Mar 14, 2008
21,440
6,141
What do you mean " compensate sinc imperfect of DAC"?
You can add a low-pass filter to smooth the steps if you are generating a sinewave with the DAC but that's about it.

5. ### xw0927 Thread Starter Member

Dec 19, 2010
114
0
as my output sine waveform generated from DDS AD9850,when frequency up to 10 MHz, the amplitude start decreasing as sinc function like figure below: ....and i have found one solution to solve it, by using post equalisation technique to compensate the sinc response like this,but this circuit seem cant work, ...so any idea to solve this sincx response?

Jun 13, 2013
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xw0927,