Hi,

I met a strange problem recently. I built a 5 order chebyshev low pass filter with a 2db ripple in the passband. The opamp used in the filter is a fully differential op with open loop gain 58dB, 3dB bandwidth 220kHz. At 1.2MHz, it still has 45dB gain.

I test the filter with 1Vpp input swing, supply is 1.2V and sweep the input frequency from 10k to 1MHz, the cutoff frequency is 1.2MHz. The filter works fine with low frequency from 0-100KHz, however, the amplitude of the output signal is attenuated when the frequency increases from 200KHz and the attenuation is more than 3dB for some frequency. The transient simulation shows that at frequency around 300KHz, the signal is distorted very much even higher than higher frequency.  I could n't find the problem, does anybody know what might be the reason for this attenuation? Any suggestion would be appreciated. Thank you very much


Best Regards
james

To James,
 According to your large input swing, the distortion and the extra attenuation may be introduced by the nonlinearity of the mos switch, if adopted.
Another reason might be the deviation of f-3dB which caused by the change of the effect resistance or capacitance.

K/R,

Jim

Hello James B.

Which topology was chosen for your filter ?
Did you use a suitable opamp macro to simulate the response ?
What is the opamp slew rate ?

Hi,

I have solved the problem, the signal is clipped in some of the stage that's why the thd is very bad.  

jimmy