Part Number:THS4509
Hello,
I'm struggling with the distortion generated by an active filter using a THS4509 and a THS4541.
The (AC coupled) low pass filter has a gain of 17dB and a cutoff frequency of 33MHz. The output of this filter is used to drive an ADS4128 12 bit 200MSPS ADC.
The input of the filter is single ended terminated at 50 Ohm and the output is differential.
Both stages are amplifiers with RLC LPF outputs in order to lower GBW requirements.
I am using TinaTI to simulate the circuit.
My problem is that I get a very very low harmonic distortion (in nV) at 10MHz (within the band pass) which is good, but a lot higher distortion at 5MHz (in tens of µV) for example (within the band pass too), which does not makes sense to me. Is this expected? How can I remedy to such variations due to the input frequency?
The PCB is already manufactured. I have a source able to generate a good 10MHz sine, and I was able to obtain the graph below. However, I don't have any other clean source, so I can't measure 5MHz for exemple in real life, I have to rely on TinaTI.
The TINA TI simulation file is attached.(Please visit the site to view this file)
Thanks in advance for your help!
Victor
