I design a 7-order linear phase low-pass filter (which is exactly the same anti-aliasing low-pass filter used in Agilent 33220A), which is:
This filter is used as an anti-aliasing filter to filter the current output of AD9744 DAC (digital to analog converter). Plus the passive filter itself, it also include some load resistors such as R1, C1, R3, R2 which are shown in the following picture. I simulate this circuit in LTSpice:
However, the magnitude and phase response is NOT flat, as can be seen in the following picture (V(ap) is the voltage at the ap node, V(bp) is at the bp node):
Where the voltage at the ap node in NOT flat, which seems like to be fluctuated, and the response at bp is also NOT flat. So what is wrong with my circuit?
The followings are my simulations using NuHertz:
And the simulation in NuHertz using the values in Agilent 33220A: