sunking said:
pay attention to you clock frequency.
the output prestage and the input nextstage pay attention to no equal R
Thanks for your reply,
What do you mean "no equal R?"
For the sampling clock, in my knowledge, they should be referred to the same
clock phase. If we define the sampling phase is phase I for first stage, and the
second one must sample in the same clock phase.
Added after 31 minutes:
I'm wondering, I have taken the loading effect of the second stage into
consideration, So, I increase the OPamp bandwidth of first stage to prevent the loading to kill the bandwidth and reduce the settling time.
By my simulation, I exchange the position of HPF and LPF alternatively, the transfer function is ok for the first stage, but it of the second stage goes wrong.
For example, if the HPF is placed at the first stage, the transfer function is ok, but
it's wrong if it is placed at second stage. If the LPF in the first, the LPF transfer
function is ok, but it's wrong if LPF is in second.
So, by this, I'm wondering is this result is because of the change of source
impedance? In the individual simulation, I put a 50 ohms to be the source impedance. The situation is different from the cascaded case.
I'm not sure the analysis is correct or not, any comment on this?
Added after 2 hours 48 minutes:
flatulent said:
My advice about the switching noise is from a board design I did using two Linear Technology switching filter ICs. The output switching noise was different depending on which chip was first and second. I had to put a RC low pass filter on the final output to get rid of most of the switching noise.
In your case, first look at the output noise on each one when it has a resistor on the input that is the same as the impedance of the circuit preceding it.
You can then decide to put a RC low pass or high pass filter after it to get rid of the out of band noise. Then try the filters together in both combinations and select the sequence with the lowest output noise.
I can't understand why the switch noise of first stage will affect the transfer function of second stage? Would you explain it in more detail?
I have tried to make the HPF be the first and LPF be the second. The transfer
function is looked better than the case LPF be the first. But it's still a little different
from the characteristic of LPF when it's alone.
In the cascaded filter design, I didn't see any textbook or reference with the
precaution of this. But it seems to happen often.