I want to drive an ADC with an OpAmp (LT1819). The op-amp is in non-inverting configuration driving the ADC pseudo-differentially.
The filter between opamp and ADC should be ("the specs"):
- -3dB at 100 MHz, 3rd order Butterworth or Bessel (or steeper)
- not load the Opamp too much (RL >1k)
- keep component values reasonable for 0402 (pF, nH)
- not introduce excessive noise (e.g., due to high series source resistance)
The classical 3rd order ladder filter (as found in literature and created by filter tables or programs like Elsie (http://www.tonnesoftware.com/elsie.html)) looks like:
This is a classical ladder filter described in literature - but it contains the termination resistor.
The first sad thing is that this filter attenuates my signal by 6dB (so I have to increase by apamp gain by a factor of 2).
Second, the signal gets distorted (clipped). It seems the LT1819 can't drive such small resistive loads.
Now I am thinking about the following options:
I could increase the resistor values. But in order to get the linearity to an acceptable level, the load resistance must be >>1k, maybe 10k. That means that the source resistance also needs to be that high and then I'm killed by noise. Besides, L and C values become unreasonable.
If I just take an ordinary RC I only get a simple 1st order rolloff.
The datasheet of the ADA4937 (Fig 67) suggests a solution:
However, using the indicated values (R=33, C1=20p, L=56n, C2=60p), the bandwidth I get is 85.77 MHz (not 125 MHz!). When I normalize the coefficients, they are
[2.07 , 2.07 , 0.0326 ] but for a 3rd order Butterworth they should be
[ 2 2 1]. Besides I do not know how this filter relates to the ladder filter described above (with resistive termination).