Do digital Anti Aliasing filters exist for traditional ADCs?
Not in the sense you are discussing. There are other forms of "aliasing", but you seem to be considering only analog to digital conversion. If the signal is already digital (so that you can filter it digitally), then it's already been aliased. It's too late.
Where usually do people put these filters?. On the IC for the ADC itself?
I'm sure you can find ADCs with integrated filters, but it's not the norm. Different designs have different filtering requirements. You may need a linear phase filter, or you may not. You may need very good performance, or you may need very low cost. You might not even need filtering at all, if you know what frequencies your analog signal can contain.
Are physically big filters (e.g. through hole capacitors, inductors and resistors) the norm?
Not really, for reasons of cost. You need bigger components if you need to handle more power. High power is not usually something you need to drive an ADC. It may also be that a particular design requires a high capacitance or high inductance that's attainable only through large components, but a good engineer will avoid it if possible. Much better to use a 2 cent SMT capacitor, than a 20 cent through-hole electrolytic, wherever possible.
First you will need to find a suitable opamp which, at 2MHz, is not the 5534. GBW should ideally be 2 orders of magnitude more than your cutoff, or at least 100MHz (or close) and keeping it stable may be an issue.
Second, you can scale R-C networks to change the frequency. So for example, R1 sets the gain of the input stage and R1C1 set the frequency. You may be able to simply scale C1 as 50/2000 * 2200pf, or 56pf. (Ditto C2)
(You may have to reduce the stage gain, in which case reduce R1. 82 ohms and 560pf would work or some intermediate value keeping R1C1 constant.)
Ditto the passive 1st order stage R3,R4,C3 scale together so you could scale C3 to ... 560pf. The relatively low resistor values here should be fine at 2MHz.
The last stage (2nd order) is a little more complex because if you scale the time constants differently you will also affect the Q, or peakiness of the stage. But again the resistor values look fine so I would simply scale the capacitors, C4=680pf, C5,6=200pf (ideally 205pf).
And simulate as Andy says.
If it doesn't behave as expected, compare the original unscaled simulation with the scaled version. Look at each stage e.g. U1 output, separately.
The opamp characteristics will interact with these ideally scaled component values and the response may not be quite as expected especially if GBW or the output slew rate is too low.. A breadboard will introduce further stray capacitances and inductances, and the PCB layout will be slightly different again...
Best Answer
If you are interested only in signals in the range DC to 3kHz, then only signals above 7kHz will alias onto those.
This means that you need a filter with ...
a passband to 3kHz
a transition band from 3kHz to 7kHz
a stopband from 7kHz upwards
Note this doesn't define the 5kHz attenuation, and doesn't need to.
The stopband must have enough attenuation to protect your signals. If you want 0.1% fidelity for your lowpass signals, you need 60dB attenuation in your stopband. You will usually find it more practical to design an elliptic filter than a classical Butterworth or Cheby to get adequate stopband attenuation.
Now comes some subtlety, follow me carefully.
What does 'if you are only interested in signals in the DC to 3kHz' really mean?
If you are bandpass analysing signals up to 3kHz, for instance estimating the power in the bandwidth 2.5kHz to 2.7kHz, using a good digital filter to isolate the band, or an FFT which is equivalent, then having a transition band from 3kHz to 7kHz is just fine.
Although it allows signals in the 5k to 7k band to alias down to below 5kHz, they are still above 3kHz, and you're going to ignore/reject those signals anyway.
If however you are plotting the samples on a scope trace, then you may have inadvertent energy above 3kHz that is still valid. For instance if you have a 1.1kHz square wave, you will have harmonics at 3.3kHz, 5.5kHz, 7.7kHz.
Now, the crucial point is that the 3.3kHz harmonic will be coherent with the fundamental, and if you plot it, it will look OK, even if its amplitude has been suppressed a little by the filter. However, the 5.5kHz harmonic will be aliased to 4.5kHz, and will be incoherent with the fundamental because its frequency has been reflected through Nyquist, and will breathe in and out as its phase changes, and will generally look wrong.
What this means is, if you are going to end up making use of energy in the 3kHz to 5kHz band anyway (even though you've said in the OP that you're not), then your transition band should only go to 5kHz, not 7kHz, so that no aliasing at all is permitted into the DC to 5kHz band.
Whether your filter has a stopband from 7kHz, or from 5kHz, depends crucially on what you intend to do with your signals. Needless to say, it's much harder to make a filter with a 3k-5k transition band, than a 3k-7k transition band.