Are low pass filters always required to be used after DAC or are they integrated?

[matrixofdynamism]

Usually a DSP diagram shows the analogue signal passing through an anti-aliasing filter then go into an ADC and then into the DSP and then come out and pass through a DAC and then a low pass filter also sometimes called the reconstruction filter.

1) Why are anti-aliasing and low pass filters not always integrated into the data converters?
2) How does one know what order filter to use for anti-aliasing or the reconstruction after DAC. Both are low pass filters.

 

[FvM]

Why are anti-aliasing and low pass filters not always integrated into the data converters?

Hardly possible, in general. With the exception of oversampling ADC (Sigma-Delta type) that mostly have a high order low-pass filter built in. They can be operated with only a simple low-pass filter removing input signal components above half the oversampling frequency. Same thing with oversampling DACs (e.g. most recent audio DACs).

The requirement for antialiasing filters and their stop-band characteristic is set by the presence of aliasing components in the input signal. If you don't have any, no filter is needed. The required filter quality depends on the ratio of highest signal to nyquist frequency, level of aliasing signals and intended ADC dynamic range. Define a filter curve based on this parameters and find the filter order and type.