Hi,
  Signal from the sensor passes through the following blocks in my system -

Ac Coupling circuit, Anti-Aliasing filter and then goes to DSP controller
where it is low-pass filtered twice.
I want to calculate the total phase shift introduced by the system at
frequency 50Hz. Following are the required data :

                              Phase Delay                Group Delay

Anti-aliasing filter    -10.603 degrees (at 50Hz)     595.264 usec (at
50Hz)

low-pass filter-1            -0.93 degrees/Hz           2.5838 msec
(in DSP controller)

low-pass filter-2            -7.4415 degrees/Hz         20.6706 msec
(in DSP controller)

Which factors need to be considered and how can the total phase shift be
calculated? Please help!

Karan M. Banthia.

Its been a week now and no one has replied till now... Please help
experts!!

(top posting fixed)

No one has answered because the question is malformed, and it looks an
awful lot like homework.  We always tend to approach malformed questions
hesitantly, and at the end of long poles.  And we _don't_ answer homework
questions directly, because we _don't_ want more incompetent engineering
managers in the world.

The reason it looks like homework is because it is laid out somewhat
artificially, it contains assumptions about closing control loops that
someone with DSP knowledge and little control systems chops would likely
make (i.e., a prof), and because if you know how to get the intermediate
answers you have, then you know how to get the final answer.

In short, it's a contrived exercise to get you to go from sufficient data
to an answer, but it's not a question that you'd come up with in the
workplace and then have trouble answering.

The reason that it looks malformed is because it gives the phase shift in
the correct dimensions for the anti-aliasing filter, but in different
dimensions for the low-pass filters.  The specifications as given would
make sense if the two low-pass filters were symmetrical FIR filters, but
using FIR filters inside a control loop is generally quite unwise.

(For that matter, entirely unlike audio applications, one should
generally avoid using an anti-alias filter in a control loop.  There are
times that they're necessary, and using an anti-alias filter is not, in
general, as dumb as trying to close your loop with FIR filters, but it's
still not generally recommended:
http://www.wescottdesign.com/articles/Sampling/sampling.pdf)

So -- what's the class, and how far have you gotten toward answering the
question?  If you know the frequency of interest, and you know the phase
shift per Hertz, then how might you find the phase shift at the frequency
of interest?  If you look at the given group delay for the FIR filters,
and you look at the given phase shift slope, are the numbers consistent?  
What equation do you use to figure this out?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com