a question on IIR low pass filter

one low pass IIR filter is implemented on a fixed-pointed DSP, say, TI
C5402.

The IIR filter itself is stable.
IF the input to the filter  is one constant value (DC signal ), eg,  X
input= 1000;

so, afte the stabilization, the output from the IIR filter is also
1000? or some value less than 1000?


Actually, I just wonder that the output will be less than 1000 because
of the finite word length effects. I am a new comer, so help me. Thanks

Simon wrote:

> one low pass IIR filter is implemented on a fixed-pointed DSP,
> say, TI C5402.
> 
> The IIR filter itself is stable.
> IF the input to the filter  is one constant value (DC signal ),
> eg,  X input= 1000;
> 
> so, afte the stabilization, the output from the IIR filter is
> also 1000? or some value less than 1000?

If H(z) is the filter's transfer function, the ideal steady-state DC 
output is H(1) * 1000 which could be any finite value, in principle. 
I guess you're making the unstated assumption that H(1) = 1?

> Actually, I just wonder that the output will be less than 1000
> because of the finite word length effects. I am a new comer, so
> help me. Thanks 

Finite arithmetic effects can disturb the values of |H(z)| in either 
direction. Consider the possibility that coefficient quantization 
shifts a nominally stable high-Q pole right onto the unit circle. The 
same may happen to a zero, and in either case the shift may be 
inwards too.


Martin

-- 
Quidquid latine scriptum sit, altum viditur.