Delta-Sigma Modulator question

Dear all,
I've created a SDM ( Sigma Delta Modulation) system.
For the signal input, I used a 3000 Hz signal, but I need a 1500 Hz LPF
(Low Pass Filter) with - 80 db / dekade slope for output signal after the
signal being modulated and in demodulation process.
my spec for SDM system:
Input frequency 3000 Hz
Output frequency 1500 Hz
frequency sampling 25000 Hz
If I used 3000 Hz LPF, the output signal become static and in bad
condition. I've heard that SDM can push the noise to upper frequency. But
why is it that I need a lower LPF for my SDM system?

For the integrator, what voltage should I use for the reference voltage?
Inside a book entitled "Feedback instrumental", the SDM block diagram is
using a 2,5 volt for reference voltage for the integrator connected to the
non-inverting pin amplifier. Why is it using me 2,5 volt for the references
instead of 0 V or 5 V ?

Thanks



		
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Victor K wrote:
> Dear all,
> I've created a SDM ( Sigma Delta Modulation) system.
> For the signal input, I used a 3000 Hz signal, but I need a 1500 Hz LPF
> (Low Pass Filter) with - 80 db / dekade slope for output signal after the
> signal being modulated and in demodulation process.
> my spec for SDM system:
> Input frequency 3000 Hz
> Output frequency 1500 Hz
> frequency sampling 25000 Hz
> If I used 3000 Hz LPF, the output signal become static and in bad
> condition. I've heard that SDM can push the noise to upper frequency. But
> why is it that I need a lower LPF for my SDM system?

It reads a signal of one frequency and puts out a signal at double that? 
I don't understand.

> For the integrator, what voltage should I use for the reference voltage?
> Inside a book entitled "Feedback instrumental", the SDM block diagram is
> using a 2,5 volt for reference voltage for the integrator connected to the
> non-inverting pin amplifier. Why is it using me 2,5 volt for the references
> instead of 0 V or 5 V ?

In order for a circuit powered from a single supply to deal with bipolar 
signals, the signals must be offset. Half the supply voltage is a 
reasonable and common amount. Sometimes it's hard to distinguish between 
an offset voltage and a reference voltage. Often, it's merely a matter 
of viewpoint.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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