decimation filter mapping

[svensl]

Hello all;

Consider a sigma delta modulator with a sinc decimation filter.
The output will be a 7 bit digital signal. Power supply is 1.5V.

What I want to do is throw away roughly 100mV from the supply rails. So I want my effective output to be in the range of 0.1 to 1.4V. Hence I want the output of the decimation filter to be b'0 for 0.1V and b'1 for 1.4V with 128 LSB steps in between.

Obviously the digital o/p for 0.1V is NOT 0000000 and similarly NOT 1111111 for 1.4V. How can make sure that 0.1V and 1.4V map to the correct digital values?

Thanks

 

[FvM]

Please pay attention the board rules. Avoid cross-posting.

I don't see the question related to CIC. It's simply about signal scaling. Scaling should be done after the decimator to achieve linearity also near the range limits. You should have one bit extra resolution to compensate for the scaling truncation error, if S/N is sufficient.

 

[svensl]

Thanks for your response. I am not sure how one would go about this signal scaling.

As a simple example consider a desired 3bit output signal with 1V as reference. Now using an additional bit at the o/p of the sinc I could distinguish between 16 states. So the LSB is 1/16V. Assuming I want to ignore 2 LSB (2/16V) off of the rails my digital 4 bit code range is 0010 to 1101. Or in analog terms 1/8V to 7/8V.

So how do I map this to a 3 bit signal 000 to 111?


Regards,

I just realized that if I remove the LSB from a 4 bit number I end up with a 3bit number with each value occurring twice. So yes, an extra LSB should work.

Thnks

 

[FvM]

I thought of scaling by multiplying the CIC output with a scale factor, substracting the offset, applying saturation logic at the range boundaries to prevent from arithmetic overflow, stripping the low bits to achieve the intended resolution. You can check the results by pencil and paper method or with a pocket calculator. To achieve an equally spaced output mapping, a higher resolution overhead (2 or 3 bits) is required before the scaling operation. In special cases, the decimation factor can be modified from power of two to an arbitray value. This also provides a scaling option.