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From: John Larkin
Subject: Re: DSP 2's complement vs straight binary
Date: Thu, 17 Oct 2002 21:09:50 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On Fri, 18 Oct 2002 03:07:07 GMT, "Thomas Magma"
>If you were to simply invert the MSB it would convert the binary number to
>something called a "sign magnitude" and not to 2's complement. 2's
>complement is how higher level programming languages treats its 'byte'
>literals, this what I need.
>"John Larkin" wrote in
>> On Fri, 18 Oct 2002 01:08:36 GMT, "Thomas Magma"
>> >I am involved in a project where we sub-sample the IF, store it to memory
>> >and then post-process it. We are at the stage of selecting a ADC for the
>> >project. I have noticed that a lot of the ADC's suitable for this design
>> >only output straight binary. All of my algorithms to date have used 2's
>> >complement (signed) values to perform DFT or FFT functions. Processing
>> >is a major concern and we don't have time to convert each binary sample
>> >2's complement. Is there an equivalent method of the DFT or FFT that uses
>> >straight binary? Or does anybody know of a 3V ADC that puts out 2's
>> >compliment and can sub-sample 10.7mhz at less than 1mhz (pipeline)?
>> You can't just invert the MSB?
No, really. Quote from an Analog Devices datasheet:
The AD9221/AD9223/AD9220 output data is presented in
positive true straight binary for all input ranges. Table IV indi-
cates the output data formats for various input ranges regardless
of the selected input range. A twos complement output data
format can be created by inverting the MSB.
So you could do this with one XOR instruction, or just stick a
hardware inverter in the MSB data line.
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