I will apologise in advance and, of course, claim that I knew this already........ because someone else told me but I forgot and you have reminded me.
Ahaaaaaaaa!!!!!!!!! there you go then. 1.22 times 2 is 2.44 and not 2.56 or
2.55.DNA
A long time ago when the earth was green we designed some stuff using
0-10V analog I/O. Back then we used a 10.24V reference for the ADC. On the surface it would seem obvious to do this because then you have a neat 10mV per step for a 10 bit ADC. This can be handy, and I guess that's the main reason why they made 10.24V values.We didn't care about having a nice round number for each ADC step, but there was another reason to use a slightly higher voltage. There was some unavoidable differential noise on the inputs. If you read a 10V input that has differential noise on it using a 10V ADC, then on average you read a value less than 10V. This is because the ADC can only see the 10V signal, plus the negative portion of the noise, or MIN(10,(Vin+Vnoise)). If you compute a rolling average to filter the noise then the average value is always less than 10V. So, if you measure a 10V input with a 10.24V ADC, then you can also measure the positive portion of the noise, and filter the readings to correctly interpret a 10V input as 10V.
These days they want to put a zillion die on each wafer, so the chips can't handle 10V. So, everything has been scaled down by a factor of
Time to go home...
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Greg Neff VP Engineering
*Microsym* Computers Inc. snipped-for-privacy@guesswhichwordgoeshere.com-- Posted via a free Usenet account from http://www.teranews.com
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