Thanks for the responce. I am trying to measure a fairly stable DC voltage in the range of .200 to 2.000 volts. I need .001 volt resolution minimum. The voltage wanders a bit but only in the realm of milliseconds, and by millivolts. I want to take many sample and average them. ----- Original Message ----- From: "Wagner Lipnharski" To: Sent: Wednesday, February 12, 2003 4:15 PM Subject: Re: [EE]: 12 bit ADC or amplify for 10 bit ADC? > Thomas Sefranek wrote: > > I think you have things backwards... > > You can not add range to an A2D with an amplifier. > > > > You want the BEST linearity in your amplifier... > > Specify what signal you intend to amplify. > > hmmm. > > I think the point is not exactly "range", but "resolution", and that is > what I believe he is looking for. > > Suppose you have a signal to measure, that can swing from zero to 1000mV. > Suppose your 10 bits unipolar ADC reference voltage is 2000mV. > It means that when the signal is at span, your ADC would be using about > half its bits, or, (01 1111 1111) 01FFh. So, in real, your system is using > only 9 bits for the whole range of the input signal, with a resolution of > 1.953mV. > > Suppose you change the ADC to an unipolar 12 bits, with the same reference > voltage of 2000mV. > Now, when reading the 1000mV signal, even that this 12 bits ADC is only > using 11 of its bits, it would be generating around (0111 1111 1111) 07FFh, > what is 2 more bits (4 times better), or, a resolution of 488uV. > > Well, if you go back to the 10 bits ADC and amplify your input signal by 2, > now, at the input signal span, the ADC would be receiving 2000mV, so it > would be using all its 10 bits, doubling also the resolution to 976uV, when > comparing to the original 1.953mV without the front end amplifier. > > The expression "ADC Range" for me, means what the ADC can embrace with its > bits, as a relation between the reference voltage and the input signal > (range). If you can extract more or less bits from an ADC using a > front-end amplifier, then you are changing the ADC range. > > Of course, it can also be done by just changing the reference voltage, > whenever possible. > > That is exactly what "auto-range" measurement units do, changing the input > signal, or, the reference voltage. > > We did it in the past with a NTC metering unit. The NTC "log" curve can be > separated in 3 parts, steep, linear and long. Depending how you amplify > each of this parts, you can get a better transfer rate between the signal > and the output resolution, I mean, as closer to 45 degrees curve, the > better. Our equipment was produced to offer an auto-range amplifier with 3 > levels, so, selected by software, at any point of the curve we had the best > possible transfer rate. We adapted the ADC range to different points of the > NTC curve. > > About the amplifier linearity, at very low resolution (10 or 12 bits), and > processing speed (low), I would not worry at all. Any regular operational > amplifier, LM324 as example, would present a linearity that would be much > better than the ADC resolution error. > > At any point in the measured signal, the 976uV resolution, lets say 1mV, > will only discriminate the signal at those steps. If the measured signal > is 100.3mV, then amplified by 2, would be 200.6mV, the binary resolution > would show 200mV or 201mV. The LM324 (not one of the best) would not > generate a linearity error bigger than the 600uV or 400uV introduced by the > ADC gross resolution. > > We usually look for BEST linearity in amplifiers, when dealing with 24 bits > ADC, as we do in our equipments. At this level, the resolution is pretty > high, around 12nV, and here yes, overall circuitry linearity is very > important, including of course, operational amplifiers. > > Wagner Lipnharski - email: wagner@ustr.net > UST Research Inc. - Development Director > http://www.ustr.net - Orlando Florida 32837 > Licensed Consultant Atmel AVR _/_/_/_/_/_/ > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics