John, I forgot to tell you a couple of things about the schematic I sent you eariler: 1) You can use any 4.095 V reference device you to; I used this device because I needed an extended temperature range and the higher precision. You should be able to find one that do not require the +12 V supply. Look at Maxim. 2) There is an odd looking arrangement of resistors on the analog input section (before the op-amp) that needed explaining (I'd forgotten that I did this): This circuit was design to provide 10 M ohms of input impedence. I had a problem with locating certain values of high-precision surface mount resisitors in the package size I needed (VERY small, to say the least), so I drew two parts options on the schematic and provided specified layout requirements to my board layout guy. In production, the board would be populated with ONLY ONE of the two options shown (either R14 OR both R12 & R16, and either R15 OR both R13 & R17). DO NOT POPULATE BOTH! 3) The diode device, D1, is available (from DigiKey) in different packaging. If you cannot find the 4-pack part, use the 2-pack part (same number, different suffix, but I don't remember off the top of my head). Actually any roughly similar diode would work; these diodes had a VERY low leakage current (this schematic is part of a device that was powered by batteries). 4) The op amp (AD623) has an offset of 10 mV (according to the datasheet). In this circuit it's actually closer to 15-17mV. For the purposes of my software I rounded up to 20 mV. This should not be a problem for you since IIRC your lower limit was 200 mV? 5) You probably won't need D5. 6) The amplifier circuit saturates somewhere around 3.5 V, so your effective range is 0.020-3.500 V. This circuit reads a very rock steady 1 mV resolution. This circuit was used to read the voltage of a half-rectified power supply, so I took 512 readings in two 60Hz AC cycles (33 mS). The ADC is capable of *MUCH* higher conversion rates than that, and the rest of the circuit is fast enough to keep up. Douglas Wood Software Engineer dbwood@kc.rr.com ICQ#: 143841506 Home of the EPICIS Development System for the PIC http://epicis.piclist.com ----- Original Message ----- From: "John Pearson" To: Sent: Wednesday, February 12, 2003 7:58 PM Subject: Re: [EE]: 12 bit ADC or amplify for 10 bit ADC? > 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 > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.