Sorry, group. That was meant to be off-list. 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: "Douglas Wood" To: Sent: Thursday, February 13, 2003 12:18 PM Subject: Re: [EE]: 12 bit ADC or amplify for 10 bit ADC? > 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. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.