----- Original Message ----- From: "Vasile Surducan" To: Sent: Wednesday, November 28, 2001 8:37 AM Subject: Re: [PIC]: Potentiometer on ADC > Martin, this offset compensation tehniques is out of use by two reasons: > > - first: it's minimize the input impedance by R1 || R2 which is wrong So use 10 Meg resistors. With TLC271 or similar that's no problem. > - second: if your OA have such offset that you must > compensate it at unity gain, you must drop it somewhere... Usually the op amp is already compensated for unity gain, but maybe I don't understand what you mean here. /\/\/\/*=Martin > > > On these buffers, offset compensation should be done even with special > offset pins or if the OA haven't such pins, on the - input using a > resistor in the negative feedback and another one from - input to a wiper > offset potentiometer which must have a low bipolar voltage on it. > The input impedance will be kept high in this way. > I think Imre does not need any repetor for his measurement, except maybe > a mains phase isolation. > > Regards, Vasile > > > > On Tue, 27 Nov 2001, Martin Peach wrote: > > > A CMOS op-amp wired as a unity-gain buffer would be close to ideal as an > > input to the PIC ADC as the source impedance could be very high. > > +5 > > < > > R3 > -/\/\--+-----|+\ > > > < | / | \ > > GND +-|-/ | > > | |/ | > > |______| > > > > R1 and R2 are equal to set the offset to midrange. R3 is whatever your input > > resistor ladder is. > > > > /\/\/\/*=Martin > > > > ----- Original Message ----- > > From: "dr. Imre Bartfai" > > To: > > Sent: Tuesday, November 27, 2001 4:14 AM > > Subject: Re: [PIC]: Potentiometer on ADC > > > > > > > Hi, > > > > > > an (maybe silly) idea: could one use instead of the capacitor a voltage > > > follower? (I want to measure the power line and it would be a divider > > > ladder with a ratio 1:150 (the Vpp is 650V = 2 * 310V = 2 * sqrt(2) * > > > 230V). > > > > > > Thank you in advance. > > > > > > Regards, > > > Imre > > > > > > > > > +-----------------------------------------------------------------------+ > > > | The information transmitted is intended only for the person or entity | > > > | to which it is addressed and may contain confidential and/or | > > > | privileged material. Any review, retransmission, dissemination or | > > > | other use of, or taking of any action in reliance upon, this | > > > | information by persons or entities other than the intended recipient | > > > | is prohibited. If you received this in error, please contact the | > > > | sender and delete the material from any computer. | > > > +-----------------------------------------------------------------------+ > > > > > > On Sun, 25 Nov 2001, Robert A. LaBudde wrote: > > > > > > > At 06:56 PM 11/24/01 -0500, Josh wrote: > > > > >How do you figure out the value of the capacitor, and how long it takes > > > > >to charge? I'm not an EE, so sometimes these things are hard for me. If > > > > >I wanted to go for simplicity, would it be best to just go for a 10K > > > > >pot? > > > > > > > > The size of the capacitor and the resistor are determined by how > > frequently > > > > you are making measurements with the ADC, if you are going to rely on > > the > > > > capacitor to do a "sample hold". > > > > > > > > For your simple setup, it's a lot easier to simply use a lower R pot so > > > > that the issue doesn't arise. The minimum size of the pot depends only > > on > > > > the amount of current you are willing to consume from the power supply. > > > > > > > > For a 5V supply, a 5000 ohm pot will pull 1 mA (I = V/R). A 1 kohm pot > > will > > > > pull 5 mA. > > > > > > > > If you are not making a low-power device (e.g., running off a 9 V > > battery), > > > > then a 1 kohm pot would be fine. > > > > > > > > Now the capacitor is present primarily to remove unwanted AC noise from > > the > > > > ADC input. The pot and the capacitor make an input low-pass filter to > > the > > > > ADC input. The roll-off frequency of the filter is at fc = 1/(2 pi R C). > > > > > > > > The problem with using too low an R for the pot is that you would need a > > > > very large C for the capacitor to protect against noise at relatively > > low > > > > frequencies (e.g., 60 Hz). For R = 1 kohm and fc = 30 Hz, you would need > > C > > > > = 5.3 uF. For fc = 3 Hz, this would increase to C = 53 uF. > > > > > > > > If you were doing a lot of high-frequency port R/Ws (which I doubt you > > > > intend in your application), you might want to bypass the filter > > capacitor > > > > with a 0.22 uF tantalum to protect against very high frequency noise. > > > > > > > > For your application (a simple low accuracy pot-controlled test jig for > > an > > > > ADC), a simple 1 kohm pot alone would probably present no problems. If > > you > > > > still want to remove noise, add a 100 uF capacitor from the ADC input to > > > > ground. Make sure the capacitor is not so large that fc is less than > > twice > > > > the sampling frequency. I.e., if you are going to read at 10 sps, make > > fc = > > > > 20 Hz or more (e.g., 4.7 uF capacitor). > > > > > > > > Also be aware that use of a large value capacitor means that you will > > not > > > > get an accurate ADC reading until 6 RC time constants have elapsed at > > start > > > > up. If R = 1 kohm and C = 100 uF, this will be 0.6 sec at start up, > > which > > > > you will have to wait out. > > > > > > > > You can tell whether or not you need the capacitor by looking at the > > input > > > > signal to the ADC with an oscilloscope while it is in its most frequent > > > > repetitive read loop. If the input is flat to the accuracy you need, > > you're > > > > home free. If not bypass with your capacitor and check that it works > > well > > > > enough. > > > > > > > > > > > > ================================================================ > > > > Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com > > > > Least Cost Formulations, Ltd. URL: http://lcfltd.com/ > > > > 824 Timberlake Drive Tel: 757-467-0954 > > > > Virginia Beach, VA 23464-3239 Fax: 757-467-2947 > > > > > > > > "Vere scire est per causas scire" > > > > ================================================================ > > > > > > > > -- > > > > http://www.piclist.com#nomail Going offline? 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