Detecting the peak value
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From: John Larkin on 1 Aug 2010 13:30 On Sun, 1 Aug 2010 17:07:46 GMT, hal(a)nospam.com wrote: > >On 1-Aug-2010, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >> > >> >On 1-Aug-2010, Jim Thompson >> ><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >> > >> >> On Sun, 1 Aug 2010 15:29:19 GMT, hal(a)nospam.com wrote: >> >> >> >> >Hello all, >> >> > >> >> >I've been following this thread and it has brought up some issues I'm >> >> >currently working on. First of all, I'm a hobbyist that likes to >> >> >learn >> >> >by >> >> >doing; as such, I'm interested in making a circuit do what I want >> >> >without >> >> >regards to a BOM - especially in regards to performance. In other >> >> >words, >> >> >I >> >> >like "best" rather than "cheapest" or "easiest." >> >> > >> >> >Currently I'm working on a LCR meter for personal use with better than >> >> >.1% >> >> >accuracy (closer to .02% if possible) for all functions. Also, all >> >> >calibration will be internal and automatic; I have quite a few of >> >> >Vishay's >> >> >S102C series resistors (.01%, 2ppm) so I have a ready internal >> >> >reference >> >> >to >> >> >use, keeping all measurements ratiometric if possible, and use >> >> >resistance >> >> >as >> >> >the "known" in all equations. >> >> > >> >> >I'm measuring capacitance using constant-current charge time, so I >> >> >already >> >> >will know the capacitance >> >> >> >> Are you not deluding yourself that you know "constant-current" to 0.1% >> >> accuracy? >> > >> >Probably so - hence my request for suggestions. I suppose that I should >> >have emphasized that this is not only something that I do for enjoyment, >> >but >> >to _learn_. As such, I would welcome feedback on not only that I *am* >> >going >> >wrong, but *how* as well. >> >> Yep. A welcome change in SED direction. >> >> Pondering that, it's going to be tricky to make a loop-stabilized >> current source that tracks a slewing capacitor's change in voltage >> (Early-effect or channel-length modulation). >> >> Perhaps _not_ use a classic current source, instead sense capacitor >> current (into a virtual ground) and adjust a voltage drive? >> >> I presume you're measuring time-to-voltage to get capacitance? > >Yes, it seemed the best way to use the high precision components that I >have. I was going to charge through one of the precision Rs and measure to >get the charging current. As long as the current source is linear over a >small range (a couple volts starting at .5 or 1V - NOT in-circuit, of >course), with an accurate clock, and using Kelvin sensing for both the >reference and DUT, I felt that I should be able to achieve very good >accuracy. I realize that this is a one-dimensional measurement of the >capacitance, but it suits for what I am trying to accomplish. The fun part, >for me, is trying to see how accurate I can get that one measurement - ditto >for ESR. One issue I'm still working on is a current source with the >required linearity. There are quite a few designs that are accurate at a >set current, but I don't have the expertise or equipment to determine if >they are accurate over a sweeping voltage range. Suggestions, anyone? > > Pch mosfet with a resistor in the source. An opamp compares the voltage drop across the resistor to some voltage reference, like an LM4040 maybe, and drives the fet gate. That will be constant to PPMs over a huge load voltage range, and will have good short-term stability. It might be a bit noisy from the reference noise, but you can use a better reference, or average most of that out. Just use something like the top half of this, with the reference up there. Or use the whole thing, drive it from a dac, and have a programmable current source. ftp://jjlarkin.lmi.net/Isrc.JPG Chopper opamps might be nice here. John
From: John Larkin on 1 Aug 2010 13:38 On Sun, 01 Aug 2010 12:14:30 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: >On Sun, 1 Aug 2010 17:07:46 GMT, hal(a)nospam.com wrote: > >> >>On 1-Aug-2010, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >> >>> > >>> >On 1-Aug-2010, Jim Thompson >>> ><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >>> > >>> >> On Sun, 1 Aug 2010 15:29:19 GMT, hal(a)nospam.com wrote: >>> >> >>> >> >Hello all, >>> >> > >>> >> >I've been following this thread and it has brought up some issues I'm >>> >> >currently working on. First of all, I'm a hobbyist that likes to >>> >> >learn >>> >> >by >>> >> >doing; as such, I'm interested in making a circuit do what I want >>> >> >without >>> >> >regards to a BOM - especially in regards to performance. In other >>> >> >words, >>> >> >I >>> >> >like "best" rather than "cheapest" or "easiest." >>> >> > >>> >> >Currently I'm working on a LCR meter for personal use with better than >>> >> >.1% >>> >> >accuracy (closer to .02% if possible) for all functions. Also, all >>> >> >calibration will be internal and automatic; I have quite a few of >>> >> >Vishay's >>> >> >S102C series resistors (.01%, 2ppm) so I have a ready internal >>> >> >reference >>> >> >to >>> >> >use, keeping all measurements ratiometric if possible, and use >>> >> >resistance >>> >> >as >>> >> >the "known" in all equations. >>> >> > >>> >> >I'm measuring capacitance using constant-current charge time, so I >>> >> >already >>> >> >will know the capacitance >>> >> >>> >> Are you not deluding yourself that you know "constant-current" to 0.1% >>> >> accuracy? >>> > >>> >Probably so - hence my request for suggestions. I suppose that I should >>> >have emphasized that this is not only something that I do for enjoyment, >>> >but >>> >to _learn_. As such, I would welcome feedback on not only that I *am* >>> >going >>> >wrong, but *how* as well. >>> >>> Yep. A welcome change in SED direction. >>> >>> Pondering that, it's going to be tricky to make a loop-stabilized >>> current source that tracks a slewing capacitor's change in voltage >>> (Early-effect or channel-length modulation). >>> >>> Perhaps _not_ use a classic current source, instead sense capacitor >>> current (into a virtual ground) and adjust a voltage drive? >>> >>> I presume you're measuring time-to-voltage to get capacitance? >> >>Yes, it seemed the best way to use the high precision components that I >>have. I was going to charge through one of the precision Rs and measure to >>get the charging current. As long as the current source is linear over a >>small range (a couple volts starting at .5 or 1V - NOT in-circuit, of >>course), with an accurate clock, and using Kelvin sensing for both the >>reference and DUT, I felt that I should be able to achieve very good >>accuracy. I realize that this is a one-dimensional measurement of the >>capacitance, but it suits for what I am trying to accomplish. The fun part, >>for me, is trying to see how accurate I can get that one measurement - ditto >>for ESR. One issue I'm still working on is a current source with the >>required linearity. There are quite a few designs that are accurate at a >>set current, but I don't have the expertise or equipment to determine if >>they are accurate over a sweeping voltage range. Suggestions, anyone? > >How are you going to calibrate your equipment (test your accuracy)? > ><...> Technically, he needs one good resistor (spring for a Vishay 0.01%, or a cheaper Susumu 0.05% part) and a decent frequency counter. If you trust your ADC linearity (and the charge distribution ones are very good) you can bootstrap a whole chain of other parts off those two. I guess you could buy one new resistor every year, and check your counter against WWV or GPS, and stay "certified." John
From: hal on 1 Aug 2010 14:09 <snip> > Technically, he needs one good resistor (spring for a Vishay 0.01%, or > a cheaper Susumu 0.05% part) and a decent frequency counter. If you > trust your ADC linearity (and the charge distribution ones are very > good) you can bootstrap a whole chain of other parts off those two. > > I guess you could buy one new resistor every year, and check your > counter against WWV or GPS, and stay "certified." > > John That was the plan - I have a hundred or so Vishay S102Cs so I figured that I might as well use a couple.
From: hal on 1 Aug 2010 14:11 <snip> On 1-Aug-2010, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >You don't really need an accurate current source, just a fairly stable > >one. Dump the current into a reference resistor (you need one of those > >anyhow) and measure it, then use the same current on your cap. > > > [snip] > > Larkin misses the Early-effect/channel-length-modulation errors as the > voltage changes. What would the magnitude of these errors be? Enough to matter? Hal
From: john1987 on 1 Aug 2010 14:27
On Jul 30, 6:41 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My- Web-Site.com> wrote: > On Fri, 30 Jul 2010 14:28:46 -0700 (PDT), john1987 > > > > > > > > <conphil...(a)hotmail.com> wrote: > >On Jul 30, 5:16 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My- > >Web-Site.com> wrote: > >> On Fri, 30 Jul 2010 14:07:46 -0700 (PDT), john1987 > > >> <conphil...(a)hotmail.com> wrote: > >> >Hi, > > >> >200pF and 82 ohm resistor gives me approximately 9 Mhz frequency and > >> >its a high pass filter. SO, does it mean that filter will pass all the > >> >frequncies less than 9MHz. > > >> >Thanks > >> >John > > >> Stop snipping content and look at my drawing. > > >> ...Jim Thompson > [snip] > > >Hi, > > >I looked at your drawing and also simulated in pspice. Why did you go > >for the High pass filter? I tried 220pF and 7.6KOhm. Is HPF more > >accurate than LPF? > > Why did you try 7.6K Ohm when I said 82 Ohms? > > It's a differentiator, so you place it much higher in frequency than > the signal. > > > > >The positive peak voltage is + 1 volts and negative peak voltage is - > >1 volts. So, 2 volts peak to peak. > > >I do not know how accurate should be the measured voltage. It would be > >desirable to have it as little as possible. > > Doesn't matter for differentiator. We're looking for sign change > (slope, at the input peak) with the comparator. > > > > >when said and done, I will read the voltage into the ADC and read the > >numbers and try to monitor the voltage across the resistor. > > >Thanks > > >John > > You said you wanted a transition... as you drew it. > > Do you want the peak voltage? > > What is it you want? > > ...Jim Thompson > -- > | James E.Thompson, CTO | mens | > | Analog Innovations, Inc. | et | > | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > | Phoenix, Arizona 85048 Skype: Contacts Only | | > | Voice:(480)460-2350begin_of_the_skype_highlighting (480)460-2350 end_of_the_skype_highlighting Fax: Available upon request | Brass Rat | > | E-mail Icon athttp://www.analog-innovations.com| 1962 | > > Spice is like a sports car... > Performance only as good as the person behind the wheel.- Hide quoted text - > > - Show quoted text - Hi, I am attaching two new drawings to show what I want. I do not know about accuracy but i would as say as much as accurate as possible. First link is as follows http://img828.imageshack.us/i/scan0001j.jpg/ Second link is as follows http://img299.imageshack.us/i/scan0002db.jpg/ I want to sample the peak value of the sine wave by using ADC (sampling rate 200ksps). I want to sample the peak value of the sine wave at the rising edge or the falling edge of the square wave that you can see in the diagram. The edges will trigger the ADC and ADC will sample that edge. The ADC can not sample the negative values as shown in the diagram. I did not mention the circuitry that can add offset to the input signal and make it all positive. The questions are 1. Is there any way other than RC circuit to get the peak sample of the sine wave at the rising and falling edge? 2. Whats the difference between in this case between using the High pass and low pass filter? 3. How a PLL can help in this case? 4. Any advice about choosing the right ADC? 5. I tested the suggested high pass filter using the 220pF and 7.6kOhm resistor and found that the edge is occuring at the right spot in pspice than the low pass filter. Thanks John |