Detecting the peak value
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From: JosephKK on 2 Aug 2010 23:59 On Mon, 02 Aug 2010 11:10:41 +1000, Grant <omg(a)grrr.id.au> wrote: >On Sun, 01 Aug 2010 09:17:59 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >>On Sun, 1 Aug 2010 16:00:54 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: >... >>>> 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). > >Wish my memory or filing system was better, there's a circuit I saw >that used an opamp to supply fixed voltage offset to an RC to get >linear cap charging. It's related to capacitor multiplier circuits >and ramp generators. > >Constant voltage across resistor == constant current into cap over >the working range of the opamps. > >But I can't find the thing, does it ring a bell? > >Grant. Actually it sounds kind of like a varaition of a Phantastron.
From: JosephKK on 3 Aug 2010 00:04 On Sun, 01 Aug 2010 10:19:51 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Sun, 01 Aug 2010 10:06:49 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Sun, 1 Aug 2010 16:00:54 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 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. >>> >>> >> >>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. > > ...Jim Thompson That is a real issue with better and best grade caps, i would be more worried about the measurement problems created by dielectrics with serious voltage non-linearities like Z5U.
From: JosephKK on 3 Aug 2010 00:28 On Sun, 1 Aug 2010 17:21:04 GMT, hal(a)nospam.com wrote: > >On 1-Aug-2010, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.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 before I measure the ESR and can account for Z. >> > I >> >originally planned to use a peak-detection (measurement) scheme with a >> >100khz low distortion sine wave, however 1-2% accuracy was the best it >> >seemed that I could do (Yes, I realize that for ESR, that is good enough >> >- >> >but remember, I'm doing this to learn and just to see if I can for the >> >fun >> >of it). So, as it stands, I am planning on using a S/H or tracking ADC >> >with >> >a high enough sample rate to detect both high and low peaks of both the >> >DUT >> >and a reference resistance and calculate the ESR from there. >> > >> >As the required additional circuitry to measure the ESR is almost as much >> >as >> >all of the rest put together, I can't help but wonder if there's an >> >easier >> >way that I've missed and I'm using a sledge hammer to drive a nail. So, >> >I >> >would welcome any suggestions, tips or hints. >> > >> >Thanks, >> > >> >Hal >> >> Really precise capacitance measurement (and "really precise" starts at >> 1% maybe) generally uses a bridge driven by a sine wave. One problem >> with caps is that C varies with frequency because of dielectric >> absorption and other effects, so the frequency is usually specified >> for accurate measurement. DA can make a ramp-type C measurement >> nonlinear. >> >> If you have a fast ADC, I'd suggest just taking a lot of samples and >> absolute value+averaging. That would be better, lower noise data than >> looking for/at the peak, and you could include a lot more samples. >> >> If you can use a capacitor for the reference, you can make a voltage >> divider with Cref and Cx, digitize the overall voltage and the >> midpoint voltage, and do the math. That's ratiometric on the >> excitation source, and works for resistors, too. I guess you could >> make a divider from a precision resistor and Cx, given a good sine >> reference. >> >> A decent 16-bit SAR ADC is cheap nowadays and they tend to be >> amazingly linear. Sampling all up and down the waveform makes it even >> better, and averaging lots of samples can extend the resolution beyond >> 16 bits. You can do synchronous detection tricks to reduce noise. The >> nice thing about an ADC approach is that the tweaks are software. >> >> To do fancy stuff, like 3/4 terminal measurements, you might buy a >> couple of old HP or GR RLC bridge manuals and study their topologies. >> Those old manuals generally had schematics and good theory sections. >> >> >> John > >A bridge was my first inclination; however, I'm wanting an all-in-one LCR >meter with one (Kelvin) set of test leads. To do so will involve fairly >complex switching circuitry and trying to make a bridge topology work with >that arrangement was a little too far beyond my comfort level. For the sine >source, I was going to use one of the ADI DDS chips. Do you think I would >achieve better results using a sine-measurement technique for the >capacitance as well as the ESR? Most of what I do is low frequency or DC so >I don't really have a need to measure the cap at 50 or 100khz. I don't want >to use a reference cap as the best I have is 1% and I was hoping to better >that. > >Hal > To the extent you can get HP user and service manuals for their (low frequency ~ 10 Hz to ~ 10 MHz) LCR meters / impedance analysers. They mostly used semibridge techniques with I&Q demodulators. Of serious interest if the methods used to measure capacitors (including with voltage bias) and inductors (including with current bias).
From: Grant on 3 Aug 2010 04:38 On Mon, 02 Aug 2010 20:59:47 -0700, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >On Mon, 02 Aug 2010 11:10:41 +1000, Grant <omg(a)grrr.id.au> wrote: > >>On Sun, 01 Aug 2010 09:17:59 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >> >>>On Sun, 1 Aug 2010 16:00:54 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: >>... >>>>> 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). >> >>Wish my memory or filing system was better, there's a circuit I saw >>that used an opamp to supply fixed voltage offset to an RC to get >>linear cap charging. It's related to capacitor multiplier circuits >>and ramp generators. >> >>Constant voltage across resistor == constant current into cap over >>the working range of the opamps. >> >>But I can't find the thing, does it ring a bell? >> >>Grant. > >Actually it sounds kind of like a varaition of a Phantastron. And that don't ring a bell, you having a lend of me? :) One day I'll put these .pdfs into some mind-mapping thingy so I can find the stuff again. Sometimes I'll spend hours going through circuits for ideas, concepts. App notes, data sheets, and so on. But the recall is a bit scrambled these days, too much information. Grant.
From: JosephKK on 3 Aug 2010 07:09
On Mon, 02 Aug 2010 20:00:37 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Mon, 02 Aug 2010 19:36:20 -0700, >"JosephKK"<quiettechblue(a)yahoo.com> wrote: > >>On Fri, 30 Jul 2010 14:28:46 -0700 (PDT), john1987 >><conphiloso(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 >>>> -- >>>> | 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. >>> >>>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? >>> >>>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. >>> >>>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 >> >>One single point blank question: Is the ADC connected to a uC that >>you can program? > >Limited ability :-( > > ...Jim Thompson I am partly trying to discover the constraints to partitioning the system. Like using a 10 MSPs converter and digitally peak detecting outside a uC. Or can we just find the peaks, several workable alternatives have been proposed, and digitize them as they occur. OP also asks about what ADC to use. Way too much how come for why actual contsraints are missing. Many of us would help OP but do not like running up blind alleys sans exit. |