Detecting the peak value
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From: hal on 1 Aug 2010 14:51 <snip> > >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 That's more or less the circuit I was thinking about - I had been playing with it in SPICE and it _seemed_ to have the required linearity, but I wasn't sure. I'll probably have 3 ranges (50ma, 200ua, 1 ua - each a dedicated source). I was also thinking about choppers, although still looking at specs - especially for the 1ua range. All voltages and currents are drived from a buffered and filtered internal reference (MAX6341), I'll have +- 5v available, +-12 (or +12) if needed for the op-amps. Any special considerations using the applicable portion of the above circuit for these current ranges? Anything special to consider in the MOSFET? Thanks Hal
From: John Devereux on 1 Aug 2010 15:15 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> writes: > 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." I made what I thought was a really nice circuit for measuring capacitance, relying only on a known precision resistance and a stable frequency. (Which are the two most easily obtained precision quantities in electronics I think). It uses a (gulp) charge pump :) Basically you use a dual low charge injection analog switch to alternately switch Cx across a reference voltage, then dump the charge into a BFC. This is then connected to an opamp virtual earth. The feedback resistor is the precision 0.01% resistor, and your ADC uses the same Vref so it's precise value doesn't matter. It's the same principle as Fig 12a in Linear Technology AN03, with the 1000pF being Cx and a quartz crystal oscillator feeding Fin. <http://cds.linear.com/docs/Application%20Note/an03f.pdf> -- John Devereux
From: krw on 1 Aug 2010 15:32 On Sun, 01 Aug 2010 10:38:22 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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. Trust, but verify. If he's going for ".02% on all functions", he'd better have a pretty damned good ADC, and everything else, right down to the amplifiers. Your .01%, or .05%, resistor just ate pretty far into his error budget. ;-) >I guess you could buy one new resistor every year, and check your >counter against WWV or GPS, and stay "certified."
From: John Larkin on 1 Aug 2010 16:03 On Sun, 1 Aug 2010 18:11:31 GMT, hal(a)nospam.com wrote: ><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 He's babbling, as usual. The mosfet current source I posted has zero errors from those effects. John
From: Jim Thompson on 1 Aug 2010 17:49
On Sun, 1 Aug 2010 18:51:09 GMT, hal(a)nospam.com wrote: > ><snip> > >> >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 > >That's more or less the circuit I was thinking about - I had been playing >with it in SPICE and it _seemed_ to have the required linearity, but I >wasn't sure. I'll probably have 3 ranges (50ma, 200ua, 1 ua - each a >dedicated source). I was also thinking about choppers, although still >looking at specs - especially for the 1ua range. All voltages and currents >are drived from a buffered and filtered internal reference (MAX6341), I'll >have +- 5v available, +-12 (or +12) if needed for the op-amps. Any special >considerations using the applicable portion of the above circuit for these >current ranges? Anything special to consider in the MOSFET? > >Thanks > >Hal I don't think such an active current mirror will hold up to your 0.1% accuracy... aforementioned channel-length modulation. ...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)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Performance only as good as the person behind the wheel. |