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From: Jim Thompson on 1 Aug 2010 21:11 On Sun, 01 Aug 2010 13:03:53 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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 Which we'll never be able to disprove, because John "The Bloviator" Larkin never gives values or part names. However, for you people trying to learn, the very network around the power FET, needed for stability (capacitive load on OpAmp) allows channel-length modulation to remain. Tutorial on channel-length modulation to follow... Can't do it tonight... already had two "1964" martinis at Benihana ;-) I'll also demonstrate the current-source-less approach I previously alluded to. For technical expertise you can always count on Jim Thompson � ...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.
From: Grant on 1 Aug 2010 21:21 On Sun, 01 Aug 2010 14:32:29 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: >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. ;-) Aiming for 1 or 2 percent is far easier for home built stuff, and buying precision resistors is one way to get a cheap 'standard'. Plus crystal-locked frequencies, derive from those, I don't see a problem. I've worked with transfer standards in the past, can get expensive to add good accuracy. > >>I guess you could buy one new resistor every year, and check your >>counter against WWV or GPS, and stay "certified." Grant.
From: krw on 1 Aug 2010 21:39 On Mon, 02 Aug 2010 11:21:25 +1000, Grant <omg(a)grrr.id.au> wrote: >On Sun, 01 Aug 2010 14:32:29 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: > >>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. ;-) > >Aiming for 1 or 2 percent is far easier for home built stuff, and >buying precision resistors is one way to get a cheap 'standard'. But he was aiming for .02%. It would be a big enough job at 1%, but five times better is a *lot* more than five times harder. >Plus crystal-locked frequencies, derive from those, I don't see a >problem. I've worked with transfer standards in the past, can get >expensive to add good accuracy. That gets you one calibration standard. You're going to need more for LCR. <...>
From: Grant on 1 Aug 2010 21:53 On Sun, 01 Aug 2010 14:49:08 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >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 And how does one overcome that? I can't see a DC effect (not saying there isn't one) because steady state it looks like the source sense resistor is seeing the drain current, disregarding gate leakage. I can see there might be a dV/dt related issue due to drain-gate capacitance, is this what you're talking about? All I can see is that there might be a voltage compliance change due to the effect you mention. You published similar circuit, CurrentSourceConceptual.pdf four years ago, but I didn't check if there's any related discussion. Grant.
From: Jim Thompson on 1 Aug 2010 21:54
On Mon, 02 Aug 2010 11:53:39 +1000, Grant <omg(a)grrr.id.au> wrote: >On Sun, 01 Aug 2010 14:49:08 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >>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 > >And how does one overcome that? > >I can't see a DC effect (not saying there isn't one) because steady >state it looks like the source sense resistor is seeing the drain >current, disregarding gate leakage. > >I can see there might be a dV/dt related issue due to drain-gate >capacitance, is this what you're talking about? > >All I can see is that there might be a voltage compliance change >due to the effect you mention. > >You published similar circuit, CurrentSourceConceptual.pdf four >years ago, but I didn't check if there's any related discussion. > >Grant. But never claimed to be good to 0.1% ...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. |