C-multiplier again
in [Design]
|
Prev: Futuristic weapon question: Anti-matter-gun, would it have a signature ?
Next: EEVblog Live Event
From: John Larkin on 27 May 2010 10:05 On Thu, 27 May 2010 06:22:10 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On May 26, 7:34�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 27 May 2010 01:27:03 GMT, Mike <s...(a)me.not> wrote: >> >dagmargoodb...(a)yahoo.com wrote: >> >> >> I was thinking about that. �Maxwell Technology makes unit with >> >> milliohm ESRs, but I wasn't sure there wasn't some funky >> >> noise problem, like electrolyte convection or who knows what. >> >> >> Oh, and they're a few cubic inches--not surface mountable. >> >> >> But as for leakage, I've seen a *really* clever dodge around that. >> >> Walt Jung, I think, in a low-noise reference IIRC. >> >> >> -- >> >> Cheers, >> >> James Arthur >> >> > �Here's Walt's article: >> >> > �http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pdf >> >> > �Here's some data on leakage in electrolytics. >> >> > �I = K * C * V � � � � � � ; leakage current >> >> > �where I, C, and V are standard values. >> >> > �Here's some values for K: >> >> > �K = 0.002 � � � � ; low leakage electrolytic spec >> > �K = 0.02 � � � � � � � � �; typical electrolytic spec >> > �K = 1.89e-5 � � � � � � � ; AVX Bestcap spec >> > �K = 3.86e-5 � � � � � � � ; HiTeck supercap spec >> > �K = 5e-7 � � � � � � � � �; measured 1 Farad supercap >> > �K = 7e-7 � � � � � � � � �; measured supercap >> > �K = 8.5e-5 � � � � � � � �; 470uf electrolytic caps measured by Win >> >> > �Supercaps can be two orders of magnitude better than the best � >> > �electrolytic. Pity the working voltage is so low. >> >> > �Mike >> >> The polymer aluminums are pretty spiffy. The 120 uF 16V one I'm using >> is 24 milliohms typ. And it's an affordable surface-mount thing. > >Is the leakage current in them very noisy? I have always assumed that >the noise current part of the leakage current was equal to the shot >noise of a semiconductor device. If it is like a resistor, the noise >would be less. > I don't know. I tried to measure the noise across a charged one and didn't see anything at microvolt resolution. They don't leak much. Unlike a wet cap, if you increase the voltage slowly, there's no apparent increase in leakage current right up to the instant it fails shorted, at about 2x rated voltage. Megohms of leakage into milliohms of ESR will attenuate that leakage noise by roughly a billion. John
From: John Larkin on 27 May 2010 10:06 On Thu, 27 May 2010 06:09:22 GMT, Mike <spam(a)me.not> wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Thu, 27 May 2010 03:12:25 GMT, Mike <spam(a)me.not> wrote: > >>>>> � Here's Walt's article: >>>>> >>>>> �http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pd > >> R3, with nearly a nV/rthz added noise, is unfortunate. > >> John > >49.9 ohms? 0.906nVrms? Is that the one you mean? Tkat's R3 all right. > >It's not clear why it's even in the circuit. I think we can simply use R1, >R2 and C1, C2, and forget the rest. > The text says something about surge protection for the opamp input. John
From: Tim Williams on 27 May 2010 10:24 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:ugusv5d7t6lbppkf2bgjsr9pnp9rfffldg(a)4ax.com... > I have a friend who manages the LED lighting division for a big > electrical products company... you buy their stuff at Home Depot. He > snuck me a developmental white LED that, at 14 volts and 0.7 amps, > looks like a welding arc. It will literally blind you for a minute if > you look straight at it. Their initial target market is street lights. Well that's rather disappointing. Admittedly I haven't tried, but I don't think you'll see anything after staring into the average welding arc for a minute. ;-) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Jan Panteltje on 27 May 2010 11:21 On a sunny day (Thu, 27 May 2010 06:19:51 -0700 (PDT)) it happened MooseFET <kensmith(a)rahul.net> wrote in <1c06ced9-19ad-466f-9d89-788a725e939f(a)u3g2000prl.googlegroups.com>: >> �http://panteltje.com/panteltje/mvp/mpv-0.2-pcb.jpg >> >> Here used as power indicator, and 1.5V reference (old LED). > >You don't want light hitting your "low noise reference diode" That is true. >I suggested a blue LED under the assumption that the higher >forward voltage would make the "SNR" better by making the >"signal" bigger. Since then, I have had a reason to rethink >that. I think that the impedance of the so called "super >bright" RED is lower than that of the BLUE. I have never measured that, next time I will have a look with the LEDs I have. >BTW: There is no such thing as a superbright LED. LEDs come >in 3 brightness classes: >(1) Are you sure this is on >(2) Way to dim >(3) I guess this will do I think there is a fourth class, AAAAUAUWW --- Way too bright. Those are the one I have, even at 10 mA, green, and blue, no way can you look into one. >I use LEDs in full sunlight. I try to use clear plastic LEDs, so if you see color it means it is 'on'. Ultimately you can flash a LED to make it stick out a bit more if it is an important indicator. After that transflective LCDs, with flashing background and beep, I have some very small 5 V beepers that almost make you jump :-)
From: Mike on 27 May 2010 11:34
dagmargoodboat(a)yahoo.com wrote: [...] > Necessary, to protect the AD797 input. > > James From the AD797 datasheet: Maximum Differential Input Voltage : +/- 0.7 V The AD797's inputs are protected by back-to-back diodes. To achieve low noise, internal current limiting resistors are not incorporated into the design of this amplifier. If the differential input voltage exceeds +/- 0.7 V, the input current should be limited to less than 25 mA by series protection resistors. Note, however, that this will degrade the low noise performance of the device. The AD587 supplies 10V. There are big electrolytics connected to both inputs of the AD797. In the event of a hard short on either input, the maximum current could be (10 - 0.7) / 49.9 = 0.186 Amp. This exceeds the rating by 7 times, so the 49.9 ohm resistor is not providing much protection against a hard failure, and it probably can be removed. Unfortunately, most modern low-noise op amps seem to have similar input restrictions. The best option seems to be to provide as much protection around the circuit as possible to prevent high input currents. And have plenty of spares on hand:) Mike |