C-multiplier again
in [Design]
|
Prev: Futuristic weapon question: Anti-matter-gun, would it have a signature ?
Next: EEVblog Live Event
From: Mike on 27 May 2010 18:23 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > > But come to think of it, an SSR could be a 4-ohm replacement for the > 50 ohm thing, with correspondingly less Johnson noise. > > John SSR - Solid State Relay? Too slow to protect the inputs, even if you could design a circuit to detect a fault instantly. Back-to-back schottkys across the inputs, with perhaps 2 ohms in series to the electrolytics, might be the best solution. Hopefully the schottkys would turn on fast enough to do the job. Noise adds as the sum of the squares. A good op amp will have about the same noise as a 50 ohm resistor. A 2 ohm resistor would add negligible noise to the system: En = sqrt(50^2 + 2^2) / sqrt(50) = 1.00079968026 = 0.0069dB Hard to measure that small an increase. Mike
From: Mike on 27 May 2010 18:26 Mike <spam(a)me.not> wrote: > En = sqrt(50^2 + 2^2) / sqrt(50) = 1.00079968026 = 0.0069dB sorry, typo: En = sqrt(50^2 + 2^2) / 50 = 1.00079968026 = 0.0069dB Mike
From: Phil Hobbs on 27 May 2010 20:23 On 5/27/2010 10:05 AM, John Larkin wrote: > 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 > Leakage current is usually Poissonian, perhaps with some 1/f noise on top. The main problem with the LEDs being photovoltaic is 120 Hz from inductive ballast fluorescents and 40 kHz - 1 MHz hash from electronic-ballast fluorescents. The ugliness of electronic ballasts has to be seen to be believed. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Mike on 27 May 2010 20:59 Mike <spam(a)me.not> wrote: > Noise adds as the sum of the squares. A good op amp will have about > the same noise as a 50 ohm resistor. A 2 ohm resistor would add > negligible noise to the system: > > En = sqrt(50^2 + 2^2) / sqrt(50) = 1.00079968026 = 0.0069dB > > Hard to measure that small an increase. > > Mike Let's start over. A 50 ohm resistor has 9.07312516170696E-10 v/sqrt(Hz) at 25C A 2 ohm resistor has 1.81462503234139E-10 v/sqrt(Hz) at 25C The total is sqrt(9.07312516170696E-10^2 + 1.81462503234139E-10^2) = 9.25280844976e-10 Cross Check: the noise of a 52 ohm resistor at 25C is 9.2528084497627E-10 The increase due to adding 2 ohm in series with 50 ohm is 20 * log(9.25280844976e-10 / 9.07312516170696E-10) = 0.17033 dB That is much easier to measure. So every ohm counts! Mike
From: George Herold on 27 May 2010 21:31
On May 27, 2:32 pm, Mike <s...(a)me.not> wrote: > George Herold <gher...(a)teachspin.com> wrote: > > OK this is still a sim. However I spiced the C-multiplier (aka Hobbs > > filter at teachspin) I use to get down to ~1nV. I haven't every > > bumped into the Early effect (I don't do much on the transistor > > level.) But this shows attenuation down near 90dB at 100kHz. (Which > > is where the switcher I use lives.) > > [...] > > Seems to be missing some components. I got the 2N4401 and what appears to > be a AC source in series with 15V. Everything else is just gaps. I browsed > the file but could not find any references. Well that figures, It's my first attempt to post a ltspice circuit. Not much to it. a two pole low pass RC filter (R-C to ground. feeds R- C to ground) and this feeds the base of a npn. the output has a minimum 1 k ohm to ground with anoter C. R is 1k and C is 100uF Al electro. There's also 10 ohms between the final R-C cap node and the base, but this is only to stop oscilations and doesn't change the spice sim much. I'm going to play hookey (sp) from work tomorrow and get the homestead ready for our annual Memorial Day party. (Else I'd try and repost) Summer's here early in Buf. NY. If you're in the area I grill up some great Polish sausage with Walt's mustard. (Walt lives down the road.) George H. > > > George H. > > Oh, and thanks again Mike for showing me how to post and read the > > LTspice stuff... I feel so empowered. > > You are welcome. > > Mike |