C-multiplier again
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From: dagmargoodboat on 28 May 2010 11:12 On May 26, 8:42 pm, Mike <s...(a)me.not> wrote: > dagmargoodb...(a)yahoo.com wrote: > > 20dB sounds about right. The advantages of this approach are low > > drop- out voltage and superior low-frequency noise cancellation > > (compared to practical passive equivalents). > > A big part of the dynamic limitation is the f.f. network rolling off. > > If you change C1 to 100uF, and tack 100uF on the output to cover the > > high-end, overall performance is much improved--nearly as good as a > > passive version using 10,000uF caps, and a lot smaller. > > I was playing with that. Still quite fiddly. If you don't get the balance > exactly right you end up with two attenuation plateaus. It would be > difficult to tell when it is adjusted correctly and to keep it there. See > the asc file for example. > > > For super massive attenuation of input noise and ripple, other > > approaches are better. > > Any examples or links? I meant filter-type approaches, like the C-mult. Cancellation approaches are inherently limited by the accuracy--gain and dynamic-- of the canceller function. Filter-type methods have no such limit. -- Cheers, James Arthur
From: dagmargoodboat on 28 May 2010 11:29 On May 26, 10:12 pm, Mike <s...(a)me.not> wrote: > dagmargoodb...(a)yahoo.com wrote: > >> Here's Walt's article: > > >> http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pdf > > > That's it! Thanks. I found in on my computer in a weird scan format, > > and was starting to ASCII it for the group... > > > Don't you love that bootstrap for the electrolytics? That's slick. > > > One thing I bet Walt didn't have--50mV of switcher ripple. > > Yes, I thought it was pretty neat. As far as switcher ripple, the article > was written in 1993. The PC had been out for about a decade, and the > switching noise was probably a lot worse than it is now. Yeah, but I doubt very much Walt fed that reference dirty. We knew better. Way back, Analog Devices said switchers plus 12+ bit SAR A/Ds didn't mix, but I did all the time, carefully, with no trouble. -- Cheers, James Arthur
From: Mike on 28 May 2010 11:32 dagmargoodboat(a)yahoo.com wrote: > It looks to me like shorting the output pulls a brief spike via C3, > then about 9mA d.c. through the protection diodes, limited by R3+R5. > I don't see any need to protect against hard shorting the inputs to > ground--don't do that. > > Walt worked at AD and knew what was inside the AD797. He's no dummy-- > whatever he did, he meant it. > > -- > Cheers, > James Arthur What's the peak current and time constant of the current spike? That is what is doing the damage, not the 9 mA constant current after the transient has dissipated. I agree, Walt is very good. But it is bad practise to exceed the manufacturer's spec at any time. Can you get Joerg to agree to exceed the manufacturer's spec? I don't think so. A pair of schottky diodes back-to-back across the input pins allows you to reduce the value of R3 to 10 ohms or less, and still meet the manufacturer's maximum rated current spec. Reducing R3 from 49.9 ohms to 10 ohms or less also reduces the Johnson noise considerably. I'll let you do the calculation. The 49.9 ohm resistor limits any improvement you can get by changing to a better op amp. So you are stuck with 1988 performance that really doesn't meet the manufacturer's max current limit. However, adding back-to-back schottky diodes and reducing the value of R3 from 49.9 ohms to 10 ohms or less means you can use a more modern op amp that has lower noise, and gain the advantage of improved performance. The back-to-back schottky diodes eliminate the problem of hard shorts, and gives better noise performance, which is the whole purpose of this exercise. Regards, Mike
From: dagmargoodboat on 28 May 2010 11:52 On May 28, 10:10 am, Mike <s...(a)me.not> wrote: > John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > The author added the resistor to prevent damage, so I assume he picked > > a value that worked. I'm sure the diodes inside the opamp can handle a > > lot more than 25 mA for a short time. > > > John > > Ask Joerg about exceeding the manufacturer's specs. > > Mike Yeah, but Walt *was* the manufacturer, designing in his own part, knowing what's inside. I see that AD's 25mA limit for the AD797 input diodes was a recommendation, not an absolute instantaneous limit. Not knowing what's inside, withstanding a worst-case spike at 7x the recommended d.c. limit seems entirely plausible. Or maybe Walt just didn't want to give up any more noise performance. Maybe he was just designing to withstand start up and shut down transients. If the circuit doesn't protect against hard dead shorts, then it wasn't supposed to. -- Cheers, James Arthur
From: Mike on 28 May 2010 12:04
dagmargoodboat(a)yahoo.com wrote: > Yeah, but Walt *was* the manufacturer, designing in his own part, > knowing what's inside. > > I see that AD's 25mA limit for the AD797 input diodes was a > recommendation, not an absolute instantaneous limit. Not knowing > what's inside, withstanding a worst-case spike at 7x the recommended > d.c. limit seems entirely plausible. Or maybe Walt just didn't want > to give up any more noise performance. > > Maybe he was just designing to withstand start up and shut down > transients. > > If the circuit doesn't protect against hard dead shorts, then it > wasn't supposed to. > > -- > Cheers, > James Arthur If you are happy with unknown fault response and 1988 noise performance, then by all means stick with that approach. Thanks, Mike |