C-multiplier again
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From: John Larkin on 28 May 2010 10:14 On Thu, 27 May 2010 22:04:34 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: > >> >I have an interesting idea. �How about a blue LED as the reference. > >> I just used two diodes in series to make a low-noise -1.5 volt >> shunt-type supply. I could have used an LED, which would be cool - >> they light up! - but I didn't want any stray light inside our box. > >GAAA! If you want low noise from AN OPTICALLY OPEN DIODE >you need to shield from light. Incandescent will cause 120 Hz >input, fluorescent 60 Hz and 120 Hz, electronic ballasts and >CCFL can go from kilohertz to megahertz. > I wonder what the numbers are like here. Suppose one used a front-panel type green LED as a power indicator and voltage reference. The dynamic impedance of the LED will be ohms. I'd guess that any fluorescent light induced current would be nanoamps. So we'd have nanovolts of optically-induced noise. A cap across the LED would kill the high frequency stuff, like from electronic ballasts. Of course, the current source would have to be a lot better than your average 5-volt-supply-resistor thing. John
From: John Larkin on 28 May 2010 10:17 On Fri, 28 May 2010 08:55:36 GMT, Mike <spam(a)me.not> wrote: > John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > > > On Fri, 28 May 2010 02:11:32 GMT, Mike <spam(a)me.not> wrote: > > [...] > > > That opamp has back-to-back diodes across its inputs and no > > current limiting resistors. The problem is too much current from > > charging those big caps. > > So you now agree that a 49.9 ohm resistor is not enough to prevent > damage, as I have claimed from the beginning. 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
From: dagmargoodboat on 28 May 2010 11:05 On May 28, 9:17 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Fri, 28 May 2010 08:55:36 GMT, Mike <s...(a)me.not> wrote: > > John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > > > On Fri, 28 May 2010 02:11:32 GMT, Mike <s...(a)me.not> wrote: > > > [...] > > > > That opamp has back-to-back diodes across its inputs and no > > > current limiting resistors. The problem is too much current from > > > charging those big caps. > > > So you now agree that a 49.9 ohm resistor is not enough to prevent > > damage, as I have claimed from the beginning. > > 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 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
From: Mike on 28 May 2010 11:09 George Herold <gherold(a)teachspin.com> wrote: [...] > That seemed to work OK, I got some error messages about the cap names, > but I clikced on ignore and it loaded. > > George H Thanks! There may some problems with line wrap. LTspice doesn't like that. In most cases it is fairly easy to fix. I'll take a look later this afternoon. Mike
From: Mike on 28 May 2010 11:10
John Larkin <jjlarkin(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 |