C-multiplier again
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From: Mike on 27 May 2010 11:42 MooseFET <kensmith(a)rahul.net> wrote: > On May 26, 6:27�pm, Mike <s...(a)me.not> wrote: > [....] >> �http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pdf > > I have a slightly improved version: > > !\U1 > Ref ------[R]--+------------! >-----+- Low noise version > ! !/ ! > ! ! > C1[C] ---[R]---+---[R]-+ > ! ! ! ! > ! ! /-!-- [R] > ---+--< ! ! > \+!----------+ > U2 ! > [C]C2 > ! > GND > > The time constant at C2 can be huge because the leakage of C2 > doesn't cause your reference to be low. > > The gain of U2 makes C1 look larger than it is. The circuit > is second order so it does have a noise peak at the corner > but that can be at a very low frequency. The resistors on > U2 are low value ones because they add directly to the noise. I'm having trouble figuring the values. Can you post that in LTspice? Mike
From: Mike on 27 May 2010 11:48 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > 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 Not much protection. The reference is 10V. Both inputs have large electrolytics. A hard short, for example on the low-noise output, will exceed the maximum current and destroy the device. If the resistor is not helping protect the inputs, why keep it? Mike
From: John Larkin on 27 May 2010 12:24 On Thu, 27 May 2010 09:24:04 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"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. ;-) That's not what I said. John
From: John Larkin on 27 May 2010 12:25 On Thu, 27 May 2010 15:48:59 GMT, Mike <spam(a)me.not> wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> 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 > >Not much protection. The reference is 10V. Both inputs have large >electrolytics. A hard short, for example on the low-noise output, will >exceed the maximum current and destroy the device. If the resistor is not >helping protect the inputs, why keep it? > >Mike Beats me. Ask the designer. John
From: John Larkin on 27 May 2010 12:27
On Thu, 27 May 2010 15:34:16 GMT, Mike <spam(a)me.not> wrote: >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. > I'm sure it's there precisely because it does protect the diodes. >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. Depletion fets or SSRs make nice current limiters and active protectors. John |