Do you sprinkle some pixie dust on the epoxy for the smaller parts?
-- SCNR, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
A while ago I suggested the Analog Devices ADP3300 and ADP3330 but you said they were too expensive. Anyway they haven't oscillated in my experience so far.
Charles Wenzel has a nice idea here:
Chris
Sort of strange, doing feedforward noise correction. Why not sense the actual output and fight the noise there?
John
The 3300 is ok in price but 50mA is a bit wimpy. The 3330 is too pricey for most of my apps (but not for all of them).
The LM833 style reminds me of a consulting job way back when. Noise, the usual. Caused by some RAM bank power surges and, like most of the time, I was called in after the fact. Layouts all done, no chance to do any serious filtering. Luckily they had placed small current sense resistors so technicians could measure the current intakes. Plus, yeehaw, a few free bus connector pins. So I current-sensed the nasty stuff and fed it back into the backplane inverted. Loop stability wasn't exactly trivial though because some of it migrated into the offending board rail again. When presenting all that at the afternoon meeting the reaction was utter disgust. "Yikes, that's ugly! I don't think it'll work." ... "Ahm, I reworked it into one system, check it out." ... They did. Noise gone. That cinched it.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Not sure why he does it like that but he is pretty clever so there is probably a subtle reason.
The only possible reasons I can think of: a) it is pretty unlikely to be unstable if it is feedforward, even with unspecified decoupling at the load, b) the amplifier might need less gain, so it could be simpler, or easier to design and/or have wider bandwidth for the same gain-bandwidth product
Chris
HC running on a squeaky-clean 5V is great. Using those with a series resistor to drive Mini-Circuits diode bridge phase detectors is the bee's knees for low close-in phase noise.
Cheers
Phil Hobbs
HC running on a squeaky-clean 5V is great. Using those with a series resistor to drive Mini-Circuits diode bridge phase detectors is the bee's knees for low close-in phase noise.
Cheers
Phil Hobbs
You can get a lot more bandwidth by using matching than feedback, because the phase shifts can be anything you like. That can help quite a bit sometimes.
I published a circuit several million years ago for making very quiet HV supplies--it used a floating supply, sensed the ripple on the hot end via a series RC and used an op amp to jiggle the cold end to keep the hot end still. Brought a 3 kV supply from 10V p-p ripple down to 100 uV or so, limited by slew rate. That was good for a 100x improvement in positioning accuracy on my piezo stage.
Cheers,
Phil Hobbs
Feedforward will sure not help AC load regulation; the series R makes it worse. Feedback onto the output node helps everything.
Nowadays you can put a ton of ceramic and polymer aluminum bypassing on a rail, so all the feedback thing has to do is stomp the lower frequency noise. That shouldn't be hard to stabilize.
John
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I have seen lots of bad or marginal ideas from Wenzel. If you are going to add transistors and losses in the name of improving PSRR, you are likely to do a lot better cleaning up the input to the LM78XX than cleaning up the output.
-------- --+------ ------+-----! LM78XX !----+--- ! \\ / e ! -------- ! ! ----- =3D=3D=3D ! =3D=3D=3D ! ! ! ! ! -/\\/\\---+----!!--+--GND---- GND
For low frequency stuff, you can use the LM78XX as the pass element and wiggle its ground wire a bit to remove any low frequency AC at the load. If you keep the corner frequency of the loop enclosing the LM78XX below about 100KHz, you can ignore its phase shift.
Well at least the reverse engineers at their competitor company will be scratching their heads trying to figure out what it's for.
Chris
For certain applications like powering the VCO in an RF synthesiser, load regulation doesn't matter since there would normally be no other loads on the VCO regulator.
Chris
Certainly PSRR is better fixed by cleaning up the supply at the input of the regulator, but that will not help the noise of the regulator itself. --------
That sounds like it should work as long as there isn't a lot of decoupling on the output of the regulator. Where you need a lot of decoupling at the output of the regulator I suspect it could get tricky.
Chris
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-+------ =A0 =A0 ------+-----! LM78XX !----+---
=A0 =A0=3D=3D=3D
=A0 =A0 !
gthe
The more bypass on the output of the regulator, the lower the gain crossover of the loop needs to be. On the other hand, the bypassing also lowers the frequency above which the capacitors eat up all the noise.
A 78xx, with internal v-set resistors, amplifies its own noise. If you use an LM317 or an LM1117 with an external divider, you can bypass the lower resistor and kill that gain. Better yet, drive the adjust pin from a smart opamp, which amounts to using the 1117 as a power amp.
Or use/build a better regulator.
As far as logic jitter goes, you don't need an unusually good regulator to be good enough that it doesn't matter. ECL is especially immune to jitter induced by Vcc variations.
John
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