When are ferrite beads the most approproate component to use?
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From: miso on 28 Dec 2009 16:30 On Dec 28, 9:40 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sun, 27 Dec 2009 22:54:43 -0800 (PST), "m...(a)sushi.com" > > > > <m...(a)sushi.com> wrote: > >On Dec 27, 9:49 am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- > >Web-Site.com/Snicker> wrote: > >> On Sun, 27 Dec 2009 09:42:18 -0800, John Larkin > > >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >On Sun, 27 Dec 2009 07:15:28 -0600, "mook johnson" <m...(a)mook.net> > >> >wrote: > > >> [snip] > > >> >>What processes do you use to predict the action of applying a ferrite bead. > >> >>Is is all cut and try? > > >> >Frankly, it's more like wearing garlic to keep away vampires. > >> [snip] > > >> >Lots of opamps have ghastly supply-noise rejection at high > >> >frequencies. We recently got bit by one that had *gain* from V- supply > >> >to output, datasheet to the contrary. > >> [snip] > >> >John > > >> That "gain" is actually quite common... which is why PSRR, on data > >> sheets, is always referred to input ;-) > > >> ...Jim Thompson > >> -- > >> | James E.Thompson, CTO | mens | > >> | Analog Innovations, Inc. | et | > >> | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > >> | Phoenix, Arizona 85048 Skype: Contacts Only | | > >> | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | > >> | E-mail Icon athttp://www.analog-innovations.com| 1962 | > > >> Help save the environment! > >> Please dispose of socialism responsibly! > > >Technically the op amp is put in a voltage follower for the PSRR test. > >I suppose that is the same as input referred. I don't find that > >cheating since the voltage follower is a common application. > > That effectively shorts out the coupling path from the rails to the > input pins, about best-case for PSRR measurements. > > > > >One problem with doing the PSRR test is you need to maintain a low > >impedance at the DUT pin while wiggling it. This generally done by > >passing the DC through a transformer. If the driving impedance is > >high, the test isn't valid. If you attach a generator to the DUT pin, > >the test is questionable since the generator impedance is probably 50 > >ohms. Even using op amps to drive the dut is questionable since their > >output impedance rises with frequency. Too bad the "damn fast buffer" > >is history. > > Once a supply pin has the desired DC voltage and desired AC ripple, > why does it matter how it got there? > > John Suppose the DUT has some capacitance to ground from a supply rail. Then the high impedance source would not be stimulating the circuit guts properly. Obviously, this become a problem at higher frequency. So not driving it hard could give a more favorable result. Also not mentioned is the applied AC for such testing. I do 100mvpp. Maybe 250mvpp is the device is really good, i.e. the output is hard to measure. Thinking about this a bit more, I can see why the voltage follower circuit is used. If you are measuring the output with a volt meter, you can't reject circuit noise from the test frequency. I use spectrum analyzers for PSRR, but this may not be a hard and fast rule in such testing. Now this makes me wonder if a circuit produces harmonics, those would be measured with a simple meter, but not a network analyzer. I could see that scenario making a device work worse in an application than the test data.
From: Joerg on 28 Dec 2009 16:40 miso(a)sushi.com wrote: > On Dec 27, 9:49 am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- > Web-Site.com/Snicker> wrote: >> On Sun, 27 Dec 2009 09:42:18 -0800, John Larkin >> >> >> >> >> >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>> On Sun, 27 Dec 2009 07:15:28 -0600, "mook johnson" <m...(a)mook.net> >>> wrote: >> [snip] >> >>>> What processes do you use to predict the action of applying a ferrite bead. >>>> Is is all cut and try? >>> Frankly, it's more like wearing garlic to keep away vampires. >> [snip] >> >>> Lots of opamps have ghastly supply-noise rejection at high >>> frequencies. We recently got bit by one that had *gain* from V- supply >>> to output, datasheet to the contrary. >> [snip] >>> John >> That "gain" is actually quite common... which is why PSRR, on data >> sheets, is always referred to input ;-) >> >> ...Jim Thompson >> -- >> | James E.Thompson, CTO | mens | >> | Analog Innovations, Inc. | et | >> | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | >> | Phoenix, Arizona 85048 Skype: Contacts Only | | >> | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | >> | E-mail Icon athttp://www.analog-innovations.com| 1962 | >> >> Help save the environment! >> Please dispose of socialism responsibly! > > Technically the op amp is put in a voltage follower for the PSRR test. > I suppose that is the same as input referred. I don't find that > cheating since the voltage follower is a common application. > > One problem with doing the PSRR test is you need to maintain a low > impedance at the DUT pin while wiggling it. This generally done by > passing the DC through a transformer. If the driving impedance is > high, the test isn't valid. If you attach a generator to the DUT pin, > the test is questionable since the generator impedance is probably 50 > ohms. Even using op amps to drive the dut is questionable since their > output impedance rises with frequency. Too bad the "damn fast buffer" > is history. Thou should have squirreled away some of those before the last order bell was rung. I did :-) Same with the SL6440 mixer from Plessey. One of the best things since the invention of pivot irrigation. So, before the company went phut I got myself a tube of these as well. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Fred Bartoli on 28 Dec 2009 16:53 Joerg a �crit : > > Thou should have squirreled away some of those before the last order > bell was rung. I did :-) > > Same with the SL6440 mixer from Plessey. One of the best things since > the invention of pivot irrigation. > So, before the company went phut ... Ah, that was you... And did the CEO kaboomed then? -- Thanks, Fred.
From: Phil Hobbs on 28 Dec 2009 17:11 On 12/28/2009 4:40 PM, Joerg wrote: > miso(a)sushi.com wrote: >> On Dec 27, 9:49 am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- >> Web-Site.com/Snicker> wrote: >>> On Sun, 27 Dec 2009 09:42:18 -0800, John Larkin >>> >>> >>> >>> >>> >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> On Sun, 27 Dec 2009 07:15:28 -0600, "mook johnson" <m...(a)mook.net> >>>> wrote: >>> [snip] >>> >>>>> What processes do you use to predict the action of applying a >>>>> ferrite bead. >>>>> Is is all cut and try? >>>> Frankly, it's more like wearing garlic to keep away vampires. >>> [snip] >>> >>>> Lots of opamps have ghastly supply-noise rejection at high >>>> frequencies. We recently got bit by one that had *gain* from V- supply >>>> to output, datasheet to the contrary. >>> [snip] >>>> John >>> That "gain" is actually quite common... which is why PSRR, on data >>> sheets, is always referred to input ;-) >>> >>> ...Jim Thompson >>> -- >>> | James E.Thompson, CTO | mens | >>> | Analog Innovations, Inc. | et | >>> | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | >>> | Phoenix, Arizona 85048 Skype: Contacts Only | | >>> | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | >>> | E-mail Icon athttp://www.analog-innovations.com| 1962 | >>> >>> Help save the environment! >>> Please dispose of socialism responsibly! >> >> Technically the op amp is put in a voltage follower for the PSRR test. >> I suppose that is the same as input referred. I don't find that >> cheating since the voltage follower is a common application. >> >> One problem with doing the PSRR test is you need to maintain a low >> impedance at the DUT pin while wiggling it. This generally done by >> passing the DC through a transformer. If the driving impedance is >> high, the test isn't valid. If you attach a generator to the DUT pin, >> the test is questionable since the generator impedance is probably 50 >> ohms. Even using op amps to drive the dut is questionable since their >> output impedance rises with frequency. Too bad the "damn fast buffer" >> is history. > > > Thou should have squirreled away some of those before the last order > bell was rung. I did :-) > > Same with the SL6440 mixer from Plessey. One of the best things since > the invention of pivot irrigation. So, before the company went phut I > got myself a tube of these as well. > I have a dozen of the LH0033 and LH0063, as well as of the lesser-known LH4009, which was one of the weird outpouring of hybrid parts in National's 1989 catalogue. Almost all were cone in a couple of years. Now I need to husband my MAT04s too. Hopefully they don't stop making BF862s! :( 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: John Larkin on 4 Jan 2010 16:38
On Mon, 28 Dec 2009 13:30:27 -0800 (PST), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Dec 28, 9:40�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Sun, 27 Dec 2009 22:54:43 -0800 (PST), "m...(a)sushi.com" >> >> >> >> <m...(a)sushi.com> wrote: >> >On Dec 27, 9:49�am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- >> >Web-Site.com/Snicker> wrote: >> >> On Sun, 27 Dec 2009 09:42:18 -0800, John Larkin >> >> >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> >On Sun, 27 Dec 2009 07:15:28 -0600, "mook johnson" <m...(a)mook.net> >> >> >wrote: >> >> >> [snip] >> >> >> >>What processes do you use to predict the action of applying a ferrite bead. >> >> >>Is is all cut and try? >> >> >> >Frankly, it's more like wearing garlic to keep away vampires. >> >> [snip] >> >> >> >Lots of opamps have ghastly supply-noise rejection at high >> >> >frequencies. We recently got bit by one that had *gain* from V- supply >> >> >to output, datasheet to the contrary. >> >> [snip] >> >> >John >> >> >> That "gain" is actually quite common... which is why PSRR, on data >> >> sheets, is always referred to input ;-) >> >> >> � � � � � � � � � � � � � � � � � � � � ...Jim Thompson >> >> -- >> >> | James E.Thompson, CTO � � � � � � � � � � � � � �| � �mens � � | >> >> | Analog Innovations, Inc. � � � � � � � � � � � � | � � et � � �| >> >> | Analog/Mixed-Signal ASIC's and Discrete Systems �| � �manus � �| >> >> | Phoenix, Arizona �85048 � �Skype: Contacts Only �| � � � � � � | >> >> | Voice:(480)460-2350 �Fax: Available upon request | �Brass Rat �| >> >> | E-mail Icon athttp://www.analog-innovations.com|� �1962 � � | >> >> >> � � � � � � � � � � Help save the environment! >> >> � � � � � � �Please dispose of socialism responsibly! >> >> >Technically the op amp is put in a voltage follower for the PSRR test. >> >I suppose that is the same as input referred. I don't find that >> >cheating since the voltage follower is a common application. >> >> That effectively shorts out the coupling path from the rails to the >> input pins, about best-case for PSRR measurements. >> >> >> >> >One problem with doing the PSRR test is you need to maintain a low >> >impedance at the DUT pin while wiggling it. This generally done by >> >passing the DC through a transformer. If the driving impedance is >> >high, the test isn't valid. If you attach a generator to the DUT pin, >> >the test is questionable since the generator impedance is probably 50 >> >ohms. Even using op amps to drive the dut is questionable since their >> >output impedance rises with frequency. Too bad the "damn fast buffer" >> >is history. >> >> Once a supply pin has the desired DC voltage and desired AC ripple, >> why does it matter how it got there? >> >> John > >Suppose the DUT has some capacitance to ground from a supply rail. >Then the high impedance source would not be stimulating the circuit >guts properly. Obviously, this become a problem at higher frequency. >So not driving it hard could give a more favorable result. If the DUT V+ pin is +15 volts and has 250 mV p-p AC superimposed on it, how is the DUT going to know how the 250 mV was generated, or what the impedance of the generator is? John |