When are ferrite beads the most approproate component to use?
in [Design]
|
Prev: Idiots
Next: Looking for photo frame that does GIF...
From: miso on 28 Dec 2009 01:54 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.
From: Fred Bartoli on 28 Dec 2009 12:15 miso(a)sushi.com a �crit : > 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. I just did such a test for a small sensor amplifier. In my case I used a 50R cable termination which made the generator impedance 25R, but that's not mandatory. Gain-phase analyzers have a reference input, so you don't mind the 1/f AC voltage resulting from the bypass caps. There's no lack of signal either since PSRR decreases at the same rate the applied AC voltage does. Also mind that PSRR is often misleading since it's (as often) well below the real circuit's PSRR which is (still as often) dominated by the opamp's PS rail to inputs parasitic caps. In my case it degraded from the opamp's 80dB down to a real 46dB. -- Thanks, Fred.
From: John Larkin on 28 Dec 2009 12:40 On Sun, 27 Dec 2009 22:54:43 -0800 (PST), "miso(a)sushi.com" <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. 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
From: Jim Thompson on 28 Dec 2009 13:09 On Mon, 28 Dec 2009 09:40:40 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 27 Dec 2009 22:54:43 -0800 (PST), "miso(a)sushi.com" ><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 >>> [snip] >>> >>> >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 >> >>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. No. What is does is divide the expected output movement by the loop gain... so it looks good :-( > >> >>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 > ...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 at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism responsibly!
From: miso on 28 Dec 2009 16:14
On Dec 28, 10:09 am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- Web-Site.com/Snicker> wrote: > On Mon, 28 Dec 2009 09:40:40 -0800, 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 > > [snip] > > >>> >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 > > >>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. > > No. What is does is divide the expected output movement by the loop > gain... so it looks good :-( > > > > > > >>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 > > ...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! But I still think the follower is a fair test circuit. What would you propose? I did a quick survey yesterday regarding PSRR test setups. LTC didn't provide a test circuit. I'd have to double check, but I think Analog didn't either. Intersil (yeah, who cares about them) had a test circuit where they put a buffer at the output of the DUT, I guess just to drive the test equipment. National had a document on op amp testing and basically showed the voltage follower. |