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From: Phil Hobbs on 19 Jun 2010 01:44 Phil Hobbs wrote: > John Larkin wrote: >> On Fri, 18 Jun 2010 20:25:36 -0700 (PDT), George Herold >> <gherold(a)teachspin.com> wrote: >> >>> >>> John Larkin wrote: >>>> On Fri, 18 Jun 2010 12:55:27 -0700 (PDT), George Herold >>>> <gherold(a)teachspin.com> wrote: >>>> >>>>> On Jun 18, 3:03?pm, Winfield Hill <Winfield_mem...(a)newsguy.com> >>>>> wrote: >>>>>> George Herold wrote... >>>>>> >>>>>>> Interesting, Thanks Phil. ?Say I noticed the other day that the >>>>>>> opa134 >>>>>>> has current noise that starts to rise dramatically with frequency >>>>>>> above 2kHz or so. ?http://focus.ti.com/lit/ds/symlink/opa134.pdf >>>>>>> See the graph at the bottom of page 4. >>>>>>> Is this common for all jfet opamps? ?I've never seen other spec >>>>>>> sheets that plot current noise vs. frequency for FET opamps. >>>>>> ?The current noise is so low, 3fA/rt-Hz, that it's scarcely >>>>>> ?significant. ?But it's still interesting to analyze. >>>>>> >>>>>> ?Often this type of very low spectral-density noise, rising >>>>>> ?proportional to frequency, is due to a capacitively-coupled >>>>>> ?signal from a spectrally-flat voltage-noise source. >>>>>> >>>>>> ?We can calculate, i_n = e_n 2pi f Cx. ?For example, assume >>>>>> ?the JFETs have 60k drain resistors, which gives about 10nV >>>>>> ?of Johnson noise. ?If this is coupled to the input via some >>>>>> ?capacitance, etc., we can calculate, Cx = I_n / 2pi f e_n. >>>>>> ?For noise at some frequency on the plot, we get Cx = 0.06pF. >>>>>> ?Is that due to Cdg, or is it some other small capacitance? >>>>>> >>>>>> ?If the opamp had a cascode input stage for the JFETs, their >>>>>> ?drains should not be exposed to high voltage noise. ?It's >>>>>> ?possible the opa134 doesn't have a cascode input. ?TI says, >>>>>> ?"The p-channel JFETs in the FET input stage exhibit a varying >>>>>> ?input capacitance with applied common-mode input voltage." >>>>>> >>>>>> -- >>>>>> ?Thanks, >>>>>> ? ? - Win >>>>> Thanks Win, The capacitive coupling makes sense. The 3 fA/rtHz is no >>>>> worry. But at 100kHz the nosie has risen to 100fA/rtHz.. If my source >>>>> impedance is 100k ohm.. that looks like 10nV/rtHz of voltage noise. >>>>> That's starting to look significant. >>>>> >>>>> any better way to measure current noise than something like this? >>>>> >>>>> +--100k ohm--+ >>>>> | |\ | >>>>> +-- \ | >>>>> | >-------+---out >>>>> +-+ / >>>>> | |/ >>>>> | >>>>> GND >>>>> >>>>> Thanks, >>>>> >>>>> George H. >>>> That setup has a huge noise gain at high frequencies. >>> Yeah a TIA with no 'external' current. A pf of capacitance to ground >>> gives me a noise gain of 2 out near 1MHz or so. I could roll the >>> gian. >>> >>> I've been using the follower circuit. (Well I usually put a bit of >>> gain in the first stage. 200 ohms to ground and a 1k feedback >>> resistor.) I see no sign of excess noise out at higher frequencies. >>> (I can only measure things with the needed accuracy out to 100kHz.) >>> So I was trying to think of some way to measure the current noise of >>> the opamp. >>> >>> But the TIA configuration looks like it gives the same answer as the >>> follower. (Well with the TIA, capacitance to ground gives noise gain >>> and more noise at higher frequencies. Whereas with the follower the C >>> to ground rolls off the signal from the DUT. Both effects tend to >>> throw a monkey wrench into the measurement. Sigh. I should try Win's >>> suggestion of a bigger resistor. I could tyr asking Burr Brown (TI) >>> how they measured the current noise. >>> >>> Thanks, >>> >>> George H. >>> >> >> The OPA134 will need a couple of megs of input or feedback resistance >> to get the current-induced noise up to the level of its voltage noise, >> where you can resolve it: 8 nv/rthz. But the Johnson noise of that >> resistor will be around 20x as much. This doesn't look good. >> >> There must be a trick somewhere. Maybe something with a capacitor. >> >> The easy was is to ask them. >> >> Maybe they calculate it, and don't measure it, based on the shot noise >> of the bias current. They claim 5 pA bias current, 3 fA/rthz noise as >> typs. The shot noise of 5 pA is 1.3 fA/rthz. Assume the bias current >> is contributed by partially canceling ESD diode leakages, and it's in >> the ballpark. >> >> John >> > > Well, the current noise is a real current that comes out of the input, > whereas the voltage noise is purely internal to the device. Thus if you > took two followers connected like George's, you could use correlation, > like this: > > 1. Leaving them separate, so all their intrinsic noise sources are > independent, cross-correlate their output noise, and integrate for long > enough that the correlation product decays below the level you want to > measure. > > 2. Connect their inputs together, so that the current noise is now 100% > correlated in both outputs, and re-do the cross-correlation with the > same integration time. The voltage noise remains uncorrelated, so the > result will be a measurement of the RMS sum of the current noise of the > two amplfiers. > Needs one more turn of the crank--use normal followers, connected to ground via large resistors. Then proceed as above. 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: George Herold on 19 Jun 2010 10:50 On Jun 19, 1:39 am, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > John Larkin wrote: > > On Fri, 18 Jun 2010 20:25:36 -0700 (PDT), George Herold > > <gher...(a)teachspin.com> wrote: > > >> John Larkin wrote: > >>> On Fri, 18 Jun 2010 12:55:27 -0700 (PDT), George Herold > >>> <gher...(a)teachspin.com> wrote: > > >>>> On Jun 18, 3:03?pm, Winfield Hill <Winfield_mem...(a)newsguy.com> > >>>> wrote: > >>>>> George Herold wrote... > > >>>>>> Interesting, Thanks Phil. ?Say I noticed the other day that the opa134 > >>>>>> has current noise that starts to rise dramatically with frequency > >>>>>> above 2kHz or so. ?http://focus.ti.com/lit/ds/symlink/opa134.pdf > >>>>>> See the graph at the bottom of page 4. > >>>>>> Is this common for all jfet opamps? ?I've never seen other spec > >>>>>> sheets that plot current noise vs. frequency for FET opamps. > >>>>> ?The current noise is so low, 3fA/rt-Hz, that it's scarcely > >>>>> ?significant. ?But it's still interesting to analyze. > > >>>>> ?Often this type of very low spectral-density noise, rising > >>>>> ?proportional to frequency, is due to a capacitively-coupled > >>>>> ?signal from a spectrally-flat voltage-noise source. > > >>>>> ?We can calculate, i_n = e_n 2pi f Cx. ?For example, assume > >>>>> ?the JFETs have 60k drain resistors, which gives about 10nV > >>>>> ?of Johnson noise. ?If this is coupled to the input via some > >>>>> ?capacitance, etc., we can calculate, Cx = I_n / 2pi f e_n. > >>>>> ?For noise at some frequency on the plot, we get Cx = 0.06pF. > >>>>> ?Is that due to Cdg, or is it some other small capacitance? > > >>>>> ?If the opamp had a cascode input stage for the JFETs, their > >>>>> ?drains should not be exposed to high voltage noise. ?It's > >>>>> ?possible the opa134 doesn't have a cascode input. ?TI says, > >>>>> ?"The p-channel JFETs in the FET input stage exhibit a varying > >>>>> ?input capacitance with applied common-mode input voltage." > > >>>>> -- > >>>>> ?Thanks, > >>>>> ? ? - Win > >>>> Thanks Win, The capacitive coupling makes sense. The 3 fA/rtHz is no > >>>> worry. But at 100kHz the nosie has risen to 100fA/rtHz.. If my source > >>>> impedance is 100k ohm.. that looks like 10nV/rtHz of voltage noise. > >>>> That's starting to look significant. > > >>>> any better way to measure current noise than something like this? > > >>>> +--100k ohm--+ > >>>> | |\ | > >>>> +-- \ | > >>>> | >-------+---out > >>>> +-+ / > >>>> | |/ > >>>> | > >>>> GND > > >>>> Thanks, > > >>>> George H. > >>> That setup has a huge noise gain at high frequencies. > >> Yeah a TIA with no 'external' current. A pf of capacitance to ground > >> gives me a noise gain of 2 out near 1MHz or so. I could roll the > >> gian. > > >> I've been using the follower circuit. (Well I usually put a bit of > >> gain in the first stage. 200 ohms to ground and a 1k feedback > >> resistor.) I see no sign of excess noise out at higher frequencies. > >> (I can only measure things with the needed accuracy out to 100kHz.) > >> So I was trying to think of some way to measure the current noise of > >> the opamp. > > >> But the TIA configuration looks like it gives the same answer as the > >> follower. (Well with the TIA, capacitance to ground gives noise gain > >> and more noise at higher frequencies. Whereas with the follower the C > >> to ground rolls off the signal from the DUT. Both effects tend to > >> throw a monkey wrench into the measurement. Sigh. I should try Win's > >> suggestion of a bigger resistor. I could tyr asking Burr Brown (TI) > >> how they measured the current noise. > > >> Thanks, > > >> George H. > > > The OPA134 will need a couple of megs of input or feedback resistance > > to get the current-induced noise up to the level of its voltage noise, > > where you can resolve it: 8 nv/rthz. But the Johnson noise of that > > resistor will be around 20x as much. This doesn't look good. > > > There must be a trick somewhere. Maybe something with a capacitor. > > > The easy was is to ask them. > > > Maybe they calculate it, and don't measure it, based on the shot noise > > of the bias current. They claim 5 pA bias current, 3 fA/rthz noise as > > typs. The shot noise of 5 pA is 1.3 fA/rthz. Assume the bias current > > is contributed by partially canceling ESD diode leakages, and it's in > > the ballpark. > > > John > > Well, the current noise is a real current that comes out of the input, > whereas the voltage noise is purely internal to the device. Thus if you > took two followers connected like George's, you could use correlation, > like this: > > 1. Leaving them separate, so all their intrinsic noise sources are > independent, cross-correlate their output noise, and integrate for long > enough that the correlation product decays below the level you want to > measure. > > 2. Connect their inputs together, so that the current noise is now 100% > correlated in both outputs, and re-do the cross-correlation with the > same integration time. The voltage noise remains uncorrelated, so the > result will be a measurement of the RMS sum of the current noise of the > two amplfiers. > > 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 nethttp://electrooptical.net- Hide quoted text - > > - Show quoted text - Wow, Phil Hobbs to the rescue! Thanks. (I think I get the above and I think I can do it.) Can I send you a bottle of my favorite Belgian style abbey ale? (are you allowed to ship alcohol via the mail?) George H.
From: George Herold on 19 Jun 2010 10:51 On Jun 19, 12:41 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Fri, 18 Jun 2010 21:18:07 -0700, John Larkin > > > > > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >On Fri, 18 Jun 2010 20:25:36 -0700 (PDT), George Herold > ><gher...(a)teachspin.com> wrote: > > >>John Larkin wrote: > >>> On Fri, 18 Jun 2010 12:55:27 -0700 (PDT), George Herold > >>> <gher...(a)teachspin.com> wrote: > > >>> >On Jun 18, 3:03?pm, Winfield Hill <Winfield_mem...(a)newsguy.com> > >>> >wrote: > >>> >> George Herold wrote... > > >>> >> > Interesting, Thanks Phil. ?Say I noticed the other day that the opa134 > >>> >> > has current noise that starts to rise dramatically with frequency > >>> >> > above 2kHz or so. ?http://focus.ti.com/lit/ds/symlink/opa134.pdf > >>> >> > See the graph at the bottom of page 4. > > >>> >> > Is this common for all jfet opamps? ?I've never seen other spec > >>> >> > sheets that plot current noise vs. frequency for FET opamps. > > >>> >> ?The current noise is so low, 3fA/rt-Hz, that it's scarcely > >>> >> ?significant. ?But it's still interesting to analyze. > > >>> >> ?Often this type of very low spectral-density noise, rising > >>> >> ?proportional to frequency, is due to a capacitively-coupled > >>> >> ?signal from a spectrally-flat voltage-noise source. > > >>> >> ?We can calculate, i_n = e_n 2pi f Cx. ?For example, assume > >>> >> ?the JFETs have 60k drain resistors, which gives about 10nV > >>> >> ?of Johnson noise. ?If this is coupled to the input via some > >>> >> ?capacitance, etc., we can calculate, Cx = I_n / 2pi f e_n. > >>> >> ?For noise at some frequency on the plot, we get Cx = 0.06pF. > >>> >> ?Is that due to Cdg, or is it some other small capacitance? > > >>> >> ?If the opamp had a cascode input stage for the JFETs, their > >>> >> ?drains should not be exposed to high voltage noise. ?It's > >>> >> ?possible the opa134 doesn't have a cascode input. ?TI says, > >>> >> ?"The p-channel JFETs in the FET input stage exhibit a varying > >>> >> ?input capacitance with applied common-mode input voltage." > > >>> >> -- > >>> >> ?Thanks, > >>> >> ? ? - Win > > >>> >Thanks Win, The capacitive coupling makes sense. The 3 fA/rtHz is no > >>> >worry. But at 100kHz the nosie has risen to 100fA/rtHz.. If my source > >>> >impedance is 100k ohm.. that looks like 10nV/rtHz of voltage noise. > >>> >That's starting to look significant. > > >>> >any better way to measure current noise than something like this? > > >>> > +--100k ohm--+ > >>> > | |\ | > >>> > +-- \ | > >>> > | >-------+---out > >>> > +-+ / > >>> > | |/ > >>> > | > >>> > GND > > >>> >Thanks, > > >>> >George H. > > >>> That setup has a huge noise gain at high frequencies. > > >>Yeah a TIA with no 'external' current. A pf of capacitance to ground > >>gives me a noise gain of 2 out near 1MHz or so. I could roll the > >>gian. > > >>I've been using the follower circuit. (Well I usually put a bit of > >>gain in the first stage. 200 ohms to ground and a 1k feedback > >>resistor.) I see no sign of excess noise out at higher frequencies. > >>(I can only measure things with the needed accuracy out to 100kHz.) > >>So I was trying to think of some way to measure the current noise of > >>the opamp. > > >>But the TIA configuration looks like it gives the same answer as the > >>follower. (Well with the TIA, capacitance to ground gives noise gain > >>and more noise at higher frequencies. Whereas with the follower the C > >>to ground rolls off the signal from the DUT. Both effects tend to > >>throw a monkey wrench into the measurement. Sigh. I should try Win's > >>suggestion of a bigger resistor. I could tyr asking Burr Brown (TI) > >>how they measured the current noise. > > >>Thanks, > > >>George H. > > >The OPA134 will need a couple of megs of input or feedback resistance > >to get the current-induced noise up to the level of its voltage noise, > >where you can resolve it: 8 nv/rthz. But the Johnson noise of that > >resistor will be around 20x as much. This doesn't look good. > > >There must be a trick somewhere. Maybe something with a capacitor. > > >The easy was is to ask them. > > >Maybe they calculate it, and don't measure it, based on the shot noise > >of the bias current. They claim 5 pA bias current, 3 fA/rthz noise as > >typs. The shot noise of 5 pA is 1.3 fA/rthz. Assume the bias current > >is contributed by partially canceling ESD diode leakages, and it's in > >the ballpark. > > >John > > Maybe make a follower but leave the + input open. The bias current, a > few pA, will charge the stray capacitance, a few pF, at roughly 1 volt > per second. Measure quickly. > > Add a Phil Hobbsian trick: cancel the bias current, in whichever > direction it turns out to be, with a 1 pF silicon photodiode > illuminated as needed. You can measure the noise, which is caused by > the opamp noise and the shot noise of the photocurrent. It will need a > tiny photodiode. At least the Johnson noise is gone. > > The capacitance will of course integrate the noise current. 5 fA/rthz > dumped into 5 pF is only around 160 uV/rthz at 1 Hz. > > Nasty problem. Ask TI! Yeah I thought I was missing something obvious. > > John- Hide quoted text - > > - Show quoted text -
From: George Herold on 19 Jun 2010 11:16 On Jun 19, 1:44 am, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > Phil Hobbs wrote: > > John Larkin wrote: > >> On Fri, 18 Jun 2010 20:25:36 -0700 (PDT), George Herold > >> <gher...(a)teachspin.com> wrote: > > >>> John Larkin wrote: > >>>> On Fri, 18 Jun 2010 12:55:27 -0700 (PDT), George Herold > >>>> <gher...(a)teachspin.com> wrote: > > >>>>> On Jun 18, 3:03?pm, Winfield Hill <Winfield_mem...(a)newsguy.com> > >>>>> wrote: > >>>>>> George Herold wrote... > > >>>>>>> Interesting, Thanks Phil. ?Say I noticed the other day that the > >>>>>>> opa134 > >>>>>>> has current noise that starts to rise dramatically with frequency > >>>>>>> above 2kHz or so. ?http://focus.ti.com/lit/ds/symlink/opa134.pdf > >>>>>>> See the graph at the bottom of page 4. > >>>>>>> Is this common for all jfet opamps? ?I've never seen other spec > >>>>>>> sheets that plot current noise vs. frequency for FET opamps. > >>>>>> ?The current noise is so low, 3fA/rt-Hz, that it's scarcely > >>>>>> ?significant. ?But it's still interesting to analyze. > > >>>>>> ?Often this type of very low spectral-density noise, rising > >>>>>> ?proportional to frequency, is due to a capacitively-coupled > >>>>>> ?signal from a spectrally-flat voltage-noise source. > > >>>>>> ?We can calculate, i_n = e_n 2pi f Cx. ?For example, assume > >>>>>> ?the JFETs have 60k drain resistors, which gives about 10nV > >>>>>> ?of Johnson noise. ?If this is coupled to the input via some > >>>>>> ?capacitance, etc., we can calculate, Cx = I_n / 2pi f e_n. > >>>>>> ?For noise at some frequency on the plot, we get Cx = 0.06pF. > >>>>>> ?Is that due to Cdg, or is it some other small capacitance? > > >>>>>> ?If the opamp had a cascode input stage for the JFETs, their > >>>>>> ?drains should not be exposed to high voltage noise. ?It's > >>>>>> ?possible the opa134 doesn't have a cascode input. ?TI says, > >>>>>> ?"The p-channel JFETs in the FET input stage exhibit a varying > >>>>>> ?input capacitance with applied common-mode input voltage." > > >>>>>> -- > >>>>>> ?Thanks, > >>>>>> ? ? - Win > >>>>> Thanks Win, The capacitive coupling makes sense. The 3 fA/rtHz is no > >>>>> worry. But at 100kHz the nosie has risen to 100fA/rtHz.. If my source > >>>>> impedance is 100k ohm.. that looks like 10nV/rtHz of voltage noise. > >>>>> That's starting to look significant. > > >>>>> any better way to measure current noise than something like this? > > >>>>> +--100k ohm--+ > >>>>> | |\ | > >>>>> +-- \ | > >>>>> | >-------+---out > >>>>> +-+ / > >>>>> | |/ > >>>>> | > >>>>> GND > > >>>>> Thanks, > > >>>>> George H. > >>>> That setup has a huge noise gain at high frequencies. > >>> Yeah a TIA with no 'external' current. A pf of capacitance to ground > >>> gives me a noise gain of 2 out near 1MHz or so. I could roll the > >>> gian. > > >>> I've been using the follower circuit. (Well I usually put a bit of > >>> gain in the first stage. 200 ohms to ground and a 1k feedback > >>> resistor.) I see no sign of excess noise out at higher frequencies.. > >>> (I can only measure things with the needed accuracy out to 100kHz.) > >>> So I was trying to think of some way to measure the current noise of > >>> the opamp. > > >>> But the TIA configuration looks like it gives the same answer as the > >>> follower. (Well with the TIA, capacitance to ground gives noise gain > >>> and more noise at higher frequencies. Whereas with the follower the C > >>> to ground rolls off the signal from the DUT. Both effects tend to > >>> throw a monkey wrench into the measurement. Sigh. I should try Win's > >>> suggestion of a bigger resistor. I could tyr asking Burr Brown (TI) > >>> how they measured the current noise. > > >>> Thanks, > > >>> George H. > > >> The OPA134 will need a couple of megs of input or feedback resistance > >> to get the current-induced noise up to the level of its voltage noise, > >> where you can resolve it: 8 nv/rthz. But the Johnson noise of that > >> resistor will be around 20x as much. This doesn't look good. > > >> There must be a trick somewhere. Maybe something with a capacitor. > > >> The easy was is to ask them. > > >> Maybe they calculate it, and don't measure it, based on the shot noise > >> of the bias current. They claim 5 pA bias current, 3 fA/rthz noise as > >> typs. The shot noise of 5 pA is 1.3 fA/rthz. Assume the bias current > >> is contributed by partially canceling ESD diode leakages, and it's in > >> the ballpark. > > >> John > > > Well, the current noise is a real current that comes out of the input, > > whereas the voltage noise is purely internal to the device. Thus if you > > took two followers connected like George's, you could use correlation, > > like this: > > > 1. Leaving them separate, so all their intrinsic noise sources are > > independent, cross-correlate their output noise, and integrate for long > > enough that the correlation product decays below the level you want to > > measure. > > > 2. Connect their inputs together, so that the current noise is now 100% > > correlated in both outputs, and re-do the cross-correlation with the > > same integration time. The voltage noise remains uncorrelated, so the > > result will be a measurement of the RMS sum of the current noise of the > > two amplfiers. > > Needs one more turn of the crank--use normal followers, connected to > ground via large resistors. Then proceed as above. > > 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 nethttp://electrooptical.net- Hide quoted text - > > - Show quoted text - OK first two follower's looking at different R's (same value). Then hook each amp to the same R. Increase in correlated noise is due to current noise from each amp. George H.
From: Phil Hobbs on 19 Jun 2010 12:25
On 6/19/2010 11:16 AM, George Herold wrote: > On Jun 19, 1:44 am, Phil Hobbs > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> Phil Hobbs wrote: >>> John Larkin wrote: >>>> On Fri, 18 Jun 2010 20:25:36 -0700 (PDT), George Herold >>>> <gher...(a)teachspin.com> wrote: >> >>>>> John Larkin wrote: >>>>>> On Fri, 18 Jun 2010 12:55:27 -0700 (PDT), George Herold >>>>>> <gher...(a)teachspin.com> wrote: >> >>>>>>> On Jun 18, 3:03?pm, Winfield Hill<Winfield_mem...(a)newsguy.com> >>>>>>> wrote: >>>>>>>> George Herold wrote... >> >>>>>>>>> Interesting, Thanks Phil. ?Say I noticed the other day that the >>>>>>>>> opa134 >>>>>>>>> has current noise that starts to rise dramatically with frequency >>>>>>>>> above 2kHz or so. ?http://focus.ti.com/lit/ds/symlink/opa134.pdf >>>>>>>>> See the graph at the bottom of page 4. >>>>>>>>> Is this common for all jfet opamps? ?I've never seen other spec >>>>>>>>> sheets that plot current noise vs. frequency for FET opamps. >>>>>>>> ?The current noise is so low, 3fA/rt-Hz, that it's scarcely >>>>>>>> ?significant. ?But it's still interesting to analyze. >> >>>>>>>> ?Often this type of very low spectral-density noise, rising >>>>>>>> ?proportional to frequency, is due to a capacitively-coupled >>>>>>>> ?signal from a spectrally-flat voltage-noise source. >> >>>>>>>> ?We can calculate, i_n = e_n 2pi f Cx. ?For example, assume >>>>>>>> ?the JFETs have 60k drain resistors, which gives about 10nV >>>>>>>> ?of Johnson noise. ?If this is coupled to the input via some >>>>>>>> ?capacitance, etc., we can calculate, Cx = I_n / 2pi f e_n. >>>>>>>> ?For noise at some frequency on the plot, we get Cx = 0.06pF. >>>>>>>> ?Is that due to Cdg, or is it some other small capacitance? >> >>>>>>>> ?If the opamp had a cascode input stage for the JFETs, their >>>>>>>> ?drains should not be exposed to high voltage noise. ?It's >>>>>>>> ?possible the opa134 doesn't have a cascode input. ?TI says, >>>>>>>> ?"The p-channel JFETs in the FET input stage exhibit a varying >>>>>>>> ?input capacitance with applied common-mode input voltage." >> >>>>>>>> -- >>>>>>>> ?Thanks, >>>>>>>> ? ? - Win >>>>>>> Thanks Win, The capacitive coupling makes sense. The 3 fA/rtHz is no >>>>>>> worry. But at 100kHz the nosie has risen to 100fA/rtHz.. If my source >>>>>>> impedance is 100k ohm.. that looks like 10nV/rtHz of voltage noise. >>>>>>> That's starting to look significant. >> >>>>>>> any better way to measure current noise than something like this? >> >>>>>>> +--100k ohm--+ >>>>>>> | |\ | >>>>>>> +-- \ | >>>>>>> |>-------+---out >>>>>>> +-+ / >>>>>>> | |/ >>>>>>> | >>>>>>> GND >> >>>>>>> Thanks, >> >>>>>>> George H. >>>>>> That setup has a huge noise gain at high frequencies. >>>>> Yeah a TIA with no 'external' current. A pf of capacitance to ground >>>>> gives me a noise gain of 2 out near 1MHz or so. I could roll the >>>>> gian. >> >>>>> I've been using the follower circuit. (Well I usually put a bit of >>>>> gain in the first stage. 200 ohms to ground and a 1k feedback >>>>> resistor.) I see no sign of excess noise out at higher frequencies.. >>>>> (I can only measure things with the needed accuracy out to 100kHz.) >>>>> So I was trying to think of some way to measure the current noise of >>>>> the opamp. >> >>>>> But the TIA configuration looks like it gives the same answer as the >>>>> follower. (Well with the TIA, capacitance to ground gives noise gain >>>>> and more noise at higher frequencies. Whereas with the follower the C >>>>> to ground rolls off the signal from the DUT. Both effects tend to >>>>> throw a monkey wrench into the measurement. Sigh. I should try Win's >>>>> suggestion of a bigger resistor. I could tyr asking Burr Brown (TI) >>>>> how they measured the current noise. >> >>>>> Thanks, >> >>>>> George H. >> >>>> The OPA134 will need a couple of megs of input or feedback resistance >>>> to get the current-induced noise up to the level of its voltage noise, >>>> where you can resolve it: 8 nv/rthz. But the Johnson noise of that >>>> resistor will be around 20x as much. This doesn't look good. >> >>>> There must be a trick somewhere. Maybe something with a capacitor. >> >>>> The easy was is to ask them. >> >>>> Maybe they calculate it, and don't measure it, based on the shot noise >>>> of the bias current. They claim 5 pA bias current, 3 fA/rthz noise as >>>> typs. The shot noise of 5 pA is 1.3 fA/rthz. Assume the bias current >>>> is contributed by partially canceling ESD diode leakages, and it's in >>>> the ballpark. >> >>>> John >> >>> Well, the current noise is a real current that comes out of the input, >>> whereas the voltage noise is purely internal to the device. Thus if you >>> took two followers connected like George's, you could use correlation, >>> like this: >> >>> 1. Leaving them separate, so all their intrinsic noise sources are >>> independent, cross-correlate their output noise, and integrate for long >>> enough that the correlation product decays below the level you want to >>> measure. >> >>> 2. Connect their inputs together, so that the current noise is now 100% >>> correlated in both outputs, and re-do the cross-correlation with the >>> same integration time. The voltage noise remains uncorrelated, so the >>> result will be a measurement of the RMS sum of the current noise of the >>> two amplfiers. >> >> Needs one more turn of the crank--use normal followers, connected to >> ground via large resistors. Then proceed as above. >> >> 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 nethttp://electrooptical.net- Hide quoted text - >> >> - Show quoted text - > > OK first two follower's looking at different R's (same value). Then > hook each amp to the same R. Increase in correlated noise is due to > current noise from each amp. > > George H. The first measurement is really just a sanity check, to make sure that the measured correlation is low enough. If it is, just use the second measurement by itself. Otherwise it'll take three or four times as long to get a measurement of the same quality. (The fluctuations in the correlation require averaging for a long time if the noise you're looking for is way way below the other noise sources.) 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 |