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From: Winfield Hill on 18 Jun 2010 15:03 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
From: George Herold on 18 Jun 2010 15:55 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.
From: bw on 18 Jun 2010 19:30 "George Herold" <gherold(a)teachspin.com> wrote in message news:8b085513-fad1-469e-9f00-88eb04e6f58f(a)y11g2000yqm.googlegroups.com... 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. ---------------------------------------------------- Some low noise circuits by VLF listeners here http://www.infiltec.com/SID-GRB(a)home/ http://www.vlf.it/ http://www.vlf.it/ewer3/spectrum1.html http://www.qsl.net/vk5br/VLFLFRx.htm I've tested some simple pre-amp/voltage amps over the years, found out 2 key points. 1. Use a discrete JFET with careful bias set by low noise resistors. 2. Then find a place FAR FAR away from any grid power lines.
From: Winfield Hill on 18 Jun 2010 20:43 bw wrote... > > any better way to measure current noise than something like this? > > +--100k ohm--+ > | |\ | > +-- \ | > | >-------+---out > +-+ / > | |/ > | > GND 100k is a bit on the low side. -- Thanks, - Win
From: John Larkin on 18 Jun 2010 21:13
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. I'd set up the opamp as a follower and dump its + input into a grounded resistor. Then amplify that voltage with another opamp. Measure the amplified noise with and without the DUT and do the math. John |