Gain peaking in driven shield
in [Design]
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From: George Herold on 5 May 2010 22:40 On May 5, 6:09 pm, Nemo <z...(a)nospam.nospam.nospam.nospam.co.uk> wrote: > The example I've seen (in a Texas app note) had a few ohms in series > with the op amp output. > > Surely the effective capacitance from inner conductor to shield, which > you call Cis, is essentially nil because of the driven shield masking > the C - i.e. the inner conductor does not see a potential difference > between itself and the outer conductor. > > I'm about to try this myself for a pair of cables about 30cm long. I > chose an AD8626 because - > > - the input current is very small > - bandwidth seemed decent, a few MHz > - it can drive a few hundred pF > - it's unity gain stable > - it has low Cin > - it can use +/-15V supplies and drive as near the rails as my signals > are likely to go > > I'm connecting the inverting input directly to the output (a voltage > follower) putting a 1 ohm resistor in series with the output, which will > drive the shield. > > If you need more info let me know and we can email directly. > -- > Nemo Hi Nemo, There are three layers. On the outside is ground, next is the shield, the center of the coax is the signal line from the resistor. Driving the middle shield, reduces the capacitance between the inner signal line and outer ground. However it adds the capacitance from the shield to the inner signal line. I'll look up the AD8626. And try adding a bit of R in the output line. Do you have a TI app note number? What sort of source impedance are you looking at? 100k looks OK, but 10k ohms shows gain peaking. George H.
From: George Herold on 5 May 2010 22:57 On May 5, 7:45 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 5 May 2010 09:26:32 -0700 (PDT), George Herold > > > > > > <ggher...(a)gmail.com> wrote: > >Im looking at the thermal noise from a resistor down the bottom of a > >probe. Cable and probe are maybe 1/2 a meter long. I added a shield > >line to try and reduce the capacitance between the active end of the > >resistor and ground. (The other end of the resistor is tied to ground > >at the bottom of the probe.) > > >The driven shield seemed to work great at the higher impedance levels > >1Meg and 100k ohms. (Though a more careful examination showed there > >were some issues.) When I tried 10k ohms there was some serious gain > >peaking at the higher frequencies...above 100kHz. I mucked about a > >bit and made sure this wasnt the common problem of an opamp driving a > >capacitive load. > > >Late yesterday it struck me that there is capacitive coupling from the > >shield back to the input. I had been mistakenly thinking of the > >shield as only a capacitance to ground. The capacitance of the inner > >conductor to the ground (Cig) is 60pF, from the inner conductor to the > >shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF. > > >I was first using an opamp follower to drive the shield, but later > >added a bit of gain...and then threw it away. > > > +-----Cis--+ > > | | > > | |\ | > >+-----+--+ \ | > >| | >----+---+----+ > >| +-- / OPA | | | > >Rmeas. | |/ 134 | R1 Csg > >Rmeas. | | R1 | > >| +---------+ | | > >| GND GND > >GND > > >R1 was 50 ohms (to get rid of Csg ringing) > >And then this, > > > +-----Cis--+ > > | | > > | |\ | > >+-----+--+ \ | > >| | >----+-R4R4--+----+ > >| +-- / OPA | | | > >Rmeas. | |/ 134 | R5 Csg > >Rmeas. | | R5 | > >| +--R3R3---+ | | > >| | GND GND > >GND R2 > > R2 > > | > > GND > > >Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms. > > >This seems like it must be a known problem and I wondered if there are > >any simple solutions. I thought that a bit of inductance (L = > >Cis*Rmeas^2) in the right place might help, but I only managed to make > >a nice oscillator. > > >Thanks for any help or advice, > >George H. > > You're trying to measure Johnson noise, right? Yup, The gain peaking is a real pain. > > One problem with any such guarding/bootstrapping scheme is that there > is a room-temp amplifier that picks up the signal and drives the > guard, and it has noise of its own. In some cases that makes adding > the guard a losing battle. 10 k ohms has 40nV/rtHz at room Temp...IIRC. The opa134 has 8nV of voltage noise. > > The OPA134 may be a bit slow, especially driving Csg. 8 MHz GBP. I only need it to work up to 1 MHz or so. Do you have a better opamp in mind? > > What sort of coax are you using? Can you reduce its capacitance? There is a cable which has ~12" of RG174 (? not sure that's the right number, small coax) and in the probe there is ~8" of SS coax from Lakeside cryo. I'm pretty much stuck with the cable I have. If you go here, http://www.teachspin.com/newsletters.shtml And click on the first item (Noise Fundamentals) there are pictures and such. George H. > > John- Hide quoted text - > > - Show quoted text -
From: John Larkin on 5 May 2010 23:25 On Wed, 5 May 2010 19:57:02 -0700 (PDT), George Herold <ggherold(a)gmail.com> wrote: >On May 5, 7:45�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 5 May 2010 09:26:32 -0700 (PDT), George Herold >> >> >> >> >> >> <ggher...(a)gmail.com> wrote: >> >I�m looking at the thermal noise from a resistor down the bottom of a >> >probe. �Cable and probe are maybe 1/2 a meter long. �I added a shield >> >line to try and reduce the capacitance between the �active� end of the >> >resistor and ground. �(The other end of the resistor is tied to ground >> >at the bottom of the probe.) >> >> >The driven shield seemed to work great at the higher impedance levels >> >1Meg and 100k ohms. (Though a more careful examination showed there >> >were some issues.) �When I tried 10k ohms there was some serious gain >> >peaking at the higher frequencies...above 100kHz. �I mucked about a >> >bit and made sure this wasn�t the common problem of an opamp driving a >> >capacitive load. >> >> >Late yesterday it struck me that there is capacitive coupling from the >> >shield back to the input. �I had been mistakenly thinking of the >> >shield as only a capacitance to ground. �The capacitance of the inner >> >conductor to the ground (Cig) is 60pF, from the inner conductor to the >> >shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF. >> >> >I was first using an opamp follower to drive the shield, but later >> >added a bit of gain...and then threw it away. >> >> > � � �+-----Cis--+ >> > � � �| � � � � �| >> > � � �| �|\ � � �| >> >+-----+--+ \ � � | >> >| � � � �| �>----+---+----+ >> >| � � �+-- / OPA | � | � �| >> >Rmeas. | |/ �134 | � R1 � Csg >> >Rmeas. | � � � � | � R1 � | >> >| � � �+---------+ � | � �| >> >| � � � � � � � � � GND � GND >> >GND >> >> >R1 was 50 ohms (to get rid of Csg ringing) >> >And then this, >> >> > � � �+-----Cis--+ >> > � � �| � � � � �| >> > � � �| �|\ � � �| >> >+-----+--+ \ � � | >> >| � � � �| �>----+-R4R4--+----+ >> >| � � �+-- / OPA | � � � | � �| >> >Rmeas. | |/ �134 | � � � R5 � Csg >> >Rmeas. | � � � � | � � � R5 � | >> >| � � �+--R3R3---+ � � � | � �| >> >| � � �| � � � � � � � � GND �GND >> >GND � �R2 >> > � � � R2 >> > � � � | >> > � � � GND >> >> >Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms. >> >> >This seems like it must be a known problem and I wondered if there are >> >any simple solutions. �I thought that a bit of inductance (L = >> >Cis*Rmeas^2) in the right place might help, but I only managed to make >> >a nice oscillator. >> >> >Thanks for any help or advice, >> >George H. >> >> You're trying to measure Johnson noise, right? > >Yup, The gain peaking is a real pain. > >> >> One problem with any such guarding/bootstrapping scheme is that there >> is a room-temp amplifier that picks up the signal and drives the >> guard, and it has noise of its own. In some cases that makes adding >> the guard a losing battle. > >10 k ohms has 40nV/rtHz at room Temp...IIRC. Closer to 13. >The opa134 has 8nV of voltage noise. 10K makes 8 nV at around 100 Kelvin. > >> >> The OPA134 may be a bit slow, especially driving Csg. > >8 MHz GBP. I only need it to work up to 1 MHz or so. Do you have a >better opamp in mind? How about a BF862 jfet? 0.8 nv/rthz and very low current noise. No opamp can do that. Cheap, too. > >> >> What sort of coax are you using? Can you reduce its capacitance? > >There is a cable which has ~12" of RG174 (? not sure that's the right >number, small coax) and in the probe there is ~8" of SS coax from >Lakeside cryo. I'm pretty much stuck with the cable I have. You can make low-C coax, like they use in scope probes, with a tiny wire inside a biggish shield, with some low density spacer like spiral monofilament or something. If you want to demonstrate Johnson noise vs temperature, it would be a lot easier to go hot! John
From: Bill Sloman on 6 May 2010 06:00 On May 5, 6:26 pm, George Herold <ggher...(a)gmail.com> wrote: > Im looking at the thermal noise from a resistor down the bottom of a > probe. Cable and probe are maybe 1/2 a meter long. I added a shield > line to try and reduce the capacitance between the active end of the > resistor and ground. (The other end of the resistor is tied to ground > at the bottom of the probe.) > > The driven shield seemed to work great at the higher impedance levels > 1Meg and 100k ohms. (Though a more careful examination showed there > were some issues.) When I tried 10k ohms there was some serious gain > peaking at the higher frequencies...above 100kHz. I mucked about a > bit and made sure this wasnt the common problem of an opamp driving a > capacitive load. > > Late yesterday it struck me that there is capacitive coupling from the > shield back to the input. I had been mistakenly thinking of the > shield as only a capacitance to ground. The capacitance of the inner > conductor to the ground (Cig) is 60pF, from the inner conductor to the > shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF. > > I was first using an opamp follower to drive the shield, but later > added a bit of gain...and then threw it away. > > +-----Cis--+ > | | > | |\ | > +-----+--+ \ | > | | >----+---+----+ > | +-- / OPA | | | > Rmeas. | |/ 134 | R1 Csg > Rmeas. | | R1 | > | +---------+ | | > | GND GND > GND > > R1 was 50 ohms (to get rid of Csg ringing) > And then this, > > +-----Cis--+ > | | > | |\ | > +-----+--+ \ | > | | >----+-R4R4--+----+ > | +-- / OPA | | | > Rmeas. | |/ 134 | R5 Csg > Rmeas. | | R5 | > | +--R3R3---+ | | > | | GND GND > GND R2 > R2 > | > GND > > Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms. > > This seems like it must be a known problem and I wondered if there are > any simple solutions. I thought that a bit of inductance (L = > Cis*Rmeas^2) in the right place might help, but I only managed to make > a nice oscillator. > > Thanks for any help or advice, I don't think that your circuit diagram shows all the necessary impedances where they ought to be. The OPA134 has finite output impedance, probably of the order of 100R, and your second circuit diagram should look more like +-----Cis----------------------+ | | | |\ | +----+-----+--+ \ | | | | >-----100R-+--R4R4--+----+ | | +-- / OPA | | | Cig Rmeas. | |/ 134 | R5 Csg | Rmeas. | | R5 | | | +--R3R3---------+ | | | | | GND GND GND GND R2 R2 | GND -- Bill Sloman, Nijmegen
From: George Herold on 6 May 2010 09:03
On May 5, 11:25 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 5 May 2010 19:57:02 -0700 (PDT), George Herold > > > > > > <ggher...(a)gmail.com> wrote: > >On May 5, 7:45 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 5 May 2010 09:26:32 -0700 (PDT), George Herold > > >> <ggher...(a)gmail.com> wrote: > >> >Im looking at the thermal noise from a resistor down the bottom of a > >> >probe. Cable and probe are maybe 1/2 a meter long. I added a shield > >> >line to try and reduce the capacitance between the active end of the > >> >resistor and ground. (The other end of the resistor is tied to ground > >> >at the bottom of the probe.) > > >> >The driven shield seemed to work great at the higher impedance levels > >> >1Meg and 100k ohms. (Though a more careful examination showed there > >> >were some issues.) When I tried 10k ohms there was some serious gain > >> >peaking at the higher frequencies...above 100kHz. I mucked about a > >> >bit and made sure this wasnt the common problem of an opamp driving a > >> >capacitive load. > > >> >Late yesterday it struck me that there is capacitive coupling from the > >> >shield back to the input. I had been mistakenly thinking of the > >> >shield as only a capacitance to ground. The capacitance of the inner > >> >conductor to the ground (Cig) is 60pF, from the inner conductor to the > >> >shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF. > > >> >I was first using an opamp follower to drive the shield, but later > >> >added a bit of gain...and then threw it away. > > >> > +-----Cis--+ > >> > | | > >> > | |\ | > >> >+-----+--+ \ | > >> >| | >----+---+----+ > >> >| +-- / OPA | | | > >> >Rmeas. | |/ 134 | R1 Csg > >> >Rmeas. | | R1 | > >> >| +---------+ | | > >> >| GND GND > >> >GND > > >> >R1 was 50 ohms (to get rid of Csg ringing) > >> >And then this, > > >> > +-----Cis--+ > >> > | | > >> > | |\ | > >> >+-----+--+ \ | > >> >| | >----+-R4R4--+----+ > >> >| +-- / OPA | | | > >> >Rmeas. | |/ 134 | R5 Csg > >> >Rmeas. | | R5 | > >> >| +--R3R3---+ | | > >> >| | GND GND > >> >GND R2 > >> > R2 > >> > | > >> > GND > > >> >Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms. > > >> >This seems like it must be a known problem and I wondered if there are > >> >any simple solutions. I thought that a bit of inductance (L = > >> >Cis*Rmeas^2) in the right place might help, but I only managed to make > >> >a nice oscillator. > > >> >Thanks for any help or advice, > >> >George H. > > >> You're trying to measure Johnson noise, right? > > >Yup, The gain peaking is a real pain. > > >> One problem with any such guarding/bootstrapping scheme is that there > >> is a room-temp amplifier that picks up the signal and drives the > >> guard, and it has noise of its own. In some cases that makes adding > >> the guard a losing battle. > > >10 k ohms has 40nV/rtHz at room Temp...IIRC. > > Closer to 13. Opps, my mistake! > > >The opa134 has 8nV of voltage noise. > > 10K makes 8 nV at around 100 Kelvin. > > > > >> The OPA134 may be a bit slow, especially driving Csg. > > >8 MHz GBP. I only need it to work up to 1 MHz or so. Do you have a > >better opamp in mind? > > How about a BF862 jfet? 0.8 nv/rthz and very low current noise. No > opamp can do that. Cheap, too. Hmmm, Roll my own? That's just going to be way too much time for very little 'gain'. I've basically got to put this to bed for the time being and get on with other things. > > > > >> What sort of coax are you using? Can you reduce its capacitance? > > >There is a cable which has ~12" of RG174 (? not sure that's the right > >number, small coax) and in the probe there is ~8" of SS coax from > >Lakeside cryo. I'm pretty much stuck with the cable I have. > > You can make low-C coax, like they use in scope probes, with a tiny > wire inside a biggish shield, with some low density spacer like spiral > monofilament or something. > > If you want to demonstrate Johnson noise vs temperature, it would be a > lot easier to go hot! Yup, the news letter doesn't show the data for Johnson noise , but we change the temperature from 77 K to 373K. > > John- Hide quoted text - > > - Show quoted text - If I get the gain peaking under control, then it will be fun to see how the driven shield changes as the Johnson noise decreases below the amp noise.... I wonder if I'll get what look likes a peak in the noise spectrum that is from the driven shield amplifier leaking it's noise in through the shield capacitance. Fortunately the whole driven shield idea is not a big part of measuring noise. George H. |