Gain peaking in driven shield
in [Design]
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From: George Herold on 6 May 2010 09:11 On May 6, 6:00 am, Bill Sloman <bill.slo...(a)ieee.org> wrote: > 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- Hide quoted text - > > - Show quoted text - Hi Bill, That is about right. I was playing around with LTspice last night and was able to get what looked like nice flat 'gain' out to near 1MHz or so, with the simpler voltage follower circuit. I tried the above circuit so I could try rolling off the gain by putting some C in parallel with R3... But this just seems to shift the gain peak to lower frequencies. George H.
From: MooseFET on 6 May 2010 09:54 On May 5, 9:26 am, George Herold <ggher...(a)gmail.com> wrote: [....] > > +-----Cis--+ > | | > | |\ | > +-----+--+ \ | > | | >----+---+----+ > | +-- / OPA | | | > Rmeas. | |/ 134 | R1 Csg > Rmeas. | | R1 | > | +---------+ | | > | GND GND > GND If the cable is very long, the simple capacitance model of the cable fails. You need to start dealing with the inductance too. I have used the LSK170 to make a very low noise non-inverting gain to make this work a little better. The noise put onto the cable by your shield drive starts to win at some high frequency. --------------- ! ! [R] U1 [V] ! !\ ! !---+--------! >----+--+---- in ---->! !/ ! !---+--[R]------------- ! R2 [R] ! R1 IIRC R1=300, R2=30 U1=gain of 100 It really matters that the gain be 1.00 and the phase be 0.00 at the frequency you need to measure. In my case, I have the option of changing the circuit between measurement bands. The circuit had a roll off at about F*100 and F/100 > > 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.
From: langwadt on 6 May 2010 12:48 On 6 Maj, 05:25, 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. > > >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. > how low, doesn't something like ADA4898 get close? -Lasse
From: Joerg on 6 May 2010 13:56 John Larkin wrote: > 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: [...] >>> 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. > Can't see George's posts because he uses gmail but this may help: Older passive car antennas (sans integrated preamp) had very low capacitance coax in order not to detune the first AM filter and not to snuff out too much signal. But I am not sure where that can be bought without the antenna. Autoparts stores used to have it but that was >20 years ago. [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Michael A. Terrell on 6 May 2010 16:17
Joerg wrote: > > John Larkin wrote: > > 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: > > [...] > > >>> 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. > > > > Can't see George's posts because he uses gmail but this may help: Older > passive car antennas (sans integrated preamp) had very low capacitance > coax in order not to detune the first AM filter and not to snuff out too > much signal. But I am not sure where that can be bought without the > antenna. Autoparts stores used to have it but that was >20 years ago. It is nothing more than RG/62, 93 ohm coax. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |