Name of screening technique
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From: Phil Hobbs on 16 Mar 2010 09:52 On 3/15/2010 8:24 PM, krw(a)att.bizzzzzzzzzzzz wrote: > On Mon, 15 Mar 2010 10:51:47 -0700 (PDT), MarkAren<markaren10(a)yahoo.com> > wrote: > >> On Mar 16, 1:42 am, "keith...(a)gmail.com"<keith...(a)gmail.com> wrote: >>> On Mar 15, 4:24 am, MarkAren<markare...(a)yahoo.com> wrote: >>> >>>> Hi All, >>> >>>> I have been scratching my head for a while and just can't remember the >>>> correct term... >>> >>>> What is the name of the technique where a very high sensitivity input >>>> is protected by a metal screen... >>> >>> Are you looking for "guarding"? I've generally use this term for >>> traces on boards used for this purpose. Leakage currents go to the >>> "guard" trace rather than the circuit. >>> >>>> The rub is that the screening can is actively driven with buffered >>>> ground reference >>> >>> "Active guarding". >>> >>> >>> >>>> I keep coming up with "force driven screening" or something similar >>> >>>> Thoughts please >>> >>>> Thanks, >>> >>>> Mark >> >> Thanks guys. >> >> "guarding", "active guarding" and "ring guarding" was what I was >> looking for. > > Yes, I've also seen "driven guard". Note that the guard usually isn't driven > at ground, but rather the common mode voltage. I think it was invented for early ASDIC (sonar) receivers--iirc it was called the 'ghost repeater'. It has most of the vices and virtues of a transimpedance amp--you can get a big bandwidth improvement, at the price of potentially severe noise degradation. Basically, you can't improve the SNR of the signal by jiggling the shield to make the frequency response flat--the resulting SNR is the same as if you used the same amplifier to buffer the output of the cable with the shield grounded. 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: krw on 16 Mar 2010 19:43
On Tue, 16 Mar 2010 09:52:31 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 3/15/2010 8:24 PM, krw(a)att.bizzzzzzzzzzzz wrote: >> On Mon, 15 Mar 2010 10:51:47 -0700 (PDT), MarkAren<markaren10(a)yahoo.com> >> wrote: >> >>> On Mar 16, 1:42 am, "keith...(a)gmail.com"<keith...(a)gmail.com> wrote: >>>> On Mar 15, 4:24 am, MarkAren<markare...(a)yahoo.com> wrote: >>>> >>>>> Hi All, >>>> >>>>> I have been scratching my head for a while and just can't remember the >>>>> correct term... >>>> >>>>> What is the name of the technique where a very high sensitivity input >>>>> is protected by a metal screen... >>>> >>>> Are you looking for "guarding"? I've generally use this term for >>>> traces on boards used for this purpose. Leakage currents go to the >>>> "guard" trace rather than the circuit. >>>> >>>>> The rub is that the screening can is actively driven with buffered >>>>> ground reference >>>> >>>> "Active guarding". >>>> >>>> >>>> >>>>> I keep coming up with "force driven screening" or something similar >>>> >>>>> Thoughts please >>>> >>>>> Thanks, >>>> >>>>> Mark >>> >>> Thanks guys. >>> >>> "guarding", "active guarding" and "ring guarding" was what I was >>> looking for. >> >> Yes, I've also seen "driven guard". Note that the guard usually isn't driven >> at ground, but rather the common mode voltage. > >I think it was invented for early ASDIC (sonar) receivers--iirc it was >called the 'ghost repeater'. It has most of the vices and virtues of a >transimpedance amp--you can get a big bandwidth improvement, at the >price of potentially severe noise degradation. > >Basically, you can't improve the SNR of the signal by jiggling the >shield to make the frequency response flat--the resulting SNR is the >same as if you used the same amplifier to buffer the output of the cable >with the shield grounded. When I used it the intention was to mitigate leakage and crosstalk in a switching network at the front end of an instrumentation amplifier. It worked well. |