Newer Model Instrumentation Amp
in [Design]
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From: Joerg on 21 Jun 2010 14:31 krw(a)att.bizzzzzzzzzzzz wrote: > On Sun, 20 Jun 2010 20:23:12 -0400, Spehro Pefhany > <speffSNIP(a)interlogDOTyou.knowwhat> wrote: > >> On Sun, 20 Jun 2010 17:07:12 -0700, the renowned Joerg >> <invalid(a)invalid.invalid> wrote: >> >>> Spehro Pefhany wrote: >>>> On Sun, 20 Jun 2010 15:28:53 -0700, the renowned John Larkin >>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>> On Sun, 20 Jun 2010 22:01:24 GMT, paulhendersen(a)qualcomm.com (Paul >>>>> Henderson) wrote: >>>>> >>>>>> On Sun, 20 Jun 2010 07:38:00 -0700, John Larkin >>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>> >>>>>>> On a current design, I had to make my own. I wanted lots of >>>>>>> overvoltage protection, logic-switchable gains from 0.05 to 256, high >>>>>>> precision, and at least +-12 volts of common-mode range, 120 dB CMRR >>>>>>> at high gain. I wound up with a classic 3-opamp diffamp, using an >>>>>>> LT1124 dual opamp, four Supertex depletion mode fets for protection, a >>>>>>> discrete string of thinfilm resistors, one DPDT gain switch relay, two >>>>>>> analog muxes, and an INA154 as the second stage. Two tiny trimpots >>>>>>> tweak cmrr. Times 16 on one board. I'd love to get all that in a SO-8! >>>>>>> >>>>>> If that's not a proprietary design John, any chance of posting a link >>>>>> to the schematic? >>>>>> >>>>>> Paul Hendersen >>>>> Yes, it is proprietary but, hell, I *am* the boss, so here it is: >>>>> >>>>> ftp://jjlarkin.lmi.net/22S490B_ch12.pdf >>>>> >>>>> in hopes that it will invoke an entertaining flurry of pecking and >>>>> clucking. >>>>> >>>>> I don't totally like the style of the schematic; I drew it on D-size >>>>> vellum "my way" and The Brat entered it into PADS. It would be too >>>>> much work to push 16 channels of stuff around at this point. >>>>> >>>>> John >>>>> >>>> That bipolar relay driver is a thing of beauty. >>>> >>> Sure is. >>> >>> But John calls them "K", as in kontactor or kool kampground :-)) >> K is the standard designator for relays. Don't know why. > > Because they kost more than an 'R'. And more than a kapacitor. I've seen K used more in power gear. In electronics it's often RL or REL. But I go with whatever the client prefers. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: langwadt on 21 Jun 2010 14:53 On 20 Jun., 19:14, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: snip > > 2. I rashly promised 8-pole digital filtering down to 1 Hz, Bessel and > Butterworth. That turns out to be non-trivial when you get to > 500,000:1 sample/cutoff ratio. Three architectures later, one > consultant hired/paid/dumped, great amounts of experiment and > simulation and debate, it seems to work. That adventure deserves a > thread of its own. One hint: the classic DSP butterfly filter explodes > at ratios like this. That is DSP 101 and kinda obvious when you think about it, for a filter to have any effect at 1Hz it needs to have "memory" it that range. with a 500kHz samplerate that either means a very high order or a very high precision standard approach would be decimate the signal to a more resonable sample rate -Lasse
From: John Larkin on 21 Jun 2010 15:58 On Mon, 21 Jun 2010 11:53:34 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >On 20 Jun., 19:14, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >snip >> >> 2. I rashly promised 8-pole digital filtering down to 1 Hz, Bessel and >> Butterworth. That turns out to be non-trivial when you get to >> 500,000:1 sample/cutoff ratio. Three architectures later, one >> consultant hired/paid/dumped, great amounts of experiment and >> simulation and debate, it seems to work. That adventure deserves a >> thread of its own. One hint: the classic DSP butterfly filter explodes >> at ratios like this. > >That is DSP 101 and kinda obvious when you think about it, for a >filter >to have any effect at 1Hz it needs to have "memory" it that range. >with a 500kHz samplerate that either means a very high order or a very >high >precision You'd think that 16 bit data and 1e6:1 sample/cutoff ratio could be accomplished with roughly 36 bit math. > >standard approach would be decimate the signal to a more resonable >sample rate That's one approach: build a chain of filters, clocked at declining rates, and use the early ones to take the stress off the later ones. Switch filters in or out as needed. We could do that, but it's harder to explain to the customers. It's easier to model the product as a lowpass filter followed by a 500 KHz ADC. The classic butterfly explodes violently as the clock:cutoff ratio goes up. Numbers have to subtract to make tiny residuals, and stage gains get absurd. Instead we used a double-integrator filter, essentially a digital simulation of an analog state-variable filter. That gets into trouble much more linearly than the butterfly, and each 2nd order section has a gain of exactly 1. It works using MAC blocks using 18x18 multiplies and a sort of coarse barrel shifter into a 48 bit accumulator. John
From: John Larkin on 21 Jun 2010 16:01 On Sun, 20 Jun 2010 17:07:12 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Spehro Pefhany wrote: >> On Sun, 20 Jun 2010 15:28:53 -0700, the renowned John Larkin >> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Sun, 20 Jun 2010 22:01:24 GMT, paulhendersen(a)qualcomm.com (Paul >>> Henderson) wrote: >>> >>>> On Sun, 20 Jun 2010 07:38:00 -0700, John Larkin >>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>> On a current design, I had to make my own. I wanted lots of >>>>> overvoltage protection, logic-switchable gains from 0.05 to 256, high >>>>> precision, and at least +-12 volts of common-mode range, 120 dB CMRR >>>>> at high gain. I wound up with a classic 3-opamp diffamp, using an >>>>> LT1124 dual opamp, four Supertex depletion mode fets for protection, a >>>>> discrete string of thinfilm resistors, one DPDT gain switch relay, two >>>>> analog muxes, and an INA154 as the second stage. Two tiny trimpots >>>>> tweak cmrr. Times 16 on one board. I'd love to get all that in a SO-8! >>>>> >>>> If that's not a proprietary design John, any chance of posting a link >>>> to the schematic? >>>> >>>> Paul Hendersen >>> >>> Yes, it is proprietary but, hell, I *am* the boss, so here it is: >>> >>> ftp://jjlarkin.lmi.net/22S490B_ch12.pdf >>> >>> in hopes that it will invoke an entertaining flurry of pecking and >>> clucking. >>> >>> I don't totally like the style of the schematic; I drew it on D-size >>> vellum "my way" and The Brat entered it into PADS. It would be too >>> much work to push 16 channels of stuff around at this point. >>> >>> John >>> >> >> That bipolar relay driver is a thing of beauty. >> > >Sure is. > >But John calls them "K", as in kontactor or kool kampground :-)) K is the ANSI/Mil designation for a relay. OK, I'm old fashioned and don't just make up stuff like LED2, TR5, RLY12, CON2, RV7, RN8, ZEN15, or other abominations. John
From: John Larkin on 21 Jun 2010 16:05
On 21 Jun 2010 11:28:54 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >dagmargoodboat(a)yahoo.com wrote... >> >> John Larkin wrote: >>> On Mon, 21 Jun 2010 09:01:55 +0100, John Devereux wrote: >> >>>> - I seem to recall you mentioning the use of the Supertex parts, not >>>> seen them used like this before. Would have guessed the fault current >>>> was too high, but in fact it looks like it is only a couple of mA. > > I've made a point about this here a few dozen times over the > years. "My favorite" LND150, I usually called it. > >>> These run pretty consistantly about 1.4 mA Idss. Small-signal Ron of >>> the pair is close to 2K, a bit higher than I'd like. 4K of Johnson >>> noise is about 8 nv/rthz, OK but not great. > > Right, 5.6nV * sqrt 2 = 8nV for two 2k parts. BTW, I like to > add a 1k resistor to control the current, in a scheme that only > needs a single resistor, hence less noise impact. Others have > also thought of this; I grabbed the drawing below from a 2002 > post by Adam Seychell. Only I use 1k, 200 ohms won't do much. > >. ,-------------, >. | | >. ------- | >. connector | | | S D >. from o---+ +-+-- 200R--+-+ +-----> to non-inv >. outside D S | | | >. world | ------- >. | | >. '-------------' How does that help? Idss is only a few mA max, and the resistor just adds to the Johnson noise. And takes up space. That does make sense if you use the bigger parts and can't stand full Idss. John |