Newer Model Instrumentation Amp
in [Design]
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From: Spehro Pefhany on 21 Jun 2010 08:02 On 21 Jun 2010 04:37:07 -0700, the renowned Winfield Hill <Winfield_member(a)newsguy.com> wrote: >Winfield Hill wrote... >> >>John Larkin wrote... >>> >>> 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. >> >> Thanks. >> >> Cluck. Peck. It doesn't have much in the way of >> RFI suppression on the input. Cluck. Cluck. > > Oh, sorry, I missed L25 and L26. What are they? Look like 0603 ferrite beads 1K @ 100MHz, so it should roll off at maybe 10MHz with the 47pF caps. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: Jan Panteltje on 21 Jun 2010 08:36 On a sunny day (Sun, 20 Jun 2010 15:38:40 -0700) it happened Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote in <uv5t161jsutpam9ncpa88pvmi4obf0rde0(a)4ax.com>: tp://jjlarkin.lmi.net/22S490B_ch12.pdf >> >>in hopes that it will invoke an entertaining flurry of pecking and >>clucking. >> >[snip] > >Well? Peck! Peck! Cluck! Cluck! If there's anything creative >there I don't see it. But it does keep the young bucks uneducated... >and that's the important part for my personal economy ;-) But you design chips no? So could you design a chip that does all that, maybe Larkin would buy 10?
From: John Larkin on 21 Jun 2010 09:50 On Mon, 21 Jun 2010 12:36:18 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Sun, 20 Jun 2010 15:38:40 -0700) it happened Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote in ><uv5t161jsutpam9ncpa88pvmi4obf0rde0(a)4ax.com>: >tp://jjlarkin.lmi.net/22S490B_ch12.pdf >>> >>>in hopes that it will invoke an entertaining flurry of pecking and >>>clucking. >>> >>[snip] >> >>Well? Peck! Peck! Cluck! Cluck! If there's anything creative >>there I don't see it. But it does keep the young bucks uneducated... >>and that's the important part for my personal economy ;-) > >But you design chips no? So could you design a chip that does all that, >maybe Larkin would buy 10? > This would be difficult to do as an IC. Especially if it had to meet my input range (10 mV to 40 volts), overload (250 volts), drift, and common-mode requirements. I even had to trim the CMRR of the INA154, which is already a laser-trimmed thinfilm thing. Note the +-16 (actually 17) supplies. But if someone did try it, they may as well include the next page: clamp, 4-pole lowpass filter, 16-bit ADC with reference. John
From: John Larkin on 21 Jun 2010 10:57 On Mon, 21 Jun 2010 09:01:55 +0100, John Devereux <john(a)devereux.me.uk> wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> writes: > >> 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. > >Thanks for posting a real-world design, it's nice to see that here. It's interesting how few designs actually get posted to s.e.d. > >Items I found "Interesting": > >- 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. 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. > >- Use of latching relays and their driver. Never used latching ones > myself (or relays at all for that matter in low power circuits)! These are the cute little surface-mount Fujitsu telecom relays. Since the coils are mostly off, there's no heat source to generate thermal EMFs in the signal path. They are a nuisance to drive, both hardware and software. I guess I could have used non-latching relays, with the activated state kicking in the HV attenuator, so they'd be off on the millivolt ranges. One ADC LSB is about 80 uV. > >- There seem to be twice as many gain setting switches/resistors as you > need. Is that for CMMR/layout symmetry reasons? It improves the AC CMRR and distortion compared to something lopsided. Layout-wise, I'd prefer fewer parts. John
From: John Larkin on 21 Jun 2010 11:05
On Mon, 21 Jun 2010 08:02:40 -0400, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On 21 Jun 2010 04:37:07 -0700, the renowned Winfield Hill ><Winfield_member(a)newsguy.com> wrote: > >>Winfield Hill wrote... >>> >>>John Larkin wrote... >>>> >>>> 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. >>> >>> Thanks. >>> >>> Cluck. Peck. It doesn't have much in the way of >>> RFI suppression on the input. Cluck. Cluck. >> >> Oh, sorry, I missed L25 and L26. What are they? > >Look like 0603 ferrite beads 1K @ 100MHz, so it should roll off at >maybe 10MHz with the 47pF caps. > > Right. That should be pretty good at zapping the sort of RF that messes up opamp front-ends. Usually just the bead helps a lot. That's all pretty much guesswork anyhow. Too much C can mess up higher frequency CMRR, so, who knows? 1/3 of the customers insist in grounding shields at the transducer, another third insist on grounding shields at the VME end, and the rest get it right. John |