Newer Model Instrumentation Amp
in [Design]
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From: krw on 20 Jun 2010 21:13 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'.
From: Robert Baer on 20 Jun 2010 22:56 Joerg wrote: > John Larkin wrote: >> On 20 Jun 2010 05:38:13 -0700, Winfield Hill >> <Winfield_member(a)newsguy.com> wrote: >> >>> Paul Henderson wrote... >>>> I have previously used AMP 01, INA114 and AD620 instrumentation amps >>>> for EEG apps. All maximum gain 10,000. >>>> >>>> Since these packages are all about ten years old now, I would like to >>>> update my design, if appropriate. >>>> >>>> Can someone advise regarding more recent and better spec'd IC's of >>>> this general type? >>> I recently finished updating our Selected Instrumentation Amps >>> table for H&H AoE III. Except for low-voltage-process designs, >>> I did not notice a lot of new activity in the last 15 years. >>> Most of the classic parts are doing fine, especially if they're >>> available in small SMT packages. >>> >>> Commenting on your choices, but for gains up to 1000 (most of >>> the data sheets don't address G=10k, that's a lot in one stage). >>> >>> The AMP01 comes in a large awkward package, but it seems to >>> do well at very high gains. However, it didn't make our list. >>> I wasn't able to find distributor inventory. Lack of interest? >>> >>> The INA114 is rather slow at high gains, the INA128 has similar >>> input characteristics, but is about 10x faster at high gains. >>> Both are fine at distributors, maybe the INA128 has the edge. >>> >>> The AD620 is a respectable part, popular, inexpensive. But >>> its 80dB CMRR at 10kHz pales compared to the INA128's 105dB. >>> The AD8221 is similar to the AD620, cheap, with 87dB CMRR. >>> The AD620 and AD8221 both let you directly bypass the input >>> transistors for RFI suppression. The INA128 may as well, but >>> TI hides the circuit details from the engineer. Too bad. >>> >>> You might want to consider some JFET parts. JFETs usually do >>> better than bipolar in the ignoring-RFI department. The AD8220 >>> is interesting, inexpensive, but for some reason distributors >>> don't have any stock. Sold out? It does well for bandwidth >>> at high gains, but not so well for CMRR at high frequency. >>> The venerable INA110, for example, beats the pant off of it. >>> >> 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! >> > > Maybe different in your case because you guys make low-volume specialty > equipment, and I have never designed an EEG. But I did design ECGs and > not in my wildest dreams would have considered instrumentation amps. > They are IMHO way overpriced, at $3-4 and up. I like to do that for less > than a buck :-) > > Sometimes it may pay off, while thinking about a change anyhow, to > ponder whether a transition to jelly-bean parts might make sense. > Over 40 years ago i designed and built an EEG-type module for brain wave monitoring and training (alpha wave if i remember correctly: OOoommm). Used a single uA709 with JFET input stage; radical compensation on the 709 and RC input lowpass filtering for net result of open-loop gain showing at the peak (pole and zero was widely separated for that). Used dimes for the pick-up pads, 2 9V batteries in case about size of cigarette package; worked like a champ.
From: John Devereux on 21 Jun 2010 04:01 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. 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. - Use of latching relays and their driver. Never used latching ones myself (or relays at all for that matter in low power circuits)! - There seem to be twice as many gain setting switches/resistors as you need. Is that for CMMR/layout symmetry reasons? -- John Devereux
From: Winfield Hill on 21 Jun 2010 07:27 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. -- Thanks, - Win
From: Winfield Hill on 21 Jun 2010 07:37
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? -- Thanks, - Win |