Newer Model Instrumentation Amp
in [Design]
|
From: Jan Panteltje on 21 Jun 2010 12:31 On a sunny day (Mon, 21 Jun 2010 09:13:14 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <b93v16pl5ur8fdmq57mc7fenau9g25p0ec(a)4ax.com>: >On Mon, 21 Jun 2010 11:58:27 -0400, Spehro Pefhany ><speffSNIP(a)interlogDOTyou.knowwhat> wrote: > >>On Mon, 21 Jun 2010 08:10:45 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >> >>>On Sun, 20 Jun 2010 15:28:53 -0700, 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 >>>> >>> >>>Young Buck Alarm: Driving Inductive loads directly from CMOS logic is >>>NOT recommended without some kind of protective elements. Allowing >>>CMOS body diodes to be exposed to flyback currents can be hazardous, >>>possibly fatal to the chip if latch-up occurs. Observe data sheet >>>current limits carefully. >>> >>> ...Jim Thompson >> >>Good advice in general. I've used emitter followers from CMOS for the >>unipolar latching type, which don't require diodes. >> >>Not sure which ones JL used, but some of those relays only draw 8mA or >>so at 4.5V, and the diodes are shunted by MOSFETs under normal >>conditions so it ought to work okay. In pathological conditions (say a >>sudden drop of Vdd to 0 during the brief pulse when the relay is >>energized) the diodes would conduct, but something like 8mA maximum is >>pretty conservative for that particular part- data sheet says 100mA or >>50mA. >> >>A 20-cent H-bridge... > >The relays are Fujitsu FTR-B3GB4.5Z-B10, 4.5 volts at 25 mA. Wonderful >little parts, second-sourced by Omron. They will turn on/off in under >a millisecond if you drive them right. > >Yes, this driver is prefectly safe from latchup, for the two reasons >you mention. We've shipped about 8000 so far, no problems. > >This is the second time in recent days that this old hen has >criticized my design suggestions without bothering to understand them >first. Or bothering to be constructive. > >John That is not fair, he just gave good advice, to check the currents. He could not know the currents, and to be fair it may work, just as did that blind shear in the blowout preventer for BP. Question is: Will it always work? Some chips can go into latchup if you spike them on pin. I am not saying this is the case in your design, but Jim's advice is sound.
From: Jan Panteltje on 21 Jun 2010 12:33 On a sunny day (Mon, 21 Jun 2010 09:10:42 -0700) it happened Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote in <bd3v1690uj0kmmevmv8jqu54fkql18k49t(a)4ax.com>: > Where is Joe McCarthy when you need him ?? You are evading the subject: Where is Osama?
From: John Larkin on 21 Jun 2010 12:46 On Mon, 21 Jun 2010 16:31:09 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Mon, 21 Jun 2010 09:13:14 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><b93v16pl5ur8fdmq57mc7fenau9g25p0ec(a)4ax.com>: > >>On Mon, 21 Jun 2010 11:58:27 -0400, Spehro Pefhany >><speffSNIP(a)interlogDOTyou.knowwhat> wrote: >> >>>On Mon, 21 Jun 2010 08:10:45 -0700, Jim Thompson >>><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >>> >>>>On Sun, 20 Jun 2010 15:28:53 -0700, 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 >>>>> >>>> >>>>Young Buck Alarm: Driving Inductive loads directly from CMOS logic is >>>>NOT recommended without some kind of protective elements. Allowing >>>>CMOS body diodes to be exposed to flyback currents can be hazardous, >>>>possibly fatal to the chip if latch-up occurs. Observe data sheet >>>>current limits carefully. >>>> >>>> ...Jim Thompson >>> >>>Good advice in general. I've used emitter followers from CMOS for the >>>unipolar latching type, which don't require diodes. >>> >>>Not sure which ones JL used, but some of those relays only draw 8mA or >>>so at 4.5V, and the diodes are shunted by MOSFETs under normal >>>conditions so it ought to work okay. In pathological conditions (say a >>>sudden drop of Vdd to 0 during the brief pulse when the relay is >>>energized) the diodes would conduct, but something like 8mA maximum is >>>pretty conservative for that particular part- data sheet says 100mA or >>>50mA. >>> >>>A 20-cent H-bridge... >> >>The relays are Fujitsu FTR-B3GB4.5Z-B10, 4.5 volts at 25 mA. Wonderful >>little parts, second-sourced by Omron. They will turn on/off in under >>a millisecond if you drive them right. >> >>Yes, this driver is prefectly safe from latchup, for the two reasons >>you mention. We've shipped about 8000 so far, no problems. >> >>This is the second time in recent days that this old hen has >>criticized my design suggestions without bothering to understand them >>first. Or bothering to be constructive. >> >>John > >That is not fair, he just gave good advice, to check the currents. >He could not know the currents, and to be fair it may work, just as >did that blind shear in the blowout preventer for BP. >Question is: Will it always work? If you look at the datasheet for these nor gates, it says in the headline text, huge font, no latchup at 100 mA. A tiny relay like this is not going to use 100 mA of coil current. And, as Spehro points out, the cmos fets are always on. >Some chips can go into latchup if you spike them on pin. This one is OK. We checked. John
From: dagmargoodboat on 21 Jun 2010 13:39 On Jun 21, 9:57 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> 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. > > 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. FWIW, Supertex's datasheet says "not recommended for new design." They suggest LND150, which looks identical. 500Vgs ... cool. -- Cheers, James Arthur
From: Winfield Hill on 21 Jun 2010 14:28
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 | ------- .. | | .. '-------------' > FWIW, Supertex's datasheet says "not recommended for new design." > They suggest LND150, which looks identical. They simply renamed the LND250, which was the elegant SOT-23 version of the TO-92 LND150, to the LND150K1, and then the old TO-92 part becomes the LND150N3, and they also added a TO-243 part, LND150N8, which can handle more power. Nice. This is all in keeping with Supertex' longstanding numbering scheme, and it makes my remark easier, because now I don't have to say, my favorite LND150 and LND250. With all the choices, now the LND150 really is my favorite! -- Thanks, - Win |