Newer Model Instrumentation Amp
in [Design]
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From: Winfield Hill on 21 Jun 2010 16:46 John Larkin wrote... > > How does that help? Idss is only a few mA max, and the > resistor just adds to the Johnson noise. It helps if you want to protect against someone connecting to say 230Vac, and leaving the connection for a while before discovering their error. We need to protect the sot-23s. -- Thanks, - Win
From: John Larkin on 21 Jun 2010 17:02 On 21 Jun 2010 13:46:18 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >John Larkin wrote... >> >> How does that help? Idss is only a few mA max, and the >> resistor just adds to the Johnson noise. > > It helps if you want to protect against someone connecting > to say 230Vac, and leaving the connection for a while before > discovering their error. We need to protect the sot-23s. Ok. 230 AC peaks at 325. Half cycle, the average voltage is 104 across each fet. Assuming 2 mA, that's 208 mW. A SOT-23 should be OK with that, but one more resistor could pull that down some. In my case, dissipation is a bit lower since the other sides of the fets are whacking the ESD diodes in the opamps, roughly +-18 volts. John
From: krw on 21 Jun 2010 19:54 On Mon, 21 Jun 2010 11:31:01 -0700, Joerg <invalid(a)invalid.invalid> wrote: >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. Dunno about that! I was just combing though some BOMs looking for a couple nickels to squeeze and just happened to notice that we're using a $7 cap (Case-D 220uF 20% 16V) on one product. I have no clue why we're using that cap because we have a similar cap (Case-D 220uF *10%* 16V) that we pay $.70 for. The relays on the $7 cap board are only a buck or two (I'm not looking at that board, so don't remember exactly). >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. I've never seen anything other than 'K' used. I'm with John, RL, REL, and all other bastardizations are just that.
From: Joel Koltner on 21 Jun 2010 20:17 <krw(a)att.bizzzzzzzzzzzz> wrote in message news:jiuv16t9njsmb99maq0gi4ittc09p945th(a)4ax.com... > Dunno about that! I was just combing though some BOMs looking for a couple > nickels to squeeze and just happened to notice that we're using a $7 cap > (Case-D 220uF 20% 16V) on one product. I have no clue why we're using that > cap because we have a similar cap (Case-D 220uF *10%* 16V) that we pay $.70 > for. In my ideal world this is the sort of thing the purchasing guys would notice. "Hey Mr. Engineer -- is there something special about this cap that makes it worth 10x this seemingly superior one over here?" Although in my ideal world the engineers at some point went over the BOM to do a little sanity checking on prices as well, and would also notice it. Before hiring you, I mean, since I imagine it's a product that existed before you were hired. :-) > I've never seen anything other than 'K' used. I'm with John, RL, REL, and > all > other bastardizations are just that. If one's not going to use 'K', 'RLY' is a lot better than RL or REL. Abbreviations are kinda strange, though. We occasionally refer to spectrum analyzers as "SpecAns" for short, but I have a radio that refers to them as "SpeAnas." Weird... ---Joel
From: Joel Koltner on 21 Jun 2010 21:15
<krw(a)att.bizzzzzzzzzzzz> wrote in message news:p120265i3rpmi20804ln9f4vbt1255hrhb(a)4ax.com... > With lead times going out into forever, we're just lucky they've not had any > purchasing disasters this year. OTOH, we have inventory of some components > that go out three years, or more. That sounds like a pretty good strategy. > Actually it did. ;-) It's used in the football version of the unit you > played with and came out a year before. The good news is that there is only > one per board, the build numbers are in the low tens per year, and the > product > has *reallY* fat margins (5-10x that of what you saw). I didn't realize there were many markets that supported fatter margains than the pro audio guys. Wow! Well, maybe jewelry or sporting goods... > The real problem is that engineering never saw prices after the original > look-see in DigiKey during design. I asked our admin to dump the purchasing > database by part number and I'm going through the BOM manually (actually, I > placing the cost info into the schematic properties pages). I'm trying to > get > the IT folks to feed back the purchasing database into our engineering > database at least monthly, but so far no one is very interested. That's a useful idea, and if it's automated you could easily run it, e.g., every night or so. We're still just working on getting ORCAD CIS tied into the Big Expensive MRP program; purportedly all the hooks and configuration bits already exist, but someone needs to sit down and define the actual fields we want to use, (more importantly) standardized methods of filling in those fields, etc. -- and then actual start filling in all that data for the parts that already exist in it. > I suspect the kids have never used them so make things up as they go. Yep, I think you're right. > As far as the "SpecAns" goes, how about "FreqDomScope", or better, "that > gizmo > over there"? We'll occasionally refer to, e.g., "the 8594" or "the 9020a," but if someone isn't remembering the particular model number, it turns into "the really expensive specan" or "the el-cheapo specan." :-) ---Joel |