Newer Model Instrumentation Amp
in [Design]
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From: Douglas Beeson on 22 Jun 2010 17:06 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > On Tue, 22 Jun 2010 12:30:19 -0500, Douglas Beeson > <dbeeson(a)videotron.ca> wrote: > >>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 >>> >>> >>> >> >>Thank you, John, for posting that real-world schematic. Newbie question: >>what do R417, R120 and R123 do for the circuit? Calibration of inputs to >>INA154? >> >>Doug Beeson > > Hi, Doug, > > Those parts do two things: > > 1. R417+R413+C113 make the first pole of a 5-pole lowpass filter. The > other 4 poles and the ADC are on the next sheet of the schematic. I > used the free TI FilterPro software to design a 5-pole Sallen-Key > lowpass filter and then ripped out the first section and made C113 do > the equivalent rolloff. > > 2. R417 and R120 and R123 (and their symmetric mates) allow fine trim > of the common-mode rejection ratio (CMRR) of U118. It's spec'd at > something like 80 dB, and we tweak it up to about 120. And hope that > will last. > > Turns out that the CMRR trim is mathematically interesting/annoying. > The amount of trim is very nonlinear on the value of R119/R120. I hate > circuits like this, trimmed dividers, bacause they are often messy to > design with available parts. > > Gain and offset trims are done digitally, in an FPGA, based on cal > tables stored in an eeprom. > > Note that the ADC, AD7699, measures voltage relative to a 2.048 volt > pseudo-differential input pin. So the reference pin of U118 is set to > VCM, namely that same 2.048 volt rail, to shift the ground-referenced > signal up. There's also clamping downstream, so the possible +-16 volt > swing of U118 doesn't blow anything up. > > None of this is remarkable. It's just connecting up boxes into > conventional circuits... carefully. A lot of engineering is like that, > declaring performance specs and then implementing it carefully. Once > in a while you get to design something really new and clever. > > My first summer electronics job was working for Ed Beeson in microwave > spectroscopy, in New Orleans. I guess there are a lot of Beesons > around. > > John > > I think there are a couple thousand Beeson families around (but only two in Canada that show up in the phonebook, anyway). We pretty much all descend from the Edward Beeson who emigrated to Pennsylvania in the late 1600s. I understand the lowpass filter section now. But don't R120 and R123, in parallel with R417, alter the filter characteristics when you trim R123? Or have you built enough attenuation overhead into the other sections that it doesn't really matter?
From: John Larkin on 22 Jun 2010 18:53 On Tue, 22 Jun 2010 16:06:47 -0500, Douglas Beeson <unkown(a)xnntp> wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Tue, 22 Jun 2010 12:30:19 -0500, Douglas Beeson >> <dbeeson(a)videotron.ca> wrote: >> >>>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 >>>> >>>> >>>> >>> >>>Thank you, John, for posting that real-world schematic. Newbie question: >>>what do R417, R120 and R123 do for the circuit? Calibration of inputs to >>>INA154? >>> >>>Doug Beeson >> >> Hi, Doug, >> >> Those parts do two things: >> >> 1. R417+R413+C113 make the first pole of a 5-pole lowpass filter. The >> other 4 poles and the ADC are on the next sheet of the schematic. I >> used the free TI FilterPro software to design a 5-pole Sallen-Key >> lowpass filter and then ripped out the first section and made C113 do >> the equivalent rolloff. >> >> 2. R417 and R120 and R123 (and their symmetric mates) allow fine trim >> of the common-mode rejection ratio (CMRR) of U118. It's spec'd at >> something like 80 dB, and we tweak it up to about 120. And hope that >> will last. >> >> Turns out that the CMRR trim is mathematically interesting/annoying. >> The amount of trim is very nonlinear on the value of R119/R120. I hate >> circuits like this, trimmed dividers, bacause they are often messy to >> design with available parts. >> >> Gain and offset trims are done digitally, in an FPGA, based on cal >> tables stored in an eeprom. >> >> Note that the ADC, AD7699, measures voltage relative to a 2.048 volt >> pseudo-differential input pin. So the reference pin of U118 is set to >> VCM, namely that same 2.048 volt rail, to shift the ground-referenced >> signal up. There's also clamping downstream, so the possible +-16 volt >> swing of U118 doesn't blow anything up. >> >> None of this is remarkable. It's just connecting up boxes into >> conventional circuits... carefully. A lot of engineering is like that, >> declaring performance specs and then implementing it carefully. Once >> in a while you get to design something really new and clever. >> >> My first summer electronics job was working for Ed Beeson in microwave >> spectroscopy, in New Orleans. I guess there are a lot of Beesons >> around. >> >> John >> >> >I think there are a couple thousand Beeson families around (but only two in >Canada that show up in the phonebook, anyway). We pretty much all descend from >the Edward Beeson who emigrated to Pennsylvania in the late 1600s. > >I understand the lowpass filter section now. But don't R120 and R123, in >parallel with R417, alter the filter characteristics when you trim R123? Or have >you built enough attenuation overhead into the other sections that it doesn't >really matter? > > > > The frequency rolloff of C113 takes into account the Thevenin impedances resulting from all the stuff that's parelleled; the 25Ks in the INA matter more than the 412K resistors. It's a 5% cap, so there's no need to get too precise here. Turning the pot has a tiny, probably unmeasurable, effect on the frequency response. John
From: krw on 22 Jun 2010 20:58 On Mon, 21 Jun 2010 19:49:20 -0700, "Joel Koltner" <zapwireDASHgroups(a)yahoo.com> wrote: >Hi Keith, > ><krw(a)att.bizzzzzzzzzzzz> wrote in message >news:7q6026hol4ihba9pm1ih626hethm7onp0b(a)4ax.com... >> Multiply an 87000 seat stadium by a $50 ticket, with free labor and come >> back >> to me. ;-) > >Hmm, good point. > >I've always figured that guys manning the hot dog stands are making pretty >much minimum wage too. Yep, that $50 doesn't include $5 hot dogs or $7 beer, either. ;-) >[Spectrum analyzers] > >> We only have one, and it's in the service department. Well, we do have FFTs >> on the scopes, witch is usually good enough for what I need. > >We have some big boxes that are effectively just fancy RF switch matrices with >filters and amplification included and we're always sniffing around them, it >seems, with spectrum analyzers trying to improve isolation, make sure things >aren't compressing, measuring intermods, etc. For the wireless stuff we >design, as long as I've been here it's always been a "roll your own" approach >using mixers, amplifiers, filters, switches, etc., and often the scopes remain >powered down for days at a time in deference to the spectrum analyzers -- we >haven't had a project yet where we felt the wireless radio modem "modules" >were the best option, even though they certainly have many compelling >features. > >Unfortunately, it's becoming harder to find general-purpose radio parts like >this -- particularly if you're after low-power operation. If you look at, >e.g., I/Q modulators, most of them are around a watt! -- Fine for >basestations, not so great for anything strapped to your hip. The iPhones and >Razrs and whatnot of the world are driving this sort of thing to a >system-of-chip approach... We don't do our own RF, so our needs in that area are much smaller. There was a proposal to do a hip-strapped base station but RF exposure was an issue. IIRC, we're just under the limit.
From: Joel Koltner on 22 Jun 2010 21:04 <krw(a)att.bizzzzzzzzzzzz> wrote in message news:anm226pri2m2q059qh2s41trpijs0iigjd(a)4ax.com... > How? Parts are ordered by part number, not by reference designator. I > doubt > our purchasing people know what an LED looks like. When I've placed orders with, e.g., Newark in the past, over the phone, I'd rattle off a list of part numbers, and then they'd read back part numbers and descriptions. So if your BOM just says, "D3, Avago AV672" and the order-taker reads back, "Avago AV672, green LED," it might raise an eyebrow. I'm confident out purchasing people know what LEDs look like. :-) These days most parts purchasing is done over the Internet, but a few things still end up being ordered by phone. ---Joel
From: krw on 22 Jun 2010 21:16
On Tue, 22 Jun 2010 18:04:33 -0700, "Joel Koltner" <zapwireDASHgroups(a)yahoo.com> wrote: ><krw(a)att.bizzzzzzzzzzzz> wrote in message >news:anm226pri2m2q059qh2s41trpijs0iigjd(a)4ax.com... >> How? Parts are ordered by part number, not by reference designator. I >> doubt >> our purchasing people know what an LED looks like. > >When I've placed orders with, e.g., Newark in the past, over the phone, I'd >rattle off a list of part numbers, and then they'd read back part numbers and >descriptions. So if your BOM just says, "D3, Avago AV672" and the order-taker >reads back, "Avago AV672, green LED," it might raise an eyebrow. The inventory control system will have all of the ordering information. Parts aren't ordered from the schematic or even the BOM. The BOM (and production forecast) just feeds the inventory control database as a gazouta. >I'm confident out purchasing people know what LEDs look like. :-) You're kind. ;-) >These days most parts purchasing is done over the Internet, but a few things >still end up being ordered by phone. But not from a schematic. The reference designator is simply a key to the various databases. |