Newer Model Instrumentation Amp
in [Design]
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From: Joel Koltner on 21 Jun 2010 22:49 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. [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... ---Joel
From: Douglas Beeson on 22 Jun 2010 13:30 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 -- ______________ Douglas Beeson Java Software Design and Development Montreal, QC dbeeson99(a)videotron.ca - remove the nines
From: John Larkin on 22 Jun 2010 14:29 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
From: Tauno Voipio on 22 Jun 2010 14:47 John Larkin 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 > Please pay attention to get a very high input impedance, to avoid common-mode problems from varying electrode series impedances. If there is any difference in the electrode contact impedance, some of the common-mode signal will be converted to differential signal if it has an opportunity to create a voltage divisor with the input impedance. My experience with ECG and EEG front-ends is that the contact impedances behave wildly. The experience is from 30 to 40 years ago, but I suspect that the test subjects are still of similar construction and design. -- Tauno Voipio tauno voipio (at) iki fi
From: John Larkin on 22 Jun 2010 15:05
On Tue, 22 Jun 2010 21:47:15 +0300, Tauno Voipio <tauno.voipio(a)notused.fi.invalid> wrote: >John Larkin 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 >> > > >Please pay attention to get a very high input impedance, >to avoid common-mode problems from varying electrode >series impedances. If there is any difference in the >electrode contact impedance, some of the common-mode signal >will be converted to differential signal if it has an >opportunity to create a voltage divisor with the input >impedance. > >My experience with ECG and EEG front-ends is that the >contact impedances behave wildly. The experience is >from 30 to 40 years ago, but I suspect that the test >subjects are still of similar construction and design. This will be connected to fairly low-impedance things: power busses, current shunts, signal conditioners, transducers, maybe raw strain gauge bridges. The initial use is for aircraft structural testing. Its DC input impedance is 500K to ground on either side. This wouldn't be suited to EKG or electrochemical sorts of things. John |