Newer Model Instrumentation Amp
in [Design]
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From: krw on 20 Jun 2010 19:18 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. Clucking_mode=on. Doesn't your common mode rejection depend on the tracking of the two DACs/resistor strings? I've done something similar, but with R407, if I understand the design correctly, being controlled (ADI 1024-tap digi-pot, in my case), rather than the two feedback resistors. In my case, the common mode rejection depends on the two fixed feedback resistors, which is simpler. Of course the gain is backwards, but that's the DSPs job. ;-) >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. Agreed. ;-)
From: John Larkin on 20 Jun 2010 19:33 On Sun, 20 Jun 2010 18:18:20 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> 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. > >Clucking_mode=on. > >Doesn't your common mode rejection depend on the tracking of the two >DACs/resistor strings? DACs? > >I've done something similar, but with R407, if I understand the design >correctly, being controlled (ADI 1024-tap digi-pot, in my case), rather than >the two feedback resistors. In my case, the common mode rejection depends on >the two fixed feedback resistors, which is simpler. Of course the gain is >backwards, but that's the DSPs job. ;-) I looked into using digi-pots as the CMRR trims, but they had poor resolution, poor TCs, and/or tons of capacitance. Plus, we were almost out of digital interface resources. The trimpots we're using are about the size of a 1206 resistor. John
From: Spehro Pefhany on 20 Jun 2010 19:39 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. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: krw on 20 Jun 2010 19:42 On Sun, 20 Jun 2010 16:33:04 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 20 Jun 2010 18:18:20 -0500, "krw(a)att.bizzzzzzzzzzzz" ><krw(a)att.bizzzzzzzzzzzz> 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. >> >>Clucking_mode=on. >> >>Doesn't your common mode rejection depend on the tracking of the two >>DACs/resistor strings? > >DACs? Resistors + switches = multiplying DAC >> >>I've done something similar, but with R407, if I understand the design >>correctly, being controlled (ADI 1024-tap digi-pot, in my case), rather than >>the two feedback resistors. In my case, the common mode rejection depends on >>the two fixed feedback resistors, which is simpler. Of course the gain is >>backwards, but that's the DSPs job. ;-) > >I looked into using digi-pots as the CMRR trims, but they had poor >resolution, poor TCs, and/or tons of capacitance. Plus, we were almost >out of digital interface resources. The trimpots we're using are about >the size of a 1206 resistor. The point is to not need trimming, particularly gain sensitive trim.
From: Jim Thompson on 20 Jun 2010 20:00
On Sun, 20 Jun 2010 19:39:31 -0400, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> 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. > > > >Best regards, >Spehro Pefhany ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Where is Joe McCarthy when you need him ?? |