On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote:> On 9/17/18 3:31 PM, John Larkin wrote: > > On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp > > <nikolaibeev@gmail.com> wrote: > > > >> > >>> Choppers are getting way better lately, but too many are low voltage > >>> things, we need a high voltage follower. We have bootstrapped the > >>> power supplies of low voltage amps to do this, but that is sort of a > >>> nightmare. Or really a nightmare. > >>> > >>> > >> > >> Indeed, such bootstraps are tough. > >> > >> I've been drooling over the datasheet of OPAx189: > >> > >> * CMRR: 168 dB (!) > >> * Open-loop gain: 170 dB (!!) > >> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz > >> * drift: 5 nV/deg C typ. > >> * huge input Z > >> * Vsupply up to 36V > >> > >> Has anyone used these yet? > >> > >> Cheers, > >> Nikolai > > > > That's a great amp, but its input common-mode range, near V+, is even > > worse than the ADA4522. All the super chopamps seem to have that > > problem. This one doesn't have input back-to-back diodes, so it won't > > have my hangup problem. > > > > DC psrr is great, but AC psrr is terrible. That could be a real-life > > noise limit. > > > > One big problem we had with bootstrapping a low-voltage chopamp was > > instability caused by psrr. > > > > > Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied > by the gain of the stage, which can be a bit of a rude shock. > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > http://hobbs-eo.comWhat's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling?
ADA4522 opamp hangup
Started by ●September 12, 2018
Reply by ●September 17, 20182018-09-17
Reply by ●September 17, 20182018-09-17
On 9/17/18 5:30 PM, Castorp wrote:> On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote: >> On 9/17/18 3:31 PM, John Larkin wrote: >>> On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp >>> <nikolaibeev@gmail.com> wrote: >>> >>>> >>>>> Choppers are getting way better lately, but too many are low voltage >>>>> things, we need a high voltage follower. We have bootstrapped the >>>>> power supplies of low voltage amps to do this, but that is sort of a >>>>> nightmare. Or really a nightmare. >>>>> >>>>> >>>> >>>> Indeed, such bootstraps are tough. >>>> >>>> I've been drooling over the datasheet of OPAx189: >>>> >>>> * CMRR: 168 dB (!) >>>> * Open-loop gain: 170 dB (!!) >>>> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz >>>> * drift: 5 nV/deg C typ. >>>> * huge input Z >>>> * Vsupply up to 36V >>>> >>>> Has anyone used these yet? >>>> >>>> Cheers, >>>> Nikolai >>> >>> That's a great amp, but its input common-mode range, near V+, is even >>> worse than the ADA4522. All the super chopamps seem to have that >>> problem. This one doesn't have input back-to-back diodes, so it won't >>> have my hangup problem. >>> >>> DC psrr is great, but AC psrr is terrible. That could be a real-life >>> noise limit. >>> >>> One big problem we had with bootstrapping a low-voltage chopamp was >>> instability caused by psrr. >>> >>> >> Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied >> by the gain of the stage, which can be a bit of a rude shock. >> >> Cheers >> >> Phil Hobbs >> >> -- >> Dr Philip C D Hobbs >> Principal Consultant >> ElectroOptical Innovations LLC / Hobbs ElectroOptics >> Optics, Electro-optics, Photonics, Analog Electronics >> Briarcliff Manor NY 10510 >> >> http://electrooptical.net >> http://hobbs-eo.com > > What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling? >Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V rails using a couple of BJTs and a zener to keep the amp from seeing more than 5V. In that topology the output feeds back to the supply rails, so there's the opportunity for feedback oscillation via the AC PS and CM sensitivity. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 17, 20182018-09-17
On Monday, September 17, 2018 at 5:41:23 PM UTC-4, Phil Hobbs wrote:> On 9/17/18 5:30 PM, Castorp wrote: > > On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote: > >> On 9/17/18 3:31 PM, John Larkin wrote: > >>> On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp > >>> <nikolaibeev@gmail.com> wrote: > >>> > >>>> > >>>>> Choppers are getting way better lately, but too many are low voltage > >>>>> things, we need a high voltage follower. We have bootstrapped the > >>>>> power supplies of low voltage amps to do this, but that is sort of a > >>>>> nightmare. Or really a nightmare. > >>>>> > >>>>> > >>>> > >>>> Indeed, such bootstraps are tough. > >>>> > >>>> I've been drooling over the datasheet of OPAx189: > >>>> > >>>> * CMRR: 168 dB (!) > >>>> * Open-loop gain: 170 dB (!!) > >>>> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz > >>>> * drift: 5 nV/deg C typ. > >>>> * huge input Z > >>>> * Vsupply up to 36V > >>>> > >>>> Has anyone used these yet? > >>>> > >>>> Cheers, > >>>> Nikolai > >>> > >>> That's a great amp, but its input common-mode range, near V+, is even > >>> worse than the ADA4522. All the super chopamps seem to have that > >>> problem. This one doesn't have input back-to-back diodes, so it won't > >>> have my hangup problem. > >>> > >>> DC psrr is great, but AC psrr is terrible. That could be a real-life > >>> noise limit. > >>> > >>> One big problem we had with bootstrapping a low-voltage chopamp was > >>> instability caused by psrr. > >>> > >>> > >> Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied > >> by the gain of the stage, which can be a bit of a rude shock. > >> > >> Cheers > >> > >> Phil Hobbs > >> > >> -- > >> Dr Philip C D Hobbs > >> Principal Consultant > >> ElectroOptical Innovations LLC / Hobbs ElectroOptics > >> Optics, Electro-optics, Photonics, Analog Electronics > >> Briarcliff Manor NY 10510 > >> > >> http://electrooptical.net > >> http://hobbs-eo.com > > > > What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling? > > > Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V > rails using a couple of BJTs and a zener to keep the amp from seeing > more than 5V. In that topology the output feeds back to the supply > rails, so there's the opportunity for feedback oscillation via the AC PS > and CM sensitivity. >Huh, OK. I never thought of 'bootstrapping' in that way. The only trick I've tried is a driven shield, which works great. (there's a dark desire to turn the gain up a bit over unity.) George H.> Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > http://hobbs-eo.com
Reply by ●September 17, 20182018-09-17
On Mon, 17 Sep 2018 18:05:52 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Monday, September 17, 2018 at 5:41:23 PM UTC-4, Phil Hobbs wrote: >> On 9/17/18 5:30 PM, Castorp wrote: >> > On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote: >> >> On 9/17/18 3:31 PM, John Larkin wrote: >> >>> On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp >> >>> <nikolaibeev@gmail.com> wrote: >> >>> >> >>>> >> >>>>> Choppers are getting way better lately, but too many are low voltage >> >>>>> things, we need a high voltage follower. We have bootstrapped the >> >>>>> power supplies of low voltage amps to do this, but that is sort of a >> >>>>> nightmare. Or really a nightmare. >> >>>>> >> >>>>> >> >>>> >> >>>> Indeed, such bootstraps are tough. >> >>>> >> >>>> I've been drooling over the datasheet of OPAx189: >> >>>> >> >>>> * CMRR: 168 dB (!) >> >>>> * Open-loop gain: 170 dB (!!) >> >>>> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz >> >>>> * drift: 5 nV/deg C typ. >> >>>> * huge input Z >> >>>> * Vsupply up to 36V >> >>>> >> >>>> Has anyone used these yet? >> >>>> >> >>>> Cheers, >> >>>> Nikolai >> >>> >> >>> That's a great amp, but its input common-mode range, near V+, is even >> >>> worse than the ADA4522. All the super chopamps seem to have that >> >>> problem. This one doesn't have input back-to-back diodes, so it won't >> >>> have my hangup problem. >> >>> >> >>> DC psrr is great, but AC psrr is terrible. That could be a real-life >> >>> noise limit. >> >>> >> >>> One big problem we had with bootstrapping a low-voltage chopamp was >> >>> instability caused by psrr. >> >>> >> >>> >> >> Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied >> >> by the gain of the stage, which can be a bit of a rude shock. >> >> >> >> Cheers >> >> >> >> Phil Hobbs >> >> >> >> -- >> >> Dr Philip C D Hobbs >> >> Principal Consultant >> >> ElectroOptical Innovations LLC / Hobbs ElectroOptics >> >> Optics, Electro-optics, Photonics, Analog Electronics >> >> Briarcliff Manor NY 10510 >> >> >> >> http://electrooptical.net >> >> http://hobbs-eo.com >> > >> > What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling? >> > >> Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V >> rails using a couple of BJTs and a zener to keep the amp from seeing >> more than 5V. In that topology the output feeds back to the supply >> rails, so there's the opportunity for feedback oscillation via the AC PS >> and CM sensitivity. >> >Huh, OK. I never thought of 'bootstrapping' in that way. >The only trick I've tried is a driven shield, which works great. >(there's a dark desire to turn the gain up a bit over unity.) >George H.I was going to post the bootstrapped chopamp circuit, but it's just too awful. The high-voltage chopper amps are a vast improvement. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●September 18, 20182018-09-18
On 9/17/18 10:32 PM, John Larkin wrote:> On Mon, 17 Sep 2018 18:05:52 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > >> On Monday, September 17, 2018 at 5:41:23 PM UTC-4, Phil Hobbs wrote: >>> On 9/17/18 5:30 PM, Castorp wrote: >>>> On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote: >>>>> On 9/17/18 3:31 PM, John Larkin wrote: >>>>>> On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp >>>>>> <nikolaibeev@gmail.com> wrote: >>>>>> >>>>>>> >>>>>>>> Choppers are getting way better lately, but too many are low voltage >>>>>>>> things, we need a high voltage follower. We have bootstrapped the >>>>>>>> power supplies of low voltage amps to do this, but that is sort of a >>>>>>>> nightmare. Or really a nightmare. >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> Indeed, such bootstraps are tough. >>>>>>> >>>>>>> I've been drooling over the datasheet of OPAx189: >>>>>>> >>>>>>> * CMRR: 168 dB (!) >>>>>>> * Open-loop gain: 170 dB (!!) >>>>>>> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz >>>>>>> * drift: 5 nV/deg C typ. >>>>>>> * huge input Z >>>>>>> * Vsupply up to 36V >>>>>>> >>>>>>> Has anyone used these yet? >>>>>>> >>>>>>> Cheers, >>>>>>> Nikolai >>>>>> >>>>>> That's a great amp, but its input common-mode range, near V+, is even >>>>>> worse than the ADA4522. All the super chopamps seem to have that >>>>>> problem. This one doesn't have input back-to-back diodes, so it won't >>>>>> have my hangup problem. >>>>>> >>>>>> DC psrr is great, but AC psrr is terrible. That could be a real-life >>>>>> noise limit. >>>>>> >>>>>> One big problem we had with bootstrapping a low-voltage chopamp was >>>>>> instability caused by psrr. >>>>>> >>>>>> >>>>> Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied >>>>> by the gain of the stage, which can be a bit of a rude shock. >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>>> >>>>> -- >>>>> Dr Philip C D Hobbs >>>>> Principal Consultant >>>>> ElectroOptical Innovations LLC / Hobbs ElectroOptics >>>>> Optics, Electro-optics, Photonics, Analog Electronics >>>>> Briarcliff Manor NY 10510 >>>>> >>>>> http://electrooptical.net >>>>> http://hobbs-eo.com >>>> >>>> What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling? >>>> >>> Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V >>> rails using a couple of BJTs and a zener to keep the amp from seeing >>> more than 5V. In that topology the output feeds back to the supply >>> rails, so there's the opportunity for feedback oscillation via the AC PS >>> and CM sensitivity. >>> >> Huh, OK. I never thought of 'bootstrapping' in that way. >> The only trick I've tried is a driven shield, which works great. >> (there's a dark desire to turn the gain up a bit over unity.) >> George H. > > I was going to post the bootstrapped chopamp circuit, but it's just > too awful. The high-voltage chopper amps are a vast improvement.Yup, especially when you're facing large signals with fairly high slew rates. It's useful to put the op amp in a socket when debugging one of those bootstrap things. You also need quite a few spares. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 18, 20182018-09-18
On Monday, 17 September 2018 23:41:23 UTC+2, Phil Hobbs wrote:> On 9/17/18 5:30 PM, Castorp wrote: > > On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote: > >> On 9/17/18 3:31 PM, John Larkin wrote: > >>> On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp > >>> <nikolaibeev@gmail.com> wrote: > >>> > >>>> > >>>>> Choppers are getting way better lately, but too many are low voltage > >>>>> things, we need a high voltage follower. We have bootstrapped the > >>>>> power supplies of low voltage amps to do this, but that is sort of a > >>>>> nightmare. Or really a nightmare. > >>>>> > >>>>> > >>>> > >>>> Indeed, such bootstraps are tough. > >>>> > >>>> I've been drooling over the datasheet of OPAx189: > >>>> > >>>> * CMRR: 168 dB (!) > >>>> * Open-loop gain: 170 dB (!!) > >>>> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz > >>>> * drift: 5 nV/deg C typ. > >>>> * huge input Z > >>>> * Vsupply up to 36V > >>>> > >>>> Has anyone used these yet? > >>>> > >>>> Cheers, > >>>> Nikolai > >>> > >>> That's a great amp, but its input common-mode range, near V+, is even > >>> worse than the ADA4522. All the super chopamps seem to have that > >>> problem. This one doesn't have input back-to-back diodes, so it won't > >>> have my hangup problem. > >>> > >>> DC psrr is great, but AC psrr is terrible. That could be a real-life > >>> noise limit. > >>> > >>> One big problem we had with bootstrapping a low-voltage chopamp was > >>> instability caused by psrr. > >>> > >>> > >> Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied > >> by the gain of the stage, which can be a bit of a rude shock. > >> > >> Cheers > >> > >> Phil Hobbs > >> > >> -- > >> Dr Philip C D Hobbs > >> Principal Consultant > >> ElectroOptical Innovations LLC / Hobbs ElectroOptics > >> Optics, Electro-optics, Photonics, Analog Electronics > >> Briarcliff Manor NY 10510 > >> > >> http://electrooptical.net > >> http://hobbs-eo.com > > > > What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling? > > > Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V > rails using a couple of BJTs and a zener to keep the amp from seeing > more than 5V. In that topology the output feeds back to the supply > rails, so there's the opportunity for feedback oscillation via the AC PS > and CM sensitivity. >Oh yes, in this case PSRR and CMRRR do matter. I was thinking more about the usual configuration of a HV chopper with fixed supply rails. Even if its PSRR is not stellar, you are on the safe side. Unless you make some blunder on the PS rails, that is. I've seen a bunch of these PS bootstrap circuits in papers from the biomedical instrumentation community, for stuff like microelectrode instrumentation and electrochemistry. They have to deal with uncomfortably high source impedances, but the signals are generally quite slow. Cheers, Nikolai
Reply by ●September 18, 20182018-09-18
On Tue, 18 Sep 2018 01:18:51 -0700 (PDT), Castorp <nikolaibeev@gmail.com> wrote:>On Monday, 17 September 2018 23:41:23 UTC+2, Phil Hobbs wrote: >> On 9/17/18 5:30 PM, Castorp wrote: >> > On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote: >> >> On 9/17/18 3:31 PM, John Larkin wrote: >> >>> On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp >> >>> <nikolaibeev@gmail.com> wrote: >> >>> >> >>>> >> >>>>> Choppers are getting way better lately, but too many are low voltage >> >>>>> things, we need a high voltage follower. We have bootstrapped the >> >>>>> power supplies of low voltage amps to do this, but that is sort of a >> >>>>> nightmare. Or really a nightmare. >> >>>>> >> >>>>> >> >>>> >> >>>> Indeed, such bootstraps are tough. >> >>>> >> >>>> I've been drooling over the datasheet of OPAx189: >> >>>> >> >>>> * CMRR: 168 dB (!) >> >>>> * Open-loop gain: 170 dB (!!) >> >>>> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz >> >>>> * drift: 5 nV/deg C typ. >> >>>> * huge input Z >> >>>> * Vsupply up to 36V >> >>>> >> >>>> Has anyone used these yet? >> >>>> >> >>>> Cheers, >> >>>> Nikolai >> >>> >> >>> That's a great amp, but its input common-mode range, near V+, is even >> >>> worse than the ADA4522. All the super chopamps seem to have that >> >>> problem. This one doesn't have input back-to-back diodes, so it won't >> >>> have my hangup problem. >> >>> >> >>> DC psrr is great, but AC psrr is terrible. That could be a real-life >> >>> noise limit. >> >>> >> >>> One big problem we had with bootstrapping a low-voltage chopamp was >> >>> instability caused by psrr. >> >>> >> >>> >> >> Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied >> >> by the gain of the stage, which can be a bit of a rude shock. >> >> >> >> Cheers >> >> >> >> Phil Hobbs >> >> >> >> -- >> >> Dr Philip C D Hobbs >> >> Principal Consultant >> >> ElectroOptical Innovations LLC / Hobbs ElectroOptics >> >> Optics, Electro-optics, Photonics, Analog Electronics >> >> Briarcliff Manor NY 10510 >> >> >> >> http://electrooptical.net >> >> http://hobbs-eo.com >> > >> > What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling? >> > >> Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V >> rails using a couple of BJTs and a zener to keep the amp from seeing >> more than 5V. In that topology the output feeds back to the supply >> rails, so there's the opportunity for feedback oscillation via the AC PS >> and CM sensitivity. >> > >Oh yes, in this case PSRR and CMRRR do matter. I was thinking more about the usual configuration of a HV chopper with fixed supply rails. Even if its PSRR is not stellar, you are on the safe side. Unless you make some blunder on the PS rails, that is. > >I've seen a bunch of these PS bootstrap circuits in papers from the biomedical instrumentation community, for stuff like microelectrode instrumentation and electrochemistry. They have to deal with uncomfortably high source impedances, but the signals are generally quite slow. > >Cheers, >NikolaiWe bootstrapped to get more voltage follower range, and bootstrapping also dramatically improves CMRR and PSRR, for slow unity-gain followers anyhow. Our problem was that we also needed speed, and the dynamics got messy. Our other innovation was to change the specs of the instrument so we don't have to follow big inputs. Just one potential customer made us design for a big voltage range, and then he didn't buy any. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●September 18, 20182018-09-18
On 9/18/18 11:31 AM, John Larkin wrote:> On Tue, 18 Sep 2018 01:18:51 -0700 (PDT), Castorp > <nikolaibeev@gmail.com> wrote: > >> On Monday, 17 September 2018 23:41:23 UTC+2, Phil Hobbs wrote: >>> On 9/17/18 5:30 PM, Castorp wrote: >>>> On Monday, 17 September 2018 21:37:50 UTC+2, Phil Hobbs wrote: >>>>> On 9/17/18 3:31 PM, John Larkin wrote: >>>>>> On Mon, 17 Sep 2018 09:46:55 -0700 (PDT), Castorp >>>>>> <nikolaibeev@gmail.com> wrote: >>>>>> >>>>>>> >>>>>>>> Choppers are getting way better lately, but too many are low voltage >>>>>>>> things, we need a high voltage follower. We have bootstrapped the >>>>>>>> power supplies of low voltage amps to do this, but that is sort of a >>>>>>>> nightmare. Or really a nightmare. >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> Indeed, such bootstraps are tough. >>>>>>> >>>>>>> I've been drooling over the datasheet of OPAx189: >>>>>>> >>>>>>> * CMRR: 168 dB (!) >>>>>>> * Open-loop gain: 170 dB (!!) >>>>>>> * en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz >>>>>>> * drift: 5 nV/deg C typ. >>>>>>> * huge input Z >>>>>>> * Vsupply up to 36V >>>>>>> >>>>>>> Has anyone used these yet? >>>>>>> >>>>>>> Cheers, >>>>>>> Nikolai >>>>>> >>>>>> That's a great amp, but its input common-mode range, near V+, is even >>>>>> worse than the ADA4522. All the super chopamps seem to have that >>>>>> problem. This one doesn't have input back-to-back diodes, so it won't >>>>>> have my hangup problem. >>>>>> >>>>>> DC psrr is great, but AC psrr is terrible. That could be a real-life >>>>>> noise limit. >>>>>> >>>>>> One big problem we had with bootstrapping a low-voltage chopamp was >>>>>> instability caused by psrr. >>>>>> >>>>>> >>>>> Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied >>>>> by the gain of the stage, which can be a bit of a rude shock. >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>>> >>>>> -- >>>>> Dr Philip C D Hobbs >>>>> Principal Consultant >>>>> ElectroOptical Innovations LLC / Hobbs ElectroOptics >>>>> Optics, Electro-optics, Photonics, Analog Electronics >>>>> Briarcliff Manor NY 10510 >>>>> >>>>> http://electrooptical.net >>>>> http://hobbs-eo.com >>>> >>>> What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling? >>>> >>> Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V >>> rails using a couple of BJTs and a zener to keep the amp from seeing >>> more than 5V. In that topology the output feeds back to the supply >>> rails, so there's the opportunity for feedback oscillation via the AC PS >>> and CM sensitivity. >>> >> >> Oh yes, in this case PSRR and CMRRR do matter. I was thinking more about the usual configuration of a HV chopper with fixed supply rails. Even if its PSRR is not stellar, you are on the safe side. Unless you make some blunder on the PS rails, that is. >> >> I've seen a bunch of these PS bootstrap circuits in papers from the biomedical instrumentation community, for stuff like microelectrode instrumentation and electrochemistry. They have to deal with uncomfortably high source impedances, but the signals are generally quite slow. >> >> Cheers, >> Nikolai > > We bootstrapped to get more voltage follower range, and bootstrapping > also dramatically improves CMRR and PSRR, for slow unity-gain > followers anyhow. Our problem was that we also needed speed, and the > dynamics got messy. > > Our other innovation was to change the specs of the instrument so we > don't have to follow big inputs. Just one potential customer made us > design for a big voltage range, and then he didn't buy any. > >The good news is that if you ignore customers, eventually they go away. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
