ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max offset, EMI hardened, pA bias current. But it has a hangup mode as a follower: if the output goes to V+ the input back-to-back diodes keep both inputs above the legal common-mode range. Pity it's not RRIO. Similar amps from TI and Maxim seem to have the same issue. Here's one fix: https://www.dropbox.com/s/tdzs1ydi7quzbs7/ADA4522_hangup.JPG?raw=1 We had the depletion fets there already, to protect the amp, so all we'll do is add the dual zener. Above 8 volts or so, small zeners [1] have tiny pre-breakdown currents, so this should be fine for +-10 volt operating inputs. We'll test some dual zeners to be sure. [1] some PITA will naturally open the zener/avalanche debate again. Everybody calls them zeners. -- John Larkin Highland Technology, Inc lunatic fringe electronics
ADA4522 opamp hangup
Started by ●September 12, 2018
Reply by ●September 12, 20182018-09-12
On Wednesday, September 12, 2018 at 11:45:38 AM UTC-4, John Larkin wrote:> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max > offset, EMI hardened, pA bias current. But it has a hangup mode as a > follower: if the output goes to V+ the input back-to-back diodes keep > both inputs above the legal common-mode range. Pity it's not RRIO.Huh that is a nice opamp. (Thanks, well slew rate not so snappy) What do you mean by hangup? Does it latch? Or take a long time to recover? George H.> > Similar amps from TI and Maxim seem to have the same issue. > > Here's one fix: > > https://www.dropbox.com/s/tdzs1ydi7quzbs7/ADA4522_hangup.JPG?raw=1 > > We had the depletion fets there already, to protect the amp, so all > we'll do is add the dual zener. Above 8 volts or so, small zeners [1] > have tiny pre-breakdown currents, so this should be fine for +-10 volt > operating inputs. We'll test some dual zeners to be sure. > > [1] some PITA will naturally open the zener/avalanche debate again. > Everybody calls them zeners. > > > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by ●September 12, 20182018-09-12
On Wed, 12 Sep 2018 11:16:42 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Wednesday, September 12, 2018 at 11:45:38 AM UTC-4, John Larkin wrote: >> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max >> offset, EMI hardened, pA bias current. But it has a hangup mode as a >> follower: if the output goes to V+ the input back-to-back diodes keep >> both inputs above the legal common-mode range. Pity it's not RRIO. >Huh that is a nice opamp. (Thanks, well slew rate not so snappy) >What do you mean by hangup? >Does it latch? Or take a long time to recover? > >George H.If the customer input is fairly high impedance, like 20K or so, and he briefly blips the voltage high, the input can hang high, even when the customer backs down. It just occurred to us that we could have that stage have a gain of +1.15 or something, which would keep the inverting input away from Vcc. That just needs two resistors and has no zener leakage concerns. We can cal around the gain. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 12, 20182018-09-12
On Wednesday, September 12, 2018 at 3:26:42 PM UTC-4, John Larkin wrote:> On Wed, 12 Sep 2018 11:16:42 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >On Wednesday, September 12, 2018 at 11:45:38 AM UTC-4, John Larkin wrote: > >> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max > >> offset, EMI hardened, pA bias current. But it has a hangup mode as a > >> follower: if the output goes to V+ the input back-to-back diodes keep > >> both inputs above the legal common-mode range. Pity it's not RRIO. > >Huh that is a nice opamp. (Thanks, well slew rate not so snappy) > >What do you mean by hangup? > >Does it latch? Or take a long time to recover? > > > >George H. > > If the customer input is fairly high impedance, like 20K or so, and he > briefly blips the voltage high, the input can hang high, even when the > customer backs down. > > It just occurred to us that we could have that stage have a gain of > +1.15 or something, which would keep the inverting input away from > Vcc. That just needs two resistors and has no zener leakage concerns. > We can cal around the gain.OK, so a little gain kills it. If you didn't care too much about the output impedance.. or you knew what it was driving, you could throw away the gain on the output. (but you know that...) George H.> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●September 12, 20182018-09-12
On Wednesday, September 12, 2018 at 7:26:13 PM UTC-4, George Herold wrote:> On Wednesday, September 12, 2018 at 3:26:42 PM UTC-4, John Larkin wrote: > > On Wed, 12 Sep 2018 11:16:42 -0700 (PDT), George Herold > > <gherold@teachspin.com> wrote: > > > > >On Wednesday, September 12, 2018 at 11:45:38 AM UTC-4, John Larkin wrote: > > >> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max > > >> offset, EMI hardened, pA bias current. But it has a hangup mode as a > > >> follower: if the output goes to V+ the input back-to-back diodes keep > > >> both inputs above the legal common-mode range. Pity it's not RRIO. > > >Huh that is a nice opamp. (Thanks, well slew rate not so snappy) > > >What do you mean by hangup? > > >Does it latch? Or take a long time to recover? > > > > > >George H. > > > > If the customer input is fairly high impedance, like 20K or so, and he > > briefly blips the voltage high, the input can hang high, even when the > > customer backs down. > > > > It just occurred to us that we could have that stage have a gain of > > +1.15 or something, which would keep the inverting input away from > > Vcc. That just needs two resistors and has no zener leakage concerns. > > We can cal around the gain. > OK, so a little gain kills it. If you didn't care too much > about the output impedance.. or you knew what it was driving, > you could throw away the gain on the output. > (but you know that...) > George H.ps let me just add this question. I like the fact that there are more HV (>36V) opamps... any ideas of what's driving the market? GH> > > > > > -- > > > > John Larkin Highland Technology, Inc > > picosecond timing precision measurement > > > > jlarkin att highlandtechnology dott com > > http://www.highlandtechnology.com
Reply by ●September 12, 20182018-09-12
On 2018-09-12 08:45, John Larkin wrote:> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max > offset, EMI hardened, pA bias current. But it has a hangup mode as a > follower: if the output goes to V+ the input back-to-back diodes keep > both inputs above the legal common-mode range. Pity it's not RRIO. > > Similar amps from TI and Maxim seem to have the same issue. > > Here's one fix: > > https://www.dropbox.com/s/tdzs1ydi7quzbs7/ADA4522_hangup.JPG?raw=1 >How could the output on a follower go to V+ if the input doesn't?> We had the depletion fets there already, to protect the amp, so all > we'll do is add the dual zener. Above 8 volts or so, small zeners [1] > have tiny pre-breakdown currents, so this should be fine for +-10 volt > operating inputs. We'll test some dual zeners to be sure. > > [1] some PITA will naturally open the zener/avalanche debate again. > Everybody calls them zeners. >Not everyone does: https://www.electro-tech-online.com/threads/5-6v-zenner-diode-ltspice-model-need.152109/ -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●September 12, 20182018-09-12
On Wed, 12 Sep 2018 16:26:08 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Wednesday, September 12, 2018 at 3:26:42 PM UTC-4, John Larkin wrote: >> On Wed, 12 Sep 2018 11:16:42 -0700 (PDT), George Herold >> <gherold@teachspin.com> wrote: >> >> >On Wednesday, September 12, 2018 at 11:45:38 AM UTC-4, John Larkin wrote: >> >> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max >> >> offset, EMI hardened, pA bias current. But it has a hangup mode as a >> >> follower: if the output goes to V+ the input back-to-back diodes keep >> >> both inputs above the legal common-mode range. Pity it's not RRIO. >> >Huh that is a nice opamp. (Thanks, well slew rate not so snappy) >> >What do you mean by hangup? >> >Does it latch? Or take a long time to recover? >> > >> >George H. >> >> If the customer input is fairly high impedance, like 20K or so, and he >> briefly blips the voltage high, the input can hang high, even when the >> customer backs down. >> >> It just occurred to us that we could have that stage have a gain of >> +1.15 or something, which would keep the inverting input away from >> Vcc. That just needs two resistors and has no zener leakage concerns. >> We can cal around the gain. >OK, so a little gain kills it. If you didn't care too much >about the output impedance.. or you knew what it was driving, >you could throw away the gain on the output. >(but you know that...) >George H.I hadn't thought of that but you're right, the voltage at the inverting input has a gain of 1.000, after the divider. Unfortunately there is a load downstream. Hmmm, I think it works! The downstream load is about 5K, so we just add one 500 ohm resistor in series with the opamp output. https://www.dropbox.com/s/d7cpms6fcde6my2/ADA4522_fix.JPG?raw=1 Gain is 1.000, opamp input voltage is constrained to the safe CM range, and the resistor tolerance and TC don't matter. Does look weird, but it should work. Good one. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 12, 20182018-09-12
On Wed, 12 Sep 2018 16:35:29 -0700, Joerg <news@analogconsultants.com> wrote:>On 2018-09-12 08:45, John Larkin wrote: >> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max >> offset, EMI hardened, pA bias current. But it has a hangup mode as a >> follower: if the output goes to V+ the input back-to-back diodes keep >> both inputs above the legal common-mode range. Pity it's not RRIO. >> >> Similar amps from TI and Maxim seem to have the same issue. >> >> Here's one fix: >> >> https://www.dropbox.com/s/tdzs1ydi7quzbs7/ADA4522_hangup.JPG?raw=1 >> > >How could the output on a follower go to V+ if the input doesn't?The concern is that a momentary input over-voltage can lock it up. That would generate phone calls.> > >> We had the depletion fets there already, to protect the amp, so all >> we'll do is add the dual zener. Above 8 volts or so, small zeners [1] >> have tiny pre-breakdown currents, so this should be fine for +-10 volt >> operating inputs. We'll test some dual zeners to be sure. >> >> [1] some PITA will naturally open the zener/avalanche debate again. >> Everybody calls them zeners. >> > >Not everyone does: > >https://www.electro-tech-online.com/threads/5-6v-zenner-diode-ltspice-model-need.152109/-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 12, 20182018-09-12
On 9/12/18 11:45 AM, John Larkin wrote:> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max > offset, EMI hardened, pA bias current. But it has a hangup mode as a > follower: if the output goes to V+ the input back-to-back diodes keep > both inputs above the legal common-mode range. Pity it's not RRIO. > > Similar amps from TI and Maxim seem to have the same issue. > > Here's one fix: > > https://www.dropbox.com/s/tdzs1ydi7quzbs7/ADA4522_hangup.JPG?raw=1 > > We had the depletion fets there already, to protect the amp, so all > we'll do is add the dual zener. Above 8 volts or so, small zeners [1] > have tiny pre-breakdown currents, so this should be fine for +-10 volt > operating inputs. We'll test some dual zeners to be sure. >Another approach would be to put a parallel RC in the feedback path. I first ran into this sort of problem with the then-new Motorola MC34084, which was sort of a turbo TL084. Its output swing was bigger than its input CM range, and it had the FET op amp "phase inversion" problem (*), so there were lots of entertaining lockup states if you used noninverting amps as part of a larger feedback loop. ;) Cheers Phil Hobbs (*) It isn't really _inversion_, of course--if the positive CM limit is exceeded, the PFET input stage turns off and the output rails low, regardless of which input is higher. 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 12, 20182018-09-12
On 9/12/18 8:09 PM, John Larkin wrote:> On Wed, 12 Sep 2018 16:26:08 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > >> On Wednesday, September 12, 2018 at 3:26:42 PM UTC-4, John Larkin wrote: >>> On Wed, 12 Sep 2018 11:16:42 -0700 (PDT), George Herold >>> <gherold@teachspin.com> wrote: >>> >>>> On Wednesday, September 12, 2018 at 11:45:38 AM UTC-4, John Larkin wrote: >>>>> ADA4522 is an amazing amp. 55 volt supplies, low noise, 5 uV max >>>>> offset, EMI hardened, pA bias current. But it has a hangup mode as a >>>>> follower: if the output goes to V+ the input back-to-back diodes keep >>>>> both inputs above the legal common-mode range. Pity it's not RRIO. >>>> Huh that is a nice opamp. (Thanks, well slew rate not so snappy) >>>> What do you mean by hangup? >>>> Does it latch? Or take a long time to recover? >>>> >>>> George H. >>> >>> If the customer input is fairly high impedance, like 20K or so, and he >>> briefly blips the voltage high, the input can hang high, even when the >>> customer backs down. >>> >>> It just occurred to us that we could have that stage have a gain of >>> +1.15 or something, which would keep the inverting input away from >>> Vcc. That just needs two resistors and has no zener leakage concerns. >>> We can cal around the gain. >> OK, so a little gain kills it. If you didn't care too much >> about the output impedance.. or you knew what it was driving, >> you could throw away the gain on the output. >> (but you know that...) >> George H. > > I hadn't thought of that but you're right, the voltage at the > inverting input has a gain of 1.000, after the divider. Unfortunately > there is a load downstream. > > Hmmm, I think it works! The downstream load is about 5K, so we just > add one 500 ohm resistor in series with the opamp output. > > https://www.dropbox.com/s/d7cpms6fcde6my2/ADA4522_fix.JPG?raw=1 > > Gain is 1.000, opamp input voltage is constrained to the safe CM > range, and the resistor tolerance and TC don't matter. > > Does look weird, but it should work. > > Good one. > >Cute! Watch out for capacitive loading though. 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