OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias current, pretty good offset specs, 5 nV noise, EMI hardened. Like most opamps, it is specified to be stable up to some capacitive load, 1 nF in this case. For bigger caps they show the usual R+C load stabilizing idea on the data sheet. That all ignores the Williams Effect, namely that a big enough cap will stabilize most anything. As a follower, handy for rail splitting and such, 47 nF to ground oscillates 1 uF ceramic rings a bit on step edges 4.7 uF cer or more is stable 56u or 180u polymer is stable Any tantalum cap is stable A 33u tantalum and a lot of ceramics looks nice. That may be the choice for a lot of opamps. https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 I have a case where I want to drive a many-bypassed rail at VCC/2 and don't want a resistor in series with the output. -- I yam what I yam - Popeye
OPA197 c-load stability
Started by ●April 9, 2022
Reply by ●April 9, 20222022-04-09
On Saturday, April 9, 2022 at 12:39:42 AM UTC-4, jla...@highlandsniptechnology.com wrote:> OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias > current, pretty good offset specs, 5 nV noise, EMI hardened. > > Like most opamps, it is specified to be stable up to some capacitive > load, 1 nF in this case. For bigger caps they show the usual R+C load > stabilizing idea on the data sheet. That all ignores the Williams > Effect, namely that a big enough cap will stabilize most anything. > > As a follower, handy for rail splitting and such, > > 47 nF to ground oscillates > > 1 uF ceramic rings a bit on step edges > > 4.7 uF cer or more is stable > > 56u or 180u polymer is stable > > Any tantalum cap is stable > > A 33u tantalum and a lot of ceramics looks nice. That may be the > choice for a lot of opamps. > > > https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 > > https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 > > https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 > > https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 > > I have a case where I want to drive a many-bypassed rail at VCC/2 and > don't want a resistor in series with the output.Not quite the low, low input bias current, but cap drive on the output is excellent. https://www.ti.com/product/LM8272 Not so easy to come by at the moment, like many parts. -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
Reply by ●April 9, 20222022-04-09
On Saturday, April 9, 2022 at 2:39:42 PM UTC+10, jla...@highlandsniptechnology.com wrote:> OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias > current, pretty good offset specs, 5 nV noise, EMI hardened. > > Like most opamps, it is specified to be stable up to some capacitive > load, 1 nF in this case. For bigger caps they show the usual R+C load > stabilizing idea on the data sheet. That all ignores the Williams > Effect, namely that a big enough cap will stabilize most anything.It doesn't. A big enough capacitor means that the oscillation isn't driving enough current into the capacitor to produce enough voltage swing to be detectable - or sometimes not even even enough to be bigger than the Johnson noise at the oscillation frequency in the series resistance of the capacitor. At that level the oscillation doesn't take the input stage out of its linear region (+/25mV, for bipolar transistors, a volts or so for FET and MOSFET inputs), so it doesn't mess up performance enough to notice, but it is still oscillating.> As a follower, handy for rail splitting and such, > > 47 nF to ground oscillates > > 1 uF ceramic rings a bit on step edges > > 4.7 uF cer or more is stable > > 56u or 180u polymer is stable > > Any tantalum cap is stable > > A 33u tantalum and a lot of ceramics looks nice. That may be the > choice for a lot of opamps. > > https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 > > https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 > > https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 > > https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 > > I have a case where I want to drive a many-bypassed rail at VCC/2 and > don't want a resistor in series with the output.Pity about that. https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html does discuss what's actually going and goes into sensible ways of tackling the problem. -- Bill Sloman, Sydney
Reply by ●April 9, 20222022-04-09
On Fri, 08 Apr 2022 21:39:25 -0700, jlarkin@highlandsniptechnology.com wrote:>OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias >current, pretty good offset specs, 5 nV noise, EMI hardened. > >Like most opamps, it is specified to be stable up to some capacitive >load, 1 nF in this case. For bigger caps they show the usual R+C load >stabilizing idea on the data sheet. That all ignores the Williams >Effect, namely that a big enough cap will stabilize most anything. > >As a follower, handy for rail splitting and such, > >47 nF to ground oscillates > >1 uF ceramic rings a bit on step edges > >4.7 uF cer or more is stable > >56u or 180u polymer is stable > >Any tantalum cap is stable > >A 33u tantalum and a lot of ceramics looks nice. That may be the >choice for a lot of opamps. > > >https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 > >https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 > >https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 > >https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 > >I have a case where I want to drive a many-bypassed rail at VCC/2 and >don't want a resistor in series with the output.Just because you don't see a voltage across a 1uF cap, doesn't mean that the thing driving current into the node isn't going unpredictably nuts. Case temperature? RL
Reply by ●April 9, 20222022-04-09
On 4/9/2022 7:39, jlarkin@highlandsniptechnology.com wrote:> OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias > current, pretty good offset specs, 5 nV noise, EMI hardened. > > Like most opamps, it is specified to be stable up to some capacitive > load, 1 nF in this case. For bigger caps they show the usual R+C load > stabilizing idea on the data sheet. That all ignores the Williams > Effect, namely that a big enough cap will stabilize most anything. > > As a follower, handy for rail splitting and such, > > 47 nF to ground oscillates > > 1 uF ceramic rings a bit on step edges > > 4.7 uF cer or more is stable > > 56u or 180u polymer is stable > > Any tantalum cap is stable > > A 33u tantalum and a lot of ceramics looks nice. That may be the > choice for a lot of opamps. > > > https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 > > https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 > > https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 > > https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 > > I have a case where I want to drive a many-bypassed rail at VCC/2 and > don't want a resistor in series with the output. > > >I'd be wary relying on that "Williams effect". Even a very small resistor - DC feedback after it, some AC prior to it - should be a lot more reliable. Or a transistor in the loop, well you know what I mean. Just not that "Williams effect", feels awful to me.
Reply by ●April 9, 20222022-04-09
On 4/9/22 12:39 AM, jlarkin@highlandsniptechnology.com wrote:> OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias > current, pretty good offset specs, 5 nV noise, EMI hardened. > > Like most opamps, it is specified to be stable up to some capacitive > load, 1 nF in this case. For bigger caps they show the usual R+C load > stabilizing idea on the data sheet. That all ignores the Williams > Effect, namely that a big enough cap will stabilize most anything. > > As a follower, handy for rail splitting and such, > > 47 nF to ground oscillates > > 1 uF ceramic rings a bit on step edges > > 4.7 uF cer or more is stable > > 56u or 180u polymer is stable > > Any tantalum cap is stable > > A 33u tantalum and a lot of ceramics looks nice. That may be the > choice for a lot of opamps. > > > https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 > > https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 > > https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 > > https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 > > I have a case where I want to drive a many-bypassed rail at VCC/2 and > don't want a resistor in series with the output.Sort of the reverse of Schawlow's law: "Anything will lase if you hit it hard enough." ;) It's worth putting a sense resistor in the supply leads to check for oscillations of very low amplitude. THat's been known to happen even when the output looks steady on a scope. 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 https://hobbs-eo.com
Reply by ●April 9, 20222022-04-09
On Sat, 09 Apr 2022 11:05:29 -0400, legg <legg@nospam.magma.ca> wrote:>On Fri, 08 Apr 2022 21:39:25 -0700, jlarkin@highlandsniptechnology.com >wrote: > >>OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias >>current, pretty good offset specs, 5 nV noise, EMI hardened. >> >>Like most opamps, it is specified to be stable up to some capacitive >>load, 1 nF in this case. For bigger caps they show the usual R+C load >>stabilizing idea on the data sheet. That all ignores the Williams >>Effect, namely that a big enough cap will stabilize most anything. >> >>As a follower, handy for rail splitting and such, >> >>47 nF to ground oscillates >> >>1 uF ceramic rings a bit on step edges >> >>4.7 uF cer or more is stable >> >>56u or 180u polymer is stable >> >>Any tantalum cap is stable >> >>A 33u tantalum and a lot of ceramics looks nice. That may be the >>choice for a lot of opamps. >> >> >>https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 >> >>https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 >> >>https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 >> >>https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 >> >>I have a case where I want to drive a many-bypassed rail at VCC/2 and >>don't want a resistor in series with the output. > >Just because you don't see a voltage across a 1uF cap, doesn't mean >that the thing driving current into the node isn't going unpredictably >nuts. Case temperature? > >RLThe 1u case seems to have a limit-cycle oscillation that dies out pretty fast. Oscillation would increase supply current, and I don't see that. The data sheet has a chart of recommended damping resistor vs cap load, table 3. The last entry is 2 ohms and 1 uF. Why did they stop there? The next step could have been 10 uF and zero ohms. And why the 47r and 100 pF point? Makes no sense. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●April 9, 20222022-04-09
On Sat, 9 Apr 2022 13:44:47 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 4/9/22 12:39 AM, jlarkin@highlandsniptechnology.com wrote: >> OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias >> current, pretty good offset specs, 5 nV noise, EMI hardened. >> >> Like most opamps, it is specified to be stable up to some capacitive >> load, 1 nF in this case. For bigger caps they show the usual R+C load >> stabilizing idea on the data sheet. That all ignores the Williams >> Effect, namely that a big enough cap will stabilize most anything. >> >> As a follower, handy for rail splitting and such, >> >> 47 nF to ground oscillates >> >> 1 uF ceramic rings a bit on step edges >> >> 4.7 uF cer or more is stable >> >> 56u or 180u polymer is stable >> >> Any tantalum cap is stable >> >> A 33u tantalum and a lot of ceramics looks nice. That may be the >> choice for a lot of opamps. >> >> >> https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 >> >> https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 >> >> https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 >> >> https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 >> >> I have a case where I want to drive a many-bypassed rail at VCC/2 and >> don't want a resistor in series with the output. > >Sort of the reverse of Schawlow's law: "Anything will lase if you hit it >hard enough." ;) > >It's worth putting a sense resistor in the supply leads to check for >oscillations of very low amplitude. THat's been known to happen even >when the output looks steady on a scope. > >Cheers > >Phil HobbsSupply current looks OK. I need to drive a net to Vcc/2, and it has a dozen 10 uF ceramics to ground, and I want a low impedance drive from DC up. Looks like the added 47 uF tantalum is prudent. That adds some ESR damping. My boss assigned me to rev this board https://www.dropbox.com/s/dnkmpdzs2va6x3z/P545_Top.jpg?raw=1 which involves picking up a bunch of ECOs and reviewing the NEXT file, where people have accumulated two pages of annoying change requests. The bottom of the board is paved with parts; there's not much room to add things. A previous fix hung a 1500 uF aluminum cap on the rail, an ugly hack on top. -- I yam what I yam - Popeye
Reply by ●April 9, 20222022-04-09
jlarkin@highlandsniptechnology.com wrote:> On Sat, 9 Apr 2022 13:44:47 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 4/9/22 12:39 AM, jlarkin@highlandsniptechnology.com wrote: >>> OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias >>> current, pretty good offset specs, 5 nV noise, EMI hardened. >>> >>> Like most opamps, it is specified to be stable up to some capacitive >>> load, 1 nF in this case. For bigger caps they show the usual R+C load >>> stabilizing idea on the data sheet. That all ignores the Williams >>> Effect, namely that a big enough cap will stabilize most anything. >>> >>> As a follower, handy for rail splitting and such, >>> >>> 47 nF to ground oscillates >>> >>> 1 uF ceramic rings a bit on step edges >>> >>> 4.7 uF cer or more is stable >>> >>> 56u or 180u polymer is stable >>> >>> Any tantalum cap is stable >>> >>> A 33u tantalum and a lot of ceramics looks nice. That may be the >>> choice for a lot of opamps. >>> >>> >>> https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 >>> >>> https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 >>> >>> https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 >>> >>> https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 >>> >>> I have a case where I want to drive a many-bypassed rail at VCC/2 and >>> don't want a resistor in series with the output. >> >> Sort of the reverse of Schawlow's law: "Anything will lase if you hit it >> hard enough." ;) >> >> It's worth putting a sense resistor in the supply leads to check for >> oscillations of very low amplitude. THat's been known to happen even >> when the output looks steady on a scope.> Supply current looks OK. > > I need to drive a net to Vcc/2, and it has a dozen 10 uF ceramics to > ground, and I want a low impedance drive from DC up. > > Looks like the added 47 uF tantalum is prudent. That adds some ESR > damping. > > My boss assigned me to rev this board > > https://www.dropbox.com/s/dnkmpdzs2va6x3z/P545_Top.jpg?raw=1 > > which involves picking up a bunch of ECOs and reviewing the NEXT file, > where people have accumulated two pages of annoying change requests. > > The bottom of the board is paved with parts; there's not much room to > add things. > > A previous fix hung a 1500 uF aluminum cap on the rail, an ugly hack > on top.I recall--the lead sneaking round the edge of the board was thrillingly gnarly. ;) (Not that I'm above doing the same, when pressed sufficiently.) The C load moves the output pole to lower frequency, and when it's too close to the zero-cross of the main+tail poles, you wind up with instability. A large, higher-ESR cap is often a good way to stabilize switchers and LDOs, too--it's a shunt version of the usual lead/lag network used in feedback amps. There's no reason that should be a problem in an op amp loop, in principle. Doing stuff outside the datasheet's guaranteed limits puts the responsibility on us, but oh, well--that's where it winds up anyway. 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 ●April 9, 20222022-04-09
On Saturday, April 9, 2022 at 5:17:38 AM UTC, bill....@ieee.org wrote:> On Saturday, April 9, 2022 at 2:39:42 PM UTC+10, jla...@highlandsniptechnology.com wrote: > > OPA197 is a great little opamp. 36 volts RRIO, 10 MHz, pA bias > > current, pretty good offset specs, 5 nV noise, EMI hardened. > > > > Like most opamps, it is specified to be stable up to some capacitive > > load, 1 nF in this case. For bigger caps they show the usual R+C load > > stabilizing idea on the data sheet. That all ignores the Williams > > Effect, namely that a big enough cap will stabilize most anything. > It doesn't. A big enough capacitor means that the oscillation isn't driving enough current into the capacitor to produce enough voltage swing to be detectable - or sometimes not even even enough to be bigger than the Johnson noise at the oscillation frequency in the series resistance of the capacitor. At that level the oscillation doesn't take the input stage out of its linear region (+/25mV, for bipolar transistors, a volts or so for FET and MOSFET inputs), so it doesn't mess up performance enough to notice, but it is still oscillating. > > As a follower, handy for rail splitting and such, > > > > 47 nF to ground oscillates > > > > 1 uF ceramic rings a bit on step edges > > > > 4.7 uF cer or more is stable > > > > 56u or 180u polymer is stable > > > > Any tantalum cap is stable > > > > A 33u tantalum and a lot of ceramics looks nice. That may be the > > choice for a lot of opamps. > > > > https://www.dropbox.com/s/bsh4i0yb2yxn2oo/Z534_1.jpg?raw=1 > > > > https://www.dropbox.com/s/psu1qsqwsb6wi6g/20220408_120126.jpg?raw=1 > > > > https://www.dropbox.com/s/6ukbwh9kj9pdl6i/20220408_112325.jpg?raw=1 > > > > https://www.dropbox.com/s/s7whu9d7gszm2im/20220408_114102.jpg?raw=1 > > > > I have a case where I want to drive a many-bypassed rail at VCC/2 and > > don't want a resistor in series with the output. > Pity about that. > > https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html > > does discuss what's actually going and goes into sensible ways of tackling the problem. > > -- > Bill Sloman, Sydneythanks for that link Bill. Although I've read the same info elsewhere this one also mentioned *external* compensation is useful. And I thought external comp was just for "old" op-amps, a previous-century idea! ;0)
