jlarkin@highlandsniptechnology.com wrote:> On Mon, 11 Apr 2022 18:56:30 +0300, Dimiter_Popoff <dp@tgi-sci.com> > wrote: > >> On 4/11/2022 17:27, jlarkin@highlandsniptechnology.com wrote: >>> On Mon, 11 Apr 2022 16:49:05 +0300, Dimiter_Popoff <dp@tgi-sci.com> >>> wrote: >>> >>>> On 4/10/2022 22:14, jlarkin@highlandsniptechnology.com wrote: >>>>> .... >>>>> >>>>> Our synchro box workes nicely with a single +24 supply from a big >>>>> wart, without a big + to - converter. So it's handy to reference >>>>> signals to a clean +12 rail. >>>>> >>>>> The original version had some channel-to-channel crosstalk via that >>>>> rail, and it was fixed with a gigantic aluminum cap to ground. I >>>>> thought I'd do something more elegant for the next rev. >>>> >>>> So why do you not want a 20-30 Ohm resistor (plus one say 1k and a >>>> few pF of a cap) if you have 12V headroom? The opamp is fast enough, >>>> what it cannot do in 1-2 uS will be done by the bypass caps you have. >>>> If this is your original setup and it took the huge aluminium cap >>>> to filter the crosstalk I very much doubt shorting the opamp's >>>> output to all the bypass caps will buy you anything. Did it? >>> >>> At low frequencies, the closed-loop output impedance of the opamp >>> follower will be less that the impedance of any reasonable cap. Adding >>> 20 ohms, well, adds 20 ohms. >>> >>> And why not do what's simplest? And learn something along the way? >>> >>> >>> >>> >>> >> >> >> Well learning something is always worth it of course. But the 20 Ohms >> closed in the loop does not mean you add 20 ohms to the output >> impedance, especially with all the 12V headroom that you have. >> To make sure we are talking about the same thing: 20 ohms between output >> and load, 1k between load and - input, a couple of pf between output and >> - input to ensure stability. > > I've done that, but it will still present a higher bus impedance at > some frequencies... assuming that the opamp doesn't peak, which it > seems not to do. The real test is to snoop the transient response to a > small load step. > > 1K and a couple of pF is a tau of a couple of ns. > > >Making an engineering evaluation isn't rocket science--find out where it stops oscillating and then swamp it some more for a safety factor. (The impedances can change with output current, of course.) 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
OPA197 c-load stability
Started by ●April 9, 2022
Reply by ●April 11, 20222022-04-11
Reply by ●April 11, 20222022-04-11
On Mon, 11 Apr 2022 13:24:41 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>jlarkin@highlandsniptechnology.com wrote: >> On Mon, 11 Apr 2022 18:56:30 +0300, Dimiter_Popoff <dp@tgi-sci.com> >> wrote: >> >>> On 4/11/2022 17:27, jlarkin@highlandsniptechnology.com wrote: >>>> On Mon, 11 Apr 2022 16:49:05 +0300, Dimiter_Popoff <dp@tgi-sci.com> >>>> wrote: >>>> >>>>> On 4/10/2022 22:14, jlarkin@highlandsniptechnology.com wrote: >>>>>> .... >>>>>> >>>>>> Our synchro box workes nicely with a single +24 supply from a big >>>>>> wart, without a big + to - converter. So it's handy to reference >>>>>> signals to a clean +12 rail. >>>>>> >>>>>> The original version had some channel-to-channel crosstalk via that >>>>>> rail, and it was fixed with a gigantic aluminum cap to ground. I >>>>>> thought I'd do something more elegant for the next rev. >>>>> >>>>> So why do you not want a 20-30 Ohm resistor (plus one say 1k and a >>>>> few pF of a cap) if you have 12V headroom? The opamp is fast enough, >>>>> what it cannot do in 1-2 uS will be done by the bypass caps you have. >>>>> If this is your original setup and it took the huge aluminium cap >>>>> to filter the crosstalk I very much doubt shorting the opamp's >>>>> output to all the bypass caps will buy you anything. Did it? >>>> >>>> At low frequencies, the closed-loop output impedance of the opamp >>>> follower will be less that the impedance of any reasonable cap. Adding >>>> 20 ohms, well, adds 20 ohms. >>>> >>>> And why not do what's simplest? And learn something along the way? >>>> >>>> >>>> >>>> >>>> >>> >>> >>> Well learning something is always worth it of course. But the 20 Ohms >>> closed in the loop does not mean you add 20 ohms to the output >>> impedance, especially with all the 12V headroom that you have. >>> To make sure we are talking about the same thing: 20 ohms between output >>> and load, 1k between load and - input, a couple of pf between output and >>> - input to ensure stability. >> >> I've done that, but it will still present a higher bus impedance at >> some frequencies... assuming that the opamp doesn't peak, which it >> seems not to do. The real test is to snoop the transient response to a >> small load step. >> >> 1K and a couple of pF is a tau of a couple of ns. >> >> >> >Making an engineering evaluation isn't rocket science--find out where it >stops oscillating and then swamp it some more for a safety factor. (The >impedances can change with output current, of course.) > >Cheers > >Phil HobbsThere are cases where a big fast load step makes it ring slew-rate limited, a dying sawtooth. Slew rate is a sort of bandwidth reduction; I think that's what's called a limit-cycle oscillation. The 1 uF case does that pretty obviously. My loads on the split rail are actually tiny, basically biasing up some diffamps. But this instrument has real 16-bit resolution, or sometimes more, and a little crosstalk between channels might be noticed. -- 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 11, 20222022-04-11
John Larkin wrote:> On Mon, 11 Apr 2022 13:24:41 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> jlarkin@highlandsniptechnology.com wrote: >>> On Mon, 11 Apr 2022 18:56:30 +0300, Dimiter_Popoff <dp@tgi-sci.com> >>> wrote: >>> >>>> On 4/11/2022 17:27, jlarkin@highlandsniptechnology.com wrote: >>>>> On Mon, 11 Apr 2022 16:49:05 +0300, Dimiter_Popoff <dp@tgi-sci.com> >>>>> wrote: >>>>> >>>>>> On 4/10/2022 22:14, jlarkin@highlandsniptechnology.com wrote: >>>>>>> .... >>>>>>> >>>>>>> Our synchro box workes nicely with a single +24 supply from a big >>>>>>> wart, without a big + to - converter. So it's handy to reference >>>>>>> signals to a clean +12 rail. >>>>>>> >>>>>>> The original version had some channel-to-channel crosstalk via that >>>>>>> rail, and it was fixed with a gigantic aluminum cap to ground. I >>>>>>> thought I'd do something more elegant for the next rev. >>>>>> >>>>>> So why do you not want a 20-30 Ohm resistor (plus one say 1k and a >>>>>> few pF of a cap) if you have 12V headroom? The opamp is fast enough, >>>>>> what it cannot do in 1-2 uS will be done by the bypass caps you have. >>>>>> If this is your original setup and it took the huge aluminium cap >>>>>> to filter the crosstalk I very much doubt shorting the opamp's >>>>>> output to all the bypass caps will buy you anything. Did it? >>>>> >>>>> At low frequencies, the closed-loop output impedance of the opamp >>>>> follower will be less that the impedance of any reasonable cap. Adding >>>>> 20 ohms, well, adds 20 ohms. >>>>> >>>>> And why not do what's simplest? And learn something along the way? >>>>> >>>>> >>>>> >>>>> >>>>> >>>> >>>> >>>> Well learning something is always worth it of course. But the 20 Ohms >>>> closed in the loop does not mean you add 20 ohms to the output >>>> impedance, especially with all the 12V headroom that you have. >>>> To make sure we are talking about the same thing: 20 ohms between output >>>> and load, 1k between load and - input, a couple of pf between output and >>>> - input to ensure stability. >>> >>> I've done that, but it will still present a higher bus impedance at >>> some frequencies... assuming that the opamp doesn't peak, which it >>> seems not to do. The real test is to snoop the transient response to a >>> small load step. >>> >>> 1K and a couple of pF is a tau of a couple of ns. >>> >>> >>> >> Making an engineering evaluation isn't rocket science--find out where it >> stops oscillating and then swamp it some more for a safety factor. (The >> impedances can change with output current, of course.)> > There are cases where a big fast load step makes it ring slew-rate > limited, a dying sawtooth. Slew rate is a sort of bandwidth reduction; > I think that's what's called a limit-cycle oscillation. The 1 uF case > does that pretty obviously. > > My loads on the split rail are actually tiny, basically biasing up > some diffamps. But this instrument has real 16-bit resolution, or > sometimes more, and a little crosstalk between channels might be > noticed. >That I believe. I sort of suspect that it's the output current limit rather than the slew rate, but if it's some fancy-schmance architecture the SR might be set by something other than the pole-splitting cap and the tail current source on the input pair. 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
