OPA197 c-load stability...

[Phil Hobbs]

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

 

[John Larkin]

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 Hobbs

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.

--

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

 

[Phil Hobbs]

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