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.
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.
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.
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.
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.
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
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
On Saturday, April 9, 2022 at 2:39:42 PM UTC+10, jla...@highlandsniptechnology.com wrote: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
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 capacitiveIt 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
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.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
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.
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
On Saturday, April 9, 2022 at 5:17:38 AM UTC, bill....@ieee.org wrote: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
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 biasIt 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
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,Pity about that.
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.
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
thanks 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)
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?
RL
The 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.
jla...@highlandsniptechnology.com wrote:
On Sat, 9 Apr 2022 13:44:47 -0400, Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:
On 4/9/22 12:39 AM, 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,
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." ;)Supply current looks OK.
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.
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.
A previous fix hung a 1500 uF aluminum cap on the rail, an ugly hack
on top.
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.
On Saturday, April 9, 2022 at 4:22:06 PM UTC-7, Phil Hobbs wrote:
jla...@highlandsniptechnology.com wrote:
On Sat, 9 Apr 2022 13:44:47 -0400, Phil Hobbs
<pcdhSpamM...@electrooptical.net> wrote:
On 4/9/22 12:39 AM, 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,
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.
In short, you don't want a power supply, you want ground, but you're making >it with an amplifier from power supply as input. Are there current surges on this
pseudo-ground? If not, current sources (by the dozen, if necessary) into >parallel RC loads are a way to get well-filtered voltage levels without power >supply ripple sensitivity. Takes one op amp and a pass transistor per branch.
Then again, why not use the ECL trick of +3.2V and -2.0V power supplies? >Signal circuitry ought to be well-characterized against power supply noise, >but the signal-splitter application has a gain of 0.5 on power noise, and >adds another round of filter capacitance, multi-branched, to boot. It's arguably better to use
split supplies (like, +1.8 and -1.8V), instead, and a single-point ground topology
like the old guys did 50 years ago.
Sort of the reverse of Schawlow's law: "Anything will lase if you hit it >> >> hard enough." ;)Supply current looks OK.
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.
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.
That 'been known' and 'looks like' means off-the-spec-sheet design.
The thought makes me... itch. Maybe it's reminding me of bugs?
...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.
A previous fix hung a 1500 uF aluminum cap on the rail, an ugly hack
on top.
Yeah, sounds like the itchy feelling isn't just me.
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.
The big reason to avoid split supplies, is ... the mindset of the student with a
small project to complete for class. He will always design a negative ground system,
usually with a microprocessor/ADC that accepts no negative signal input.
Next year, he'll be writing up applications literature, based on his 'experience',
which will guide the next generation of students.
....
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.
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?
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?
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.
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.
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.
jlarkin@highlandsniptechnology.com wrote:
On Mon, 11 Apr 2022 18:56:30 +0300, Dimiter_Popoff <dp@tgi-sci.com>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.)
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.
Cheers
Phil Hobbs
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>Making an engineering evaluation isn't rocket science--find out where it
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.
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.
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