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Datasheet Jargon Translation Required

Started by Cursitor Doom July 11, 2021
Can some kind sould tell me what the following load of old cobblers
from the datasheet of a FET opamp  means in plain English?

********************************************************************************

"As with most amplifiers,care should be taken with lead
dress,component placement and supply decoupling in order to ensure
stability.For example,resistors from the output to an input should be
placed with the body close to the input to minimize pick-up and
maximize the frequency of the feedback pole by minimizing the
capacitance from the input to ground.

"A feedback pole is created when the feedback around any amplifier is
resistive.The parallel resistance and capacitance from the input of
the device(usually the inverting input) to AC ground set the frequency
of the pole.In many instances the frequency of this pole is much
greater than the expected 3-dB frequency of the closed loop gain and
consequently there is negligible effect on stability margin.However,
if the feedback pole is less than approximately six times the expected
3-dB frequency a lead capacitor should be placed from the output to
the input of the op amp.The value of the added capacitor should be
such that the RC time constant of this capacitor and the resistance it
parallels is greater than or equal to the original feedback pole time
constant."

********************************************************************************

Nothing shows up when I Google "feedback pole" so knowing WTF that
alone means would be a help!
On Sun, 11 Jul 2021 16:55:37 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

For context, this comes from page 13 of the datasheet found here:

 https://www.ti.com/lit/ds/symlink/lf356-mil.pdf
On Sunday, July 11, 2021 at 8:55:51 AM UTC-7, Cursitor Doom wrote:
> Can some kind sould tell me what the following load of old cobblers > from the datasheet of a FET opamp means in plain English? > > ******************************************************************************** > > "As with most amplifiers,care should be taken with lead > dress,component placement and supply decoupling in order to ensure > stability.For example,resistors from the output to an input should be > placed with the body close to the input to minimize pick-up and > maximize the frequency of the feedback pole by minimizing the > capacitance from the input to ground. > > "A feedback pole is created when the feedback around any amplifier is > resistive.The parallel resistance and capacitance from the input of > the device(usually the inverting input) to AC ground set the frequency > of the pole.In many instances the frequency of this pole is much > greater than the expected 3-dB frequency of the closed loop gain and > consequently there is negligible effect on stability margin.However, > if the feedback pole is less than approximately six times the expected > 3-dB frequency a lead capacitor should be placed from the output to > the input of the op amp.The value of the added capacitor should be > such that the RC time constant of this capacitor and the resistance it > parallels is greater than or equal to the original feedback pole time > constant." > > ******************************************************************************** > > Nothing shows up when I Google "feedback pole" so knowing WTF that > alone means would be a help!
Pole is the opposite of zero. Namely, resonance pole.
On Sunday, July 11, 2021 at 9:04:24 AM UTC-7, Ed Lee wrote:
> On Sunday, July 11, 2021 at 8:55:51 AM UTC-7, Cursitor Doom wrote: > > Can some kind sould tell me what the following load of old cobblers > > from the datasheet of a FET opamp means in plain English? > > > > ******************************************************************************** > > > > "As with most amplifiers,care should be taken with lead > > dress,component placement and supply decoupling in order to ensure > > stability.For example,resistors from the output to an input should be > > placed with the body close to the input to minimize pick-up and > > maximize the frequency of the feedback pole by minimizing the > > capacitance from the input to ground. > > > > "A feedback pole is created when the feedback around any amplifier is > > resistive.The parallel resistance and capacitance from the input of > > the device(usually the inverting input) to AC ground set the frequency > > of the pole.In many instances the frequency of this pole is much > > greater than the expected 3-dB frequency of the closed loop gain and > > consequently there is negligible effect on stability margin.However, > > if the feedback pole is less than approximately six times the expected > > 3-dB frequency a lead capacitor should be placed from the output to > > the input of the op amp.The value of the added capacitor should be > > such that the RC time constant of this capacitor and the resistance it > > parallels is greater than or equal to the original feedback pole time > > constant." > > > > ******************************************************************************** > > > > Nothing shows up when I Google "feedback pole" so knowing WTF that > > alone means would be a help! > Pole is the opposite of zero. Namely, resonance pole.
Here is more technical explanation. https://web.mit.edu/2.14/www/Handouts/PoleZero.pdf
On Monday, July 12, 2021 at 1:55:51 AM UTC+10, Cursitor Doom wrote:
> Can some kind sould tell me what the following load of old cobblers > from the datasheet of a FET opamp means in plain English?
There's always a bit of capacitance between the op amp input and ground. If you use a big resistor from the op amp output to the input where you are applying the feedback, this can give you enough lag at a frequency where the op amp still has gain to let it oscillate at that frequency. To prevent this put a small capacitor in parallel with the feed-back resistor. It will knock off some gain at high frequency, but it will stop it oscillating. It's op amps 101. -- Bill Sloman, Sydney
On Sun, 11 Jul 2021 16:55:37 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>Can some kind sould tell me what the following load of old cobblers >from the datasheet of a FET opamp means in plain English? > >******************************************************************************** > >"As with most amplifiers,care should be taken with lead >dress,component placement and supply decoupling in order to ensure >stability.For example,resistors from the output to an input should be >placed with the body close to the input to minimize pick-up and >maximize the frequency of the feedback pole by minimizing the >capacitance from the input to ground. > >"A feedback pole is created when the feedback around any amplifier is >resistive.The parallel resistance and capacitance from the input of >the device(usually the inverting input) to AC ground set the frequency >of the pole.In many instances the frequency of this pole is much >greater than the expected 3-dB frequency of the closed loop gain and >consequently there is negligible effect on stability margin.However, >if the feedback pole is less than approximately six times the expected >3-dB frequency a lead capacitor should be placed from the output to >the input of the op amp.The value of the added capacitor should be >such that the RC time constant of this capacitor and the resistance it >parallels is greater than or equal to the original feedback pole time >constant." > >******************************************************************************** > >Nothing shows up when I Google "feedback pole" so knowing WTF that >alone means would be a help!
A pole is basically a frequency rolloff corner, a resistor and a capacitor as a lowpass filter. The corner frequency is 1/(2*pi*R*C) The term actually refers to LaPlace transforms and complex plane plots and stuff. Opamps usually have an internal pole in the 10 Hz sort of range. A pole has phase shift, 45 degrees lag at the corner frequency, creeping up towards 90 at high frequencies. Two poles in a loop approaches 180 degrees shift, so negative feedback becomes positive and things can get unstable. Your feedback network might create a significant pole. Got a circuit sketch? You can Spice this. The part about supply decoupling and pcb capacitance is usually no big deal. Data sheets like to be alarmist about that stuff. -- John Larkin Highland Technology, Inc The best designs are necessarily accidental.
On Sun, 11 Jul 2021 11:04:57 -0700, jlarkin@highlandsniptechnology.com
wrote:
> >A pole is basically a frequency rolloff corner, a resistor and a >capacitor as a lowpass filter. The corner frequency is 1/(2*pi*R*C) > >The term actually refers to LaPlace transforms and complex plane plots >and stuff. > >Opamps usually have an internal pole in the 10 Hz sort of range. > >A pole has phase shift, 45 degrees lag at the corner frequency, >creeping up towards 90 at high frequencies. > >Two poles in a loop approaches 180 degrees shift, so negative feedback >becomes positive and things can get unstable. Your feedback network >might create a significant pole. > >Got a circuit sketch? You can Spice this.
I'm just following page 16 of this document (diagram 2 at the bottom of the page). Couldn't be simpler - or so I thought til they started on about feedback poles. https://www.ti.com/lit/ds/symlink/lf356-mil.pdf
>The part about supply decoupling and pcb capacitance is usually no big >deal. Data sheets like to be alarmist about that stuff.
Is that what the C1 'virtual capacitor' between the inverting and non-inverting inputs is all about?
Cursitor Doom wrote:
> On Sun, 11 Jul 2021 11:04:57 -0700, jlarkin@highlandsniptechnology.com > wrote: >> >> A pole is basically a frequency rolloff corner, a resistor and a >> capacitor as a lowpass filter. The corner frequency is 1/(2*pi*R*C) >> >> The term actually refers to LaPlace transforms and complex plane plots >> and stuff. >> >> Opamps usually have an internal pole in the 10 Hz sort of range. >> >> A pole has phase shift, 45 degrees lag at the corner frequency, >> creeping up towards 90 at high frequencies. >> >> Two poles in a loop approaches 180 degrees shift, so negative feedback >> becomes positive and things can get unstable. Your feedback network >> might create a significant pole. >> >> Got a circuit sketch? You can Spice this. > > I'm just following page 16 of this document (diagram 2 at the bottom > of the page). Couldn't be simpler - or so I thought til they started > on about feedback poles. > > https://www.ti.com/lit/ds/symlink/lf356-mil.pdf > >> The part about supply decoupling and pcb capacitance is usually no big >> deal. Data sheets like to be alarmist about that stuff. > > Is that what the C1 'virtual capacitor' between the inverting and > non-inverting inputs is all about? >
It means, "Watch out for stray capacitance on the summing junction or across the feedback resistor." ;) 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
On Sun, 11 Jul 2021 19:51:59 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>On Sun, 11 Jul 2021 11:04:57 -0700, jlarkin@highlandsniptechnology.com >wrote: >> >>A pole is basically a frequency rolloff corner, a resistor and a >>capacitor as a lowpass filter. The corner frequency is 1/(2*pi*R*C) >> >>The term actually refers to LaPlace transforms and complex plane plots >>and stuff. >> >>Opamps usually have an internal pole in the 10 Hz sort of range. >> >>A pole has phase shift, 45 degrees lag at the corner frequency, >>creeping up towards 90 at high frequencies. >> >>Two poles in a loop approaches 180 degrees shift, so negative feedback >>becomes positive and things can get unstable. Your feedback network >>might create a significant pole. >> >>Got a circuit sketch? You can Spice this. > >I'm just following page 16 of this document (diagram 2 at the bottom >of the page). Couldn't be simpler - or so I thought til they started >on about feedback poles. > >https://www.ti.com/lit/ds/symlink/lf356-mil.pdf > >>The part about supply decoupling and pcb capacitance is usually no big >>deal. Data sheets like to be alarmist about that stuff. > >Is that what the C1 'virtual capacitor' between the inverting and >non-inverting inputs is all about?
That 3 pF is internal to the opamp. PCB pads are typically a fraction of a pF. 1/16" thick FR4 is around 15 pF per square inch, so a typical pad is pretty small. Vias are usually a fraction of a pF too. If R1||R2 were, say, 1K, and C1 were 3 pF, the feedback pole would be tau=3 ns, corner frequency around 50 MHz, assuming pi=6. That's way above the freq where this opamp is out of gain, so it's OK. But if the resistance were 100K, there could be trouble. -- John Larkin Highland Technology, Inc The best designs are necessarily accidental.
On Sun, 11 Jul 2021 13:58:12 -0700, jlarkin@highlandsniptechnology.com
wrote:

>On Sun, 11 Jul 2021 19:51:59 +0100, Cursitor Doom <cd@nowhere.com> >wrote: > >>On Sun, 11 Jul 2021 11:04:57 -0700, jlarkin@highlandsniptechnology.com >>wrote: >>> >>>A pole is basically a frequency rolloff corner, a resistor and a >>>capacitor as a lowpass filter. The corner frequency is 1/(2*pi*R*C) >>> >>>The term actually refers to LaPlace transforms and complex plane plots >>>and stuff. >>> >>>Opamps usually have an internal pole in the 10 Hz sort of range. >>> >>>A pole has phase shift, 45 degrees lag at the corner frequency, >>>creeping up towards 90 at high frequencies. >>> >>>Two poles in a loop approaches 180 degrees shift, so negative feedback >>>becomes positive and things can get unstable. Your feedback network >>>might create a significant pole. >>> >>>Got a circuit sketch? You can Spice this. >> >>I'm just following page 16 of this document (diagram 2 at the bottom >>of the page). Couldn't be simpler - or so I thought til they started >>on about feedback poles. >> >>https://www.ti.com/lit/ds/symlink/lf356-mil.pdf >> >>>The part about supply decoupling and pcb capacitance is usually no big >>>deal. Data sheets like to be alarmist about that stuff. >> >>Is that what the C1 'virtual capacitor' between the inverting and >>non-inverting inputs is all about? > >That 3 pF is internal to the opamp. PCB pads are typically a fraction >of a pF. > >1/16" thick FR4 is around 15 pF per square inch, so a typical pad is >pretty small. Vias are usually a fraction of a pF too. > >If R1||R2 were, say, 1K, and C1 were 3 pF, the feedback pole would be >tau=3 ns, corner frequency around 50 MHz, assuming pi=6. That's way >above the freq where this opamp is out of gain, so it's OK. But if the >resistance were 100K, there could be trouble.
Seems I needn't worry, then. I'm only going to be using this as the first stage of an amp for a dynamic microphone and the 20hz-20khz audio band is way too low for tiny stray capacitances to matter. There may be other pitfalls, but stray caps isn't something I need to worry about here.