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Datasheet Jargon Translation Required

Started by Cursitor Doom July 11, 2021
On Sun, 11 Jul 2021 20:21:31 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>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! > > > >The op-amp inverting input has some capacitance (you can think of it >as to ground) Some op-amps are worse than others (esp. those that use >large mosaic of input transistors internally). Also there is stray >capacitance from the 'input' end of the feedback resistor and input >resistor to ground (especially if you have ground or power planes >under the connections or parts). > >That acts like a low-pass filter to the inverting input so the >negative feedback signal lags and is attenuated at higher frequencies. >That can cause overshoot or even oscillation. > >In micropower circuits you typically want to have very high value >feedback resistors so the effect of a given capacitance is higher. >Similarly, high frequency op-amps can have plenty of gain at hundreds >of MHz so even a tiny capacitance can have a significant effect. It's >not typically an issue with jellybean op-amps and normal resistance >values. > >Poles (and zeros) refer to the numerator and denominator of the >transfer function (in the complex plane), but for the purposes of a >simple low-pass filter, the feedback pole is just the cutoff frequency >of the filter. > >H(s) = Vout(s)/Vin(s) = w0/(w0+s) so there's a pole at s = -w0 = >-1/(RC), where the denominator becomes zero. > >A 1M ohm resistance and a 5pF capacitance has a pole at f = >1/(2*pi*R*C) ~= 32kHz. > >If you have a low-power inverting amplifier with an input+stray >capacitance of 5pF, with a 10M feedback resistor and a 1.11M input >resistor (gain of -9, parallel resistance of 1.0M), you can parallel >the 10M resistor with 0.5pF to compensate. That's a fairly small >capacitance so you might have to cut and try. If the capacitance is >too high you get more sluggish response than necessary, if it's too >low you may get overshoot. > >Much like adjusting the probe compensation on an oscilloscope to get a >nice square wave from the reference square wave test point.
.... and all is revealed. Thanks, Speff!
On Mon, 12 Jul 2021 21:11:29 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>On Sun, 11 Jul 2021 18:18:45 -0700, jlarkin@highlandsniptechnology.com >wrote: > >>On Sun, 11 Jul 2021 23:59:07 +0100, Cursitor Doom <cd@nowhere.com> >>wrote: >> >>>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. >> >>Post a sketch of the circuit. Words don't tell the story. > >It's on page 16 of the datasheet I linked to (diagram 2). Really >couldn't be any simpler; just a case of observing the layout >directions and chosing the right support components for the op-amp >itself. > >https://www.ti.com/lit/ds/symlink/lf356-mil.pdf
Or "figure 27" as they also show it as. Bottom of page 16 anyway.
On Mon, 12 Jul 2021 21:27:09 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>On Mon, 12 Jul 2021 21:11:29 +0100, Cursitor Doom <cd@nowhere.com> >wrote: > >>On Sun, 11 Jul 2021 18:18:45 -0700, jlarkin@highlandsniptechnology.com >>wrote: >> >>>On Sun, 11 Jul 2021 23:59:07 +0100, Cursitor Doom <cd@nowhere.com> >>>wrote: >>> >>>>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. >>> >>>Post a sketch of the circuit. Words don't tell the story. >> >>It's on page 16 of the datasheet I linked to (diagram 2). Really >>couldn't be any simpler; just a case of observing the layout >>directions and chosing the right support components for the op-amp >>itself. >> >>https://www.ti.com/lit/ds/symlink/lf356-mil.pdf > >Or "figure 27" as they also show it as. Bottom of page 16 anyway.
No use trying to help you.
On Sun, 11 Jul 2021 20:21:31 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>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! > > > >The op-amp inverting input has some capacitance (you can think of it >as to ground) Some op-amps are worse than others (esp. those that use >large mosaic of input transistors internally). Also there is stray >capacitance from the 'input' end of the feedback resistor and input >resistor to ground (especially if you have ground or power planes >under the connections or parts). > >That acts like a low-pass filter to the inverting input so the >negative feedback signal lags and is attenuated at higher frequencies. >That can cause overshoot or even oscillation. > >In micropower circuits you typically want to have very high value >feedback resistors so the effect of a given capacitance is higher. >Similarly, high frequency op-amps can have plenty of gain at hundreds >of MHz so even a tiny capacitance can have a significant effect. It's >not typically an issue with jellybean op-amps and normal resistance >values. > >Poles (and zeros) refer to the numerator and denominator of the >transfer function (in the complex plane), but for the purposes of a >simple low-pass filter, the feedback pole is just the cutoff frequency >of the filter. > >H(s) = Vout(s)/Vin(s) = w0/(w0+s) so there's a pole at s = -w0 = >-1/(RC), where the denominator becomes zero. > >A 1M ohm resistance and a 5pF capacitance has a pole at f = >1/(2*pi*R*C) ~= 32kHz. > >If you have a low-power inverting amplifier with an input+stray >capacitance of 5pF, with a 10M feedback resistor and a 1.11M input >resistor (gain of -9, parallel resistance of 1.0M), you can parallel >the 10M resistor with 0.5pF to compensate. That's a fairly small >capacitance so you might have to cut and try. If the capacitance is >too high you get more sluggish response than necessary, if it's too >low you may get overshoot. > >Much like adjusting the probe compensation on an oscilloscope to get a >nice square wave from the reference square wave test point.
Ri on that opamp includes a dynamic microphone!
On 13/7/21 6:51 am, John Larkin wrote:
> On Mon, 12 Jul 2021 21:27:09 +0100, Cursitor Doom <cd@nowhere.com> > wrote: > >> On Mon, 12 Jul 2021 21:11:29 +0100, Cursitor Doom <cd@nowhere.com> >> wrote: >> >>> On Sun, 11 Jul 2021 18:18:45 -0700, jlarkin@highlandsniptechnology.com >>> wrote: >>> >>>> On Sun, 11 Jul 2021 23:59:07 +0100, Cursitor Doom <cd@nowhere.com> >>>> wrote: >>>> >>>>> 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. >>>> >>>> Post a sketch of the circuit. Words don't tell the story. >>> >>> It's on page 16 of the datasheet I linked to (diagram 2). Really >>> couldn't be any simpler; just a case of observing the layout >>> directions and chosing the right support components for the op-amp >>> itself. >>> >>> https://www.ti.com/lit/ds/symlink/lf356-mil.pdf >> >> Or "figure 27" as they also show it as. Bottom of page 16 anyway. > > No use trying to help you.
Please stop trying to educate it then. This is a creature who still thinks resistors must have leads. Fully Neanderthal...
On Tue, 13 Jul 2021 10:01:49 +1000, Clifford Heath
<no.spam@please.net> wrote:

>On 13/7/21 6:51 am, John Larkin wrote: >> On Mon, 12 Jul 2021 21:27:09 +0100, Cursitor Doom <cd@nowhere.com> >> wrote: >> >>> On Mon, 12 Jul 2021 21:11:29 +0100, Cursitor Doom <cd@nowhere.com> >>> wrote: >>> >>>> On Sun, 11 Jul 2021 18:18:45 -0700, jlarkin@highlandsniptechnology.com >>>> wrote: >>>> >>>>> On Sun, 11 Jul 2021 23:59:07 +0100, Cursitor Doom <cd@nowhere.com> >>>>> wrote: >>>>> >>>>>> 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. >>>>> >>>>> Post a sketch of the circuit. Words don't tell the story. >>>> >>>> It's on page 16 of the datasheet I linked to (diagram 2). Really >>>> couldn't be any simpler; just a case of observing the layout >>>> directions and chosing the right support components for the op-amp >>>> itself. >>>> >>>> https://www.ti.com/lit/ds/symlink/lf356-mil.pdf >>> >>> Or "figure 27" as they also show it as. Bottom of page 16 anyway. >> >> No use trying to help you. > >Please stop trying to educate it then. This is a creature who still >thinks resistors must have leads. Fully Neanderthal...
Poles are jargon.
On 13/07/2021 01:21, John Larkin wrote:
> On Tue, 13 Jul 2021 10:01:49 +1000, Clifford Heath > <no.spam@please.net> wrote: > >> On 13/7/21 6:51 am, John Larkin wrote:
>>> No use trying to help you. >> >> Please stop trying to educate it then. This is a creature who still >> thinks resistors must have leads. Fully Neanderthal... > > Poles are jargon.
Poles come from complex analysis representing zeroes in the denominator of a transfer function. Zeroes in the numerator are just called zeros. Electronic engineers have adopted the language of mathematicians here. -- Regards, Martin Brown
On 2021-07-13 09:51, Martin Brown wrote:
> On 13/07/2021 01:21, John Larkin wrote: >> On Tue, 13 Jul 2021 10:01:49 +1000, Clifford Heath >> <no.spam@please.net> wrote: >> >>> On 13/7/21 6:51 am, John Larkin wrote: > >>>> No use trying to help you. >>> >>> Please stop trying to educate it then. This is a creature who >>> still thinks resistors must have leads. Fully Neanderthal... >> >> Poles are jargon. > > Poles come from complex analysis representing zeroes in the > denominator of a transfer function. Zeroes in the numerator are just > called zeros. Electronic engineers have adopted the language of > mathematicians here. >
The name makes sense too. If you plot the function, it looks like tent poles. Jeroen Belleman
On 13/07/2021 09:12, Jeroen Belleman wrote:
> On 2021-07-13 09:51, Martin Brown wrote: >> On 13/07/2021 01:21, John Larkin wrote: >>> On Tue, 13 Jul 2021 10:01:49 +1000, Clifford Heath >>> <no.spam@please.net> wrote: >>> >>>> On 13/7/21 6:51 am, John Larkin wrote: >> >>>>> No use trying to help you. >>>> >>>> Please stop trying to educate it then. This is a creature who >>>> still thinks resistors must have leads. Fully Neanderthal... >>> >>> Poles are jargon. >> >> Poles come from complex analysis representing zeroes in the >> denominator of a transfer function. Zeroes in the numerator are just >> called zeros. Electronic engineers have adopted the language of >> mathematicians here. >> > > The name makes sense too. If you plot the function, it looks like > tent poles.
I'd never really thought about it like that. First ran into them in complex analysis and contour integration methods for solving various diverse physics problems more elegantly via Cauchy's Residue Theorem. Introduction for anyone interested in the gory details here: https://en.wikipedia.org/wiki/Residue_theorem -- Regards, Martin Brown
On Tue, 13 Jul 2021 08:51:31 +0100, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 13/07/2021 01:21, John Larkin wrote: >> On Tue, 13 Jul 2021 10:01:49 +1000, Clifford Heath >> <no.spam@please.net> wrote: >> >>> On 13/7/21 6:51 am, John Larkin wrote: > >>>> No use trying to help you. >>> >>> Please stop trying to educate it then. This is a creature who still >>> thinks resistors must have leads. Fully Neanderthal... >> >> Poles are jargon. > >Poles come from complex analysis representing zeroes in the denominator >of a transfer function. Zeroes in the numerator are just called zeros. >Electronic engineers have adopted the language of mathematicians here.
A dynamic mic driving an inverting opamp gets much more interesting. Kind of a tent village. -- John Larkin Highland Technology, Inc The best designs are necessarily accidental.