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George Herold
Guest
Wed Jan 18, 2012 7:09 pm
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Bitrex
Guest
Wed Jan 18, 2012 7:09 pm
On 1/18/2012 12:09 PM, George Herold wrote:
Quote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Would an "ideal" circuit even oscillate in that configuration? I think
the only reason it's oscillating at all is because there's some
parasitic capacitive feedback from the output to the non inverting
input, and that will affect the oscillation frequency.
John Larkin
Guest
Wed Jan 18, 2012 7:09 pm
On Wed, 18 Jan 2012 09:09:43 -0800 (PST), George Herold
<gherold_at_teachspin.com> wrote:
Quote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Cool. Sounds like capacitance between opamp pins is making a
series-resonant circuit with the L. The opamp's phase lag and the new
LC network's lead cancel out. You only need a small resonant boost to
push the loop gain over the top.
Hohn
amdx
Guest
Wed Jan 18, 2012 7:09 pm
On 1/18/2012 11:09 AM, George Herold wrote:
Quote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | |>-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
Thanks
George H.
If the SRF of your 1mh inductor is 2.1Mhz, that calculates to a self
capacitance of 5.7 picofarads.
The opa2134 spec sheet shows an input capacitance
of 5 picofarads. (common mode).
That, plus a little stray capacitance would lower your oscillator
frequency to about 1/2 of SRF.
That's my take on it.
Mikek
Tim Wescott
Guest
Wed Jan 18, 2012 7:52 pm
On Wed, 18 Jan 2012 09:09:43 -0800, George Herold wrote:
Quote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self resonant
frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/ Freq. about 3.4
MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/ ~10MHz, which is
beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
In general, operational amplifiers are not perfect. They have phase lag,
they have coupling between pins, they have non-zero open-loop output
impedance, they have finite open-loop gain, etc.
Often this does not matter. But any time you hang a highly reactive load
off the end -- it does.
I rather suspect that if you put a healthy amount of resistance in series
with your inductor -- "healthy" meaning 10k or maybe even 1k -- that
you'll kill the oscillation, without killing the Q of your circuit as
much as you're doing when you load things down with the 100k-ohm shunt.
Further, it shouldn't have a whole lot of bad effects on the circuit as a
whole (with the possible exception of lowering your bandwidth).
If you really want to isolate the inductor consider using a source
follower. Assuming that you can find a small-enough signal FET with low
enough capacitance, it should do a pretty good job of isolating things
without killing the Q.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
George Herold
Guest
Wed Jan 18, 2012 8:54 pm
On Jan 18, 12:21 pm, Bitrex <bit...@de.lete.earthlink.net> wrote:
Quote:
On 1/18/2012 12:09 PM, George Herold wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
" Would an "ideal" circuit even oscillate in that configuration? "
That's what I don't understand... where's the feed back?
I think
Quote:
the only reason it's oscillating at all is because there's some
parasitic capacitive feedback from the output to the non inverting
input, and that will affect the oscillation frequency.
OK I can try tightening up my layout a little bit.
Or how about if I add some capacitance from output to
non-inverting input.
From the... "If you can't make it better, try making it worse." ..
school of thought.
George H.
- Hide quoted text -
Quote:
- Show quoted text -
John Larkin
Guest
Wed Jan 18, 2012 8:56 pm
On Wed, 18 Jan 2012 10:57:06 -0800 (PST), George Herold
<gherold_at_teachspin.com> wrote:
Quote:
On Jan 18, 12:30 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 18 Jan 2012 09:09:43 -0800 (PST), George Herold
gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Cool. Sounds like capacitance between opamp pins is making a
series-resonant circuit with the L. The opamp's phase lag and the new
LC network's lead cancel out. You only need a small resonant boost to
push the loop gain over the top.
Hohn- Hide quoted text -
- Show quoted text -
Does that suggest any way to make it stop? (Without loading the
buffer down too much.)
George H.
1. Kill the Q with a smaller resistor
2. Use an opamp with less capacitance or less phase shift
3. Use a jfet follower or some such ahead of the opamp
4. Neutralize it with a "negative capacitor"
5. Possibly improve the physical layout.
6. Try a different inductor, or just swap the leads. I tested some
transformers recently for adjacent magnetic and electrostatic
coupling, and the electrostatic coupling was much less if I grounded
the "outside" of the winding.
John
George Herold
Guest
Wed Jan 18, 2012 8:57 pm
On Jan 18, 12:30 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Wed, 18 Jan 2012 09:09:43 -0800 (PST), George Herold
gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Cool. Sounds like capacitance between opamp pins is making a
series-resonant circuit with the L. The opamp's phase lag and the new
LC network's lead cancel out. You only need a small resonant boost to
push the loop gain over the top.
Hohn- Hide quoted text -
- Show quoted text -
Does that suggest any way to make it stop? (Without loading the
buffer down too much.)
George H.
George Herold
Guest
Wed Jan 18, 2012 8:57 pm
On Jan 18, 12:34 pm, amdx <a...@knology.net> wrote:
Quote:
On 1/18/2012 11:09 AM, George Herold wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | |>-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
Thanks
George H.
If the SRF of your 1mh inductor is 2.1Mhz, that calculates to a self
capacitance of 5.7 picofarads.
The opa2134 spec sheet shows an input capacitance
of 5 picofarads. (common mode).
That, plus a little stray capacitance would lower your oscillator
frequency to about 1/2 of SRF.
That's my take on it.
Mikek- Hide quoted text -
- Show quoted text -
Hmm OK, I'll try adding some C to ground from the non-inverting
input. (Again to make it worse.)
George H.
George Herold
Guest
Wed Jan 18, 2012 11:30 pm
On Jan 18, 2:56 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Wed, 18 Jan 2012 10:57:06 -0800 (PST), George Herold
gher...@teachspin.com> wrote:
On Jan 18, 12:30 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 18 Jan 2012 09:09:43 -0800 (PST), George Herold
gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Cool. Sounds like capacitance between opamp pins is making a
series-resonant circuit with the L. The opamp's phase lag and the new
LC network's lead cancel out. You only need a small resonant boost to
push the loop gain over the top.
Hohn- Hide quoted text -
- Show quoted text -
Does that suggest any way to make it stop? (Without loading the
buffer down too much.)
George H.
Thanks for the suggestions!
Quote:
1. Kill the Q with a smaller resistor
I added 1 k ohm in series.. no change. (I can't afford 10 k.)
Quote:
2. Use an opamp with less capacitance or less phase shift
Well I tried a few different amps. Only the AD827 'video' amp did not
show the oscillation. (But it's only got 300k ohm of input input
impedance.)
Quote:
3. Use a jfet follower or some such ahead of the opamp
Yeah I thought something like that might work...
but it complicates the front end. I could try and see if it stops it.
Quote:
4. Neutralize it with a "negative capacitor"
ughh.
Quote:
5. Possibly improve the physical layout.
I tightened up the output. Less than 1" of buss wire from the output
to the BNC input. This actually seemed to make it worse.
a bit higher oscillation freq and I had to load it down with more than
100k ohm to get the oscillations to stop.
Quote:
6. Try a different inductor, or just swap the leads. I tested some
transformers recently for adjacent magnetic and electrostatic
coupling, and the electrostatic coupling was much less if I grounded
the "outside" of the winding.
My inductor selection is a bit limited. The others I've tried all
have the same pathology.
I also tried adding a bit of gain to the buffer.. but still the same
with a gain of two.
I'm not really convinced it's capacitive feedback from the output.
On the 'scope traces the output is always negative. Almost bumping
into the negative rail.
George H.
Quote:
John- Hide quoted text -
- Show quoted text -
Whoa, look at this,
http://imageshack.us/photo/my-images/513/tek0043.png/
I put a x10 scope probe on the input.
(chan. 2 upper about 30 Vp-p)
Chan. 1 in the middle is the output (15Vp-p and always negative)
Ref A is the output without the 16pF scope probe.
Geo...
whit3rd
Guest
Thu Jan 19, 2012 12:28 am
On Wednesday, January 18, 2012 10:54:23 AM UTC-8, George Herold wrote:
Quote:
On Jan 18, 12:21 pm, Bitrex <bit...@de.lete.earthlink.net> wrote:
On 1/18/2012 12:09 PM, George Herold wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
" Would an "ideal" circuit even oscillate in that configuration? "
That's what I don't understand... where's the feed back?
The circuit loop from output to (-) input can inductively couple to the
field around the inductor, of course.
More to the point, the two op amp input pins are sourced from very
different impedances; try putting another inductor in the negative-feedback
link; you likely have to account for the input capacitance of the op amp
to get the phase shifts calculated correctly. Like as not, the 'high
input impedance' assumption for this amplifier is ... not operative at this time.
George Herold
Guest
Thu Jan 19, 2012 1:31 am
On Jan 18, 5:28 pm, whit3rd <whit...@gmail.com> wrote:
Quote:
On Wednesday, January 18, 2012 10:54:23 AM UTC-8, George Herold wrote:
On Jan 18, 12:21 pm, Bitrex <bit...@de.lete.earthlink.net> wrote:
On 1/18/2012 12:09 PM, George Herold wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
" Would an "ideal" circuit even oscillate in that configuration? "
That's what I don't understand... where's the feed back?
The circuit loop from output to (-) input can inductively couple to the
field around the inductor, of course.
OK, I'll try moving the inductor around and see if that changes
anything.
Quote:
More to the point, the two op amp input pins are sourced from very
different impedances; try putting another inductor in the negative-feedback
link; you likely have to account for the input capacitance of the op amp
to get the phase shifts calculated correctly.
Oh, maybe I can cut and try first and calculate second?
Like as not, the 'high
Quote:
input impedance' assumption for this amplifier is ... not operative at this time.-
Ahh, Indeed I didn't calculate that.. 3pF is close to 50k...(at 1
meg).
(I shouldn't have sneered at the 300k ohms for the AD827.)
George H.
Hide quoted text -
Quote:
- Show quoted text -
Fred Bartoli
Guest
Thu Jan 19, 2012 4:19 pm
George Herold a écrit :
Quote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
OK, George I see you're still at it.
I've have that very pb when I designed my 200pV/rtHz preamplifier
(several paralleled huge input jets which means lots of parasitic
capacitance) and I think I reported it here in its time.
That pb comes from when you use a closed loop amplifier with a
gain-bandwidth product GBW, which means that, at a high enough frequency
above the low frequency pole, the amplifier differential input voltage
has 90degree phase shift WRT to the input (or output) voltage.
This phase shifted voltage then injects some current into the opamp
positive input node, thanks to the opamp differential input capacitance.
You can do the math for a perfect single pole opamp and a cd capacitor
shunting the inputs.
You then obtain a synthesized input impedance which is:
Zin= - (2 pi GBW + j w)/(cd w^2)
When you decompose this in parallel real and imaginary impedances, that
gives you:
Rp = -(4 pi^2 GBW^2 + w^2)/(2 pi cd GWB w)
Cp = cd w^2 /(4 pi^2 GBW^2 + w^2)
As you can see your stage input impedance has an always negative
parallel admittance which depends only on your opamp GBW and capacitance.
Now for the oscillating frequency, apart from the obvious additional
parasitic capacitance you also have to add the opamp CM input capacitance.
And now again, when you take the limit for w -> infinity, the parallel
resistance limit is:
RpLimit = -1/(2 pi cd GBW)
which is roughly -10K for your example OPA2134 and have no high
frequency limit!
That is, no frequency limit for a perfect one pole opamp model plus
differential input capacitance.
The additional poles above GBW for real world opamps change that
somewhere above the GBW frequency, but it's no surprising at all that
you see some oscillation above the 8MHz GBW limit of your 2134.
Have fun with that :-)
--
Thanks,
Fred.
amdx
Guest
Thu Jan 19, 2012 5:49 pm
On 1/19/2012 10:35 AM, George Herold wrote:
Quote:
On Jan 18, 12:09 pm, George Herold<gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | |>-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Hello Gentlemen (and any Ladies), Just want a ‘reality’ check.
+---cf---+
| |
+ |\ | opa2134
+---+--+---|+\ |
| | | |>-+out
C L R +|-/ |
C L R ||/ |
C L R +-----+
| | |
+---+--+
GND
So here’s my current model of the oscillations and feedback. (thanks
for the help in letting me see it.)
CCC is the capacitance in the coil, opamp and any strays to ground.
cf is some feed back capacitance from output to input.
RRR is a parallel resistance that I add to kill the oscillations. In
a hand-wavy way the added resistance should about equal the cf
impedance.
As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of
the dip sticking out from the copper tape.)
http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/
So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input. Oscillation frequency were all near 1.5 MHz. (a tad
lower with 2.2 pF C)
Test RRR
shield 45.7k ohm
no shield 38.9k
2.2pF 27k
The numbers don’t quite add up.. but the trend is pretty clear.
So I’ll search for a 1mH inductor with a high SRF, and also a bit
better opamp.
Thanx again for the help,
George H
Hi George,
Looks like my hypothesis missed the mark, (I didn't note it wasn't
supposed to be an osc.)
However, does the frequency get lower with additional capacitance
across the inductor.
Mikek
George Herold
Guest
Thu Jan 19, 2012 6:35 pm
On Jan 18, 12:09 pm, George Herold <gher...@teachspin.com> wrote:
Quote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
|\ opa2134
+--+---|+\
| | | >-+out
L R +|-/ |
L R ||/ |
L R +-----+
| |
+--+
GND
R = 1 Meg ohm
With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.
The inductors are 2200R series from Murata,http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf
But I’ve got some wimpier 1mH’s that show similar behavior.
Here’s a ‘scope shot. frequency about 1.1 MHz.http://imageshack.us/photo/my-images/718/tek0039.png/
The upper trace is with the supply voltage lowered to +/-6 volts. The
other is +/-15V.
Here’s a 100uH inductor.http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.
and a 10uH. (scale change)http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)
If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.
Can anyone give me a hint as to what the heck is going on?
The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)
Thanks
George H.
Hello Gentlemen (and any Ladies), Just want a ‘reality’ check.
+---cf---+
| |
+ |\ | opa2134
+---+--+---|+\ |
| | | | >-+out
C L R +|-/ |
C L R ||/ |
C L R +-----+
| | |
+---+--+
GND
So here’s my current model of the oscillations and feedback. (thanks
for the help in letting me see it.)
CCC is the capacitance in the coil, opamp and any strays to ground.
cf is some feed back capacitance from output to input.
RRR is a parallel resistance that I add to kill the oscillations. In
a hand-wavy way the added resistance should about equal the cf
impedance.
As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of
the dip sticking out from the copper tape.)
http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/
So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input. Oscillation frequency were all near 1.5 MHz. (a tad
lower with 2.2 pF C)
Test RRR
shield 45.7k ohm
no shield 38.9k
2.2pF 27k
The numbers don’t quite add up.. but the trend is pretty clear.
So I’ll search for a 1mH inductor with a high SRF, and also a bit
better opamp.
Thanx again for the help,
George H
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