Op amp noise from Inductors

Discussion in 'Electronic Basics' started by pmlonline@gmail.com, Apr 8, 2006.

1. Guest

Sorry for the double reply, but Google Groups isn't displaying this on
their site because it associated the title with another post. Hopefully
it will appear on deja.com by changing the title.

Kevin Aylward wrote:
> wrote:
> > Hi,

> > I'm looking at an op-amp graph of total noise vs source resistance. The
> > noise increases with an increase of source resistance. Does this also
> > apply to reactance?

> > The way I understand it is resistance is thermal noise, but reactance
> > from inductors and caps don't caused any noise. Real L's and C's have
> > some R, but in that sense they have noise.

> > ---------------------------------

> > Some details:

> > I have an input coil antenna that's 22 ohms R and about 10 mH. Nothing
> > fancy, just a round loop coil with a lot of turns. Typical frequencies:
> > from 1KHz to 1MHz. At say 1MHz the reactance would be just over 6 M
> > ohms. The chart, "Total Noise vs Matched Source Resistance" at

> > According to the chart a 6 M ohm source resistance would have
> > outrageous noise. Is it safe to say the 6 M ohm reactance (for 1MHz
> > signals) will cause no noise? Rather the noise will come from the 22
> > ohms R? According to the chart, 22 R at 1KHz is ~ 1 nV/SqrtHz. Not sure
> > what it would be at 1 MHz, but it seems lower than 1KHz.

> > The charts is for "Matched Source Resistance." The coil is one
> > continuous resister, so if I place the 2 coil leads directly to the
> > op-amp then is that considered matched? I'm using a typical
> > differential op-amp has two input R's. So if the coil's total R is 22
> > ohms then would that equate to two 11 ohm input R's? Very confusing,
> > lol.

> Noise is due to thermal noise *and* shot curent noise. The shot noise
> current of an amplifier is droped accross the source impedance (and
> Rin). If the source is an inducter, the voltage noise caused by the amp
> input current shot noise will increase with frequency despite the fact
> that the inductor itself does not generate any noise.

> Kevin Aylward B.Sc.
>
> http://www.anasoft.co.uk
> SuperSpice, a very affordable Mixed-Mode
> Windows Simulator with Schematic Capture,
> Waveform Display, FFT's and Filter Design.

> "There are none more ignorant and useless,than they that seek answers
> on their knees, with their eyes closed"

Let me see if I understand this. A pure inductor cause shot noise, but
no thermal noise? How can I calculate a differential op-amp's output
noise if the source is mostly inductive? ->
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html

Note that in my case R1 & R2 are also reactive-- 22 ohm resistance and
6 M ohm inductance.

The capacitance is extremely small. So there's no resonance.

I presume that the noise would be a lot greater if it was 6 M ohm
resisters rather than inductors.

Thanks for any help,
Paul

, Apr 8, 2006

2. John LarkinGuest

On 8 Apr 2006 08:32:18 -0700, wrote:

>Sorry for the double reply, but Google Groups isn't displaying this on
>their site because it associated the title with another post. Hopefully
>it will appear on deja.com by changing the title.
>
>
>Kevin Aylward wrote:
>> wrote:
>> > Hi,

>
>> > I'm looking at an op-amp graph of total noise vs source resistance. The
>> > noise increases with an increase of source resistance. Does this also
>> > apply to reactance?

>
>> > The way I understand it is resistance is thermal noise, but reactance
>> > from inductors and caps don't caused any noise. Real L's and C's have
>> > some R, but in that sense they have noise.

>
>> > ---------------------------------

>
>> > Some details:

>
>> > I have an input coil antenna that's 22 ohms R and about 10 mH. Nothing
>> > fancy, just a round loop coil with a lot of turns. Typical frequencies:
>> > from 1KHz to 1MHz. At say 1MHz the reactance would be just over 6 M
>> > ohms. The chart, "Total Noise vs Matched Source Resistance" at

>
>
>> > According to the chart a 6 M ohm source resistance would have
>> > outrageous noise. Is it safe to say the 6 M ohm reactance (for 1MHz
>> > signals) will cause no noise? Rather the noise will come from the 22
>> > ohms R? According to the chart, 22 R at 1KHz is ~ 1 nV/SqrtHz. Not sure
>> > what it would be at 1 MHz, but it seems lower than 1KHz.

>
>> > The charts is for "Matched Source Resistance." The coil is one
>> > continuous resister, so if I place the 2 coil leads directly to the
>> > op-amp then is that considered matched? I'm using a typical
>> > differential op-amp has two input R's. So if the coil's total R is 22
>> > ohms then would that equate to two 11 ohm input R's? Very confusing,
>> > lol.

>
>> Noise is due to thermal noise *and* shot curent noise. The shot noise
>> current of an amplifier is droped accross the source impedance (and
>> Rin). If the source is an inducter, the voltage noise caused by the amp
>> input current shot noise will increase with frequency despite the fact
>> that the inductor itself does not generate any noise.

>
>> Kevin Aylward B.Sc.
>>
>> http://www.anasoft.co.uk
>> SuperSpice, a very affordable Mixed-Mode
>> Windows Simulator with Schematic Capture,
>> Waveform Display, FFT's and Filter Design.

>
>> "There are none more ignorant and useless,than they that seek answers
>> on their knees, with their eyes closed"

>
>
>Let me see if I understand this. A pure inductor cause shot noise, but
>no thermal noise?

A pure inductance *generates* no noise. That would violate
conservation of energy.

> How can I calculate a differential op-amp's output
>noise if the source is mostly inductive? ->
>http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html
>

At any given frequency, multiply the opamp's input current noise
density (in amps per root Hz) by the inductor's impedance; that's the
input noise voltage density, units of volts per root Hz. Multiply that
by amp gain at that frequency to get output noise density. Looks like
you'll need a fet amp here.

To get net noise voltage, you have to integrate the noise density
curve over frequency. Note that, unless the opamp gain rolls off, or
the coil has shunt capacitance, the output noise density (at least
this In*Zin*gain thing) here will increase linearly with frequency,
pretty much forever.

>Note that in my case R1 & R2 are also reactive-- 22 ohm resistance and
>6 M ohm inductance.
>
>The capacitance is extremely small. So there's no resonance.
>
>I presume that the noise would be a lot greater if it was 6 M ohm
>resisters rather than inductors.

The resistor would add its own noise, too. At some point, in real
life, pickup from ambient magnetic fields will actually dominate.
That's why HF receivers don't need good noise figures: atmospheric
noise is huge below about 30 MHz. A little 60 Hz hum, or one good AM
station, will entirely blow away any considerations of thermal noise.

John

John Larkin, Apr 8, 2006

3. JoergGuest

Hello John,

>
> The resistor would add its own noise, too. At some point, in real
> life, pickup from ambient magnetic fields will actually dominate.
> That's why HF receivers don't need good noise figures: atmospheric
> noise is huge below about 30 MHz. A little 60 Hz hum, or one good AM
> station, will entirely blow away any considerations of thermal noise.
>

That's why it can be a good idea to use toroids or for very large
inductances pot cores. They don't "talk" much.

Regards, Joerg

http://www.analogconsultants.com

Joerg, Apr 8, 2006
4. Guest

John Larkin wrote:
> >Sorry for the double reply, but Google Groups isn't displaying this on
> >their site because it associated the title with another post. Hopefully
> >it will appear on deja.com by changing the title.
> >
> >
> >Kevin Aylward wrote:
> >> wrote:
> >> > Hi,

> >
> >> > I'm looking at an op-amp graph of total noise vs source resistance. The
> >> > noise increases with an increase of source resistance. Does this also
> >> > apply to reactance?

> >
> >> > The way I understand it is resistance is thermal noise, but reactance
> >> > from inductors and caps don't caused any noise. Real L's and C's have
> >> > some R, but in that sense they have noise.

> >
> >> > ---------------------------------

> >
> >> > Some details:

> >
> >> > I have an input coil antenna that's 22 ohms R and about 10 mH. Nothing
> >> > fancy, just a round loop coil with a lot of turns. Typical frequencies:
> >> > from 1KHz to 1MHz. At say 1MHz the reactance would be just over 6 M
> >> > ohms. The chart, "Total Noise vs Matched Source Resistance" at

> >
> >
> >> > According to the chart a 6 M ohm source resistance would have
> >> > outrageous noise. Is it safe to say the 6 M ohm reactance (for 1MHz
> >> > signals) will cause no noise? Rather the noise will come from the 22
> >> > ohms R? According to the chart, 22 R at 1KHz is ~ 1 nV/SqrtHz. Not sure
> >> > what it would be at 1 MHz, but it seems lower than 1KHz.

> >
> >> > The charts is for "Matched Source Resistance." The coil is one
> >> > continuous resister, so if I place the 2 coil leads directly to the
> >> > op-amp then is that considered matched? I'm using a typical
> >> > differential op-amp has two input R's. So if the coil's total R is 22
> >> > ohms then would that equate to two 11 ohm input R's? Very confusing,
> >> > lol.

> >
> >> Noise is due to thermal noise *and* shot curent noise. The shot noise
> >> current of an amplifier is droped accross the source impedance (and
> >> Rin). If the source is an inducter, the voltage noise caused by the amp
> >> input current shot noise will increase with frequency despite the fact
> >> that the inductor itself does not generate any noise.

> >
> >> Kevin Aylward B.Sc.
> >>
> >> http://www.anasoft.co.uk
> >> SuperSpice, a very affordable Mixed-Mode
> >> Windows Simulator with Schematic Capture,
> >> Waveform Display, FFT's and Filter Design.

> >
> >> "There are none more ignorant and useless,than they that seek answers
> >> on their knees, with their eyes closed"

> >
> >
> >Let me see if I understand this. A pure inductor cause shot noise, but
> >no thermal noise?

>
> A pure inductance *generates* no noise. That would violate
> conservation of energy.
>
> > How can I calculate a differential op-amp's output
> >noise if the source is mostly inductive? ->
> >http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html
> >

>
> At any given frequency, multiply the opamp's input current noise
> density (in amps per root Hz) by the inductor's impedance; that's the
> input noise voltage density, units of volts per root Hz.

Thanks for info. Using that equation and lets say the input source is
100% inductive. At 1pA/SqrtHz and 600 ohms impedance = 600 pV/SqrtHz.
Or using 6 M ohm impedance (from inductor) = 6 uV/SqrtHz. ... I don't
understand why the noise should increase from an increase in pure
inductance. If inductors don't cause noise then how would the output
noise be a factor of input source impedance? If we double the
inductance, then according to the above equation, the noise doubles. Is
this absolutely correct?

I'm trying to understand where the noise would come from. In the case
of pure inductance, wouldn't the noise only come from the op-amp itself
and the input shot current noise? If by chance the output noise is a
function of input current in this case, then shouldn't the noise go
down by increasing the input reactance? Let me look at this from
another angle. If the source impedance in this type of op-amp doubles,
then the gain should cut in half. Either viewpoint, how can an increase
in impedance on this op-amps input source cause an increase in output
noise?

Thanks for any help,
Paul

, Apr 9, 2006
5. colinGuest

> I'm trying to understand where the noise would come from. In the case
> of pure inductance, wouldn't the noise only come from the op-amp itself
> and the input shot current noise? If by chance the output noise is a
> function of input current in this case, then shouldn't the noise go
> down by increasing the input reactance? Let me look at this from
> another angle. If the source impedance in this type of op-amp doubles,
> then the gain should cut in half. Either viewpoint, how can an increase
> in impedance on this op-amps input source cause an increase in output
> noise?
>
>
> Thanks for any help,
> Paul

The noise you are talking about is just one source of the overal noise and
it is there already in the bias current coming out(or in) of the op amp
inputs, it has to go somewhere, the resultant voltage noise at the input is
therefore proportional to the effective impedance seen at the input, from
simple aplication of ohms law, it is not generated by the inductor,
noise).
Hope this helps to make it clearer.

A resistor in parallel with the inductor will lower the impedance seen at
the input, however it will also reduce the output, so it will not benefit
the SNR however half the turns on the coil will reduce the voltage by 2, but
will reduce the impedance and resultant noise by 4, at some point either the
voltage noise source or the current noise source will dominate, an op amp
with much lower input current ie jfet/mosfet will probably give a better
compromise.

Colin =^.^=

colin, Apr 9, 2006
6. Kevin AylwardGuest

wrote:
> Sorry for the double reply, but Google Groups isn't displaying this on
> their site because it associated the title with another post.
> Hopefully it will appear on deja.com by changing the title.
>

>
> Let me see if I understand this. A pure inductor cause shot noise, but
> no thermal noise?

Err... no, you missed the bit "the fact that the inductor itself does
not generate any noise" then?

The input transistors generate the shot noise. This noise is dropped
across the source impedance.

Well the other dudes have answerd the rest, so I'll leave it there.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

Kevin Aylward, Apr 9, 2006
7. John LarkinGuest

On 8 Apr 2006 17:08:34 -0700, wrote:

>
>
>Thanks for info. Using that equation and lets say the input source is
>100% inductive. At 1pA/SqrtHz and 600 ohms impedance = 600 pV/SqrtHz.
>Or using 6 M ohm impedance (from inductor) = 6 uV/SqrtHz. ... I don't
>understand why the noise should increase from an increase in pure
>inductance. If inductors don't cause noise then how would the output
>noise be a factor of input source impedance? If we double the
>inductance, then according to the above equation, the noise doubles. Is
>this absolutely correct?
>
>I'm trying to understand where the noise would come from. In the case
>of pure inductance, wouldn't the noise only come from the op-amp itself
>and the input shot current noise? If by chance the output noise is a
>function of input current in this case, then shouldn't the noise go
>down by increasing the input reactance? Let me look at this from
>another angle. If the source impedance in this type of op-amp doubles,
>then the gain should cut in half. Either viewpoint, how can an increase
>in impedance on this op-amps input source cause an increase in output
>noise?
>
>
>Thanks for any help,
>Paul

We may be a bit confused here. I was assuming, maybe wrongly, that you
wanted to amplify the signal created by a magnetic-field pickup coil,
from something like 1KHz to 1MHz. If that's the case, you don't want
to load the coil, because that would kill the signal level at higher
frequencies.

If you do connect the coil to a high-impedance amp, like the n.i.
input of an opamp, the opamp input current noise (pretty constant over
frequency) will produce voltage noise across the impedance of the
coil, and the impedance hence noise increase linearly with frequency.

As in all such cases, it would help a lot if you explained the
application, what you're trying to accomplish, and what actual circuit
you propose to use. A schematic (post to a.b.s.e., or a web site)
would help.

John

John Larkin, Apr 9, 2006
8. Guest

John Larkin wrote:
> On 8 Apr 2006 08:32:18 -0700, wrote:
>
> >Sorry for the double reply, but Google Groups isn't displaying this on
> >their site because it associated the title with another post. Hopefully
> >it will appear on deja.com by changing the title.
> >
> >
> >Kevin Aylward wrote:
> >> wrote:
> >> > Hi,

> >
> >> > I'm looking at an op-amp graph of total noise vs source resistance. The
> >> > noise increases with an increase of source resistance. Does this also
> >> > apply to reactance?

> >
> >> > The way I understand it is resistance is thermal noise, but reactance
> >> > from inductors and caps don't caused any noise. Real L's and C's have
> >> > some R, but in that sense they have noise.

> >
> >> > ---------------------------------

> >
> >> > Some details:

> >
> >> > I have an input coil antenna that's 22 ohms R and about 10 mH. Nothing
> >> > fancy, just a round loop coil with a lot of turns. Typical frequencies:
> >> > from 1KHz to 1MHz. At say 1MHz the reactance would be just over 6 M
> >> > ohms. The chart, "Total Noise vs Matched Source Resistance" at

> >
> >
> >> > According to the chart a 6 M ohm source resistance would have
> >> > outrageous noise. Is it safe to say the 6 M ohm reactance (for 1MHz
> >> > signals) will cause no noise? Rather the noise will come from the 22
> >> > ohms R? According to the chart, 22 R at 1KHz is ~ 1 nV/SqrtHz. Not sure
> >> > what it would be at 1 MHz, but it seems lower than 1KHz.

> >
> >> > The charts is for "Matched Source Resistance." The coil is one
> >> > continuous resister, so if I place the 2 coil leads directly to the
> >> > op-amp then is that considered matched? I'm using a typical
> >> > differential op-amp has two input R's. So if the coil's total R is 22
> >> > ohms then would that equate to two 11 ohm input R's? Very confusing,
> >> > lol.

> >
> >> Noise is due to thermal noise *and* shot curent noise. The shot noise
> >> current of an amplifier is droped accross the source impedance (and
> >> Rin). If the source is an inducter, the voltage noise caused by the amp
> >> input current shot noise will increase with frequency despite the fact
> >> that the inductor itself does not generate any noise.

> >
> >> Kevin Aylward B.Sc.
> >>
> >> http://www.anasoft.co.uk
> >> SuperSpice, a very affordable Mixed-Mode
> >> Windows Simulator with Schematic Capture,
> >> Waveform Display, FFT's and Filter Design.

> >
> >> "There are none more ignorant and useless,than they that seek answers
> >> on their knees, with their eyes closed"

> >
> >
> >Let me see if I understand this. A pure inductor cause shot noise, but
> >no thermal noise?

>
> A pure inductance *generates* no noise. That would violate
> conservation of energy.
>
> > How can I calculate a differential op-amp's output
> >noise if the source is mostly inductive? ->
> >http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html
> >

>
> At any given frequency, multiply the opamp's input current noise
> density (in amps per root Hz) by the inductor's impedance; that's the
> input noise voltage density, units of volts per root Hz. Multiply that
> by amp gain at that frequency to get output noise density.

> Looks like you'll need a fet amp here.

That seems to be the consistent answer. Great! I only need to amplify
one polarity, and linearity is of very little concern. Would a simple
Common-Source JFET Amp work best?

Some op-amps have 10^2 input fet resistance. That's nice due to my high
input reactance and you also get some good gain. I'm wondering if that
would also work well for my case?

Given R resistance, X reactance, gain of G, may I ask what the output
voltage noise will be for such a jfet amp? I heard that paralleling
multiple fets would decrease the noise.

Again, thanks for the great help,
Paul

, Apr 9, 2006