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noise in instrumentation amplifier

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  noise in instrumentation amplifier val <a@aa> - 2016-11-03 12:26 +0000
    Re: noise in instrumentation amplifier Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> - 2016-11-03 09:21 -0400
      Re: noise in instrumentation amplifier George Herold <gherold@teachspin.com> - 2016-11-03 07:21 -0700
        Re: noise in instrumentation amplifier val <a@aa> - 2016-11-03 16:25 +0000
          Re: noise in instrumentation amplifier George Herold <gherold@teachspin.com> - 2016-11-03 10:41 -0700
        Re: noise in instrumentation amplifier Winfield Hill <hill@rowland.harvard.edu> - 2016-11-03 10:58 -0700
          Re: noise in instrumentation amplifier George Herold <gherold@teachspin.com> - 2016-11-03 15:36 -0700
            Re: noise in instrumentation amplifier val <a@aa> - 2016-11-04 10:34 +0000
              Re: noise in instrumentation amplifier George Herold <gherold@teachspin.com> - 2016-11-04 06:14 -0700
              Re: noise in instrumentation amplifier bill.sloman@ieee.org - 2016-11-04 20:39 -0700
              Re: noise in instrumentation amplifier val <a@aa> - 2016-11-06 16:16 +0000
          Re: noise in instrumentation amplifier Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> - 2016-11-03 21:30 -0400
      Re: noise in instrumentation amplifier Kevin Glover <kevinfglover@yahoo.com> - 2016-11-04 10:23 -0700
        Re: noise in instrumentation amplifier George Herold <gherold@teachspin.com> - 2016-11-04 10:40 -0700
        Re: noise in instrumentation amplifier Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> - 2016-11-04 18:16 -0400
          Re: noise in instrumentation amplifier George Herold <gherold@teachspin.com> - 2016-11-04 16:10 -0700
        Re: noise in instrumentation amplifier Phil Allison <pallison49@gmail.com> - 2016-11-04 20:43 -0700
        Re: noise in instrumentation amplifier val <a@aa> - 2016-11-05 10:54 +0000
          Re: noise in instrumentation amplifier Phil Allison <pallison49@gmail.com> - 2016-11-05 05:37 -0700
            Re: noise in instrumentation amplifier John S <Sophi.2@invalid.org> - 2016-11-05 09:00 -0500
              Re: noise in instrumentation amplifier Phil Allison <pallison49@gmail.com> - 2016-11-05 18:23 -0700
            Re: noise in instrumentation amplifier val <a@aa> - 2016-11-06 16:11 +0000
          Re: noise in instrumentation amplifier Robert Baer <robertbaer@localnet.com> - 2016-11-07 20:32 -0800
    Re: noise in instrumentation amplifier gravpoet <tciacciarelli@gmail.com> - 2016-11-03 06:26 -0700
      Re: noise in instrumentation amplifier John Larkin <jjlarkin@highlandtechnology.com> - 2016-11-03 09:08 -0700

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#433613 — noise in instrumentation amplifier

Hello all,

I have a basic instrumentation amplifier made with three MAX 44245 op amps.
My problem is that I have too much noise in the 50 to 100 kHz bandwidth and
I don't know how to reduce it. Maybe I don't have enough phase margin
around 100 kHz (the GBW product of the 44245 is 1 MHz and the gain of the
first stage of the instr. amplifier is about 10). However, the data sheet
says that the MAX 44245 is unity gain stable so I wonder what I did wrong
to get these small oscillations.

Do you have any idea on how this noise could be reduced without altering
the 3 dB bandwidth of 50 kHz ?

Thank you for any help !

val

PS. I put a schematics here : http://www.smacc.fr/download/IA.pdf

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#433617

On Thu, 03 Nov 16 12:26:32 +0000, the renowned val <a@aa> wrote:

>Hello all,
>
>I have a basic instrumentation amplifier made with three MAX 44245 op amps.
>My problem is that I have too much noise in the 50 to 100 kHz bandwidth and
>I don't know how to reduce it. Maybe I don't have enough phase margin
>around 100 kHz (the GBW product of the 44245 is 1 MHz and the gain of the
>first stage of the instr. amplifier is about 10). However, the data sheet
>says that the MAX 44245 is unity gain stable so I wonder what I did wrong
>to get these small oscillations.
>
>Do you have any idea on how this noise could be reduced without altering
>the 3 dB bandwidth of 50 kHz ?
>
>Thank you for any help !
>
>val
>
>PS. I put a schematics here : http://www.smacc.fr/download/IA.pdf


If you look at the datasheet, that op-amp has a strong peak of noise
around 40kHz (130nV/rtHz) vs. less than 50nV/rtHz at 1kHz (still
pretty noisy). You have a gain of what, 20, or something like that and
don't forget noise is RMS, so P-P will be many times higher (maybe 6
or 8x) 

Maybe try another op-amp. Your resistor values do seem a bit low for a
90uA op-amp. 

--sp 


-- 
Best regards, 
Spehro Pefhany
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8

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#433625

On Thursday, November 3, 2016 at 9:21:55 AM UTC-4, Spehro Pefhany wrote:
> On Thu, 03 Nov 16 12:26:32 +0000, the renowned val <a@aa> wrote:
> 
> >Hello all,
> >
> >I have a basic instrumentation amplifier made with three MAX 44245 op amps.
> >My problem is that I have too much noise in the 50 to 100 kHz bandwidth and
> >I don't know how to reduce it. Maybe I don't have enough phase margin
> >around 100 kHz (the GBW product of the 44245 is 1 MHz and the gain of the
> >first stage of the instr. amplifier is about 10). However, the data sheet
> >says that the MAX 44245 is unity gain stable so I wonder what I did wrong
> >to get these small oscillations.
> >
> >Do you have any idea on how this noise could be reduced without altering
> >the 3 dB bandwidth of 50 kHz ?
> >
> >Thank you for any help !
> >
> >val
> >
> >PS. I put a schematics here : http://www.smacc.fr/download/IA.pdf
> 
> 
> If you look at the datasheet, that op-amp has a strong peak of noise
> around 40kHz (130nV/rtHz) vs. less than 50nV/rtHz at 1kHz (still
> pretty noisy). You have a gain of what, 20, or something like that and
> don't forget noise is RMS, so P-P will be many times higher (maybe 6
> or 8x) 
> 
> Maybe try another op-amp. Your resistor values do seem a bit low for a
> 90uA op-amp. 
> 
> --sp 
> 
50 nV/rtHz, Yuck!  And they call it a low noise opamp.  
If it's a strain gauge or something you don't really need a low bias current opamp.  
You could even try an instrument amp.  I use a lot of AD620's but the noise is not 
that good ~10nV/rtHz.  There is a nice list of Int. amps on page 363 of AoE3.  

So I agree with Spehro, try another IC.  

George H.   
> 
> -- 
> Best regards, 
> Spehro Pefhany
> Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8

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#433655

Le 03/11/2016 à 15:21, George Herold a écrit :
> On Thursday, November 3, 2016 at 9:21:55 AM UTC-4, Spehro Pefhany wrote:
>> On Thu, 03 Nov 16 12:26:32 +0000, the renowned val <a@aa> wrote:
>> 
>> >Hello all,
>> >
>> >I have a basic instrumentation amplifier made with three MAX 44245 op
>> amps.
>> >My problem is that I have too much noise in the 50 to 100 kHz bandwidth
>> and
>> >I don't know how to reduce it. Maybe I don't have enough phase margin
>> >around 100 kHz (the GBW product of the 44245 is 1 MHz and the gain of the
>> >first stage of the instr. amplifier is about 10). However, the data sheet
>> >says that the MAX 44245 is unity gain stable so I wonder what I did wrong
>> >to get these small oscillations.
>> >
>> >Do you have any idea on how this noise could be reduced without altering
>> >the 3 dB bandwidth of 50 kHz ?
>> >
>> >Thank you for any help !
>> >
>> >val
>> >
>> >PS. I put a schematics here : http://www.smacc.fr/download/IA.pdf
>> 
>> 
>> If you look at the datasheet, that op-amp has a strong peak of noise
>> around 40kHz (130nV/rtHz) vs. less than 50nV/rtHz at 1kHz (still
>> pretty noisy). You have a gain of what, 20, or something like that and
>> don't forget noise is RMS, so P-P will be many times higher (maybe 6
>> or 8x) 
>> 
>> Maybe try another op-amp. Your resistor values do seem a bit low for a
>> 90uA op-amp. 
>> 
>> --sp 
>> 
> 50 nV/rtHz, Yuck!  And they call it a low noise opamp.  
> If it's a strain gauge or something you don't really need a low bias current
> opamp.  
> You could even try an instrument amp.  I use a lot of AD620's but the noise
> is not 
> that good ~10nV/rtHz.  There is a nice list of Int. amps on page 363 of
> AoE3.  
> 
> So I agree with Spehro, try another IC.  
> 
> George H.   
>> 
>> -- 
>> Best regards, 
>> Spehro Pefhany
>> Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8
> 

Thank you for all the advices. Effectively it seems that the opamp is very
noisy and I was not looking in the right direction. If I do the calculation
I get about 1.5mV which is close to the measure I got.

In fact I was using the OP462 before and the noise was much lower, about
0.3 mV rms. I moved to the MAX 44245 because it has a very good EMI
rejection Ratio (like 60 dB at 100 MHz). With the OP462 I saw DC
rectification above 100 MHz. I need to find a low power op amp with decent
noise and good EMI rejection.

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#433673

On Thursday, November 3, 2016 at 12:27:10 PM UTC-4, val wrote:
> Le 03/11/2016 à 15:21, George Herold a écrit :
> > On Thursday, November 3, 2016 at 9:21:55 AM UTC-4, Spehro Pefhany wrote:
> >> On Thu, 03 Nov 16 12:26:32 +0000, the renowned val <a@aa> wrote:
> >> 
> >> >Hello all,
> >> >
> >> >I have a basic instrumentation amplifier made with three MAX 44245 op
> >> amps.
> >> >My problem is that I have too much noise in the 50 to 100 kHz bandwidth
> >> and
> >> >I don't know how to reduce it. Maybe I don't have enough phase margin
> >> >around 100 kHz (the GBW product of the 44245 is 1 MHz and the gain of the
> >> >first stage of the instr. amplifier is about 10). However, the data sheet
> >> >says that the MAX 44245 is unity gain stable so I wonder what I did wrong
> >> >to get these small oscillations.
> >> >
> >> >Do you have any idea on how this noise could be reduced without altering
> >> >the 3 dB bandwidth of 50 kHz ?
> >> >
> >> >Thank you for any help !
> >> >
> >> >val
> >> >
> >> >PS. I put a schematics here : http://www.smacc.fr/download/IA.pdf
> >> 
> >> 
> >> If you look at the datasheet, that op-amp has a strong peak of noise
> >> around 40kHz (130nV/rtHz) vs. less than 50nV/rtHz at 1kHz (still
> >> pretty noisy). You have a gain of what, 20, or something like that and
> >> don't forget noise is RMS, so P-P will be many times higher (maybe 6
> >> or 8x) 
> >> 
> >> Maybe try another op-amp. Your resistor values do seem a bit low for a
> >> 90uA op-amp. 
> >> 
> >> --sp 
> >> 
> > 50 nV/rtHz, Yuck!  And they call it a low noise opamp.  
> > If it's a strain gauge or something you don't really need a low bias current
> > opamp.  
> > You could even try an instrument amp.  I use a lot of AD620's but the noise
> > is not 
> > that good ~10nV/rtHz.  There is a nice list of Int. amps on page 363 of
> > AoE3.  
> > 
> > So I agree with Spehro, try another IC.  
> > 
> > George H.   
> >> 
> >> -- 
> >> Best regards, 
> >> Spehro Pefhany
> >> Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8
> > 
> 
> Thank you for all the advices. Effectively it seems that the opamp is very
> noisy and I was not looking in the right direction. If I do the calculation
> I get about 1.5mV which is close to the measure I got.
> 
> In fact I was using the OP462 before and the noise was much lower, about
> 0.3 mV rms. I moved to the MAX 44245 because it has a very good EMI
> rejection Ratio (like 60 dB at 100 MHz). With the OP462 I saw DC
> rectification above 100 MHz. I need to find a low power op amp with decent
> noise and good EMI rejection.

OK, I've never had EMI getting into the front end.  The "word on the street" 
(that I got from Joerg) is that only bjt opamps will have that problem.  
So ignore what I said before and get an FET or Cmos opamp.  
(They are typically a little noisier, but something less than 
10 nV is not hard to find.)  

George H.

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#433686

George Herold wrote...
> Spehro Pefhany wrote:
>> On Thu, 03 Nov 16, the renowned val <a@aa> wrote:
>> 
>>> I have a basic instrumentation amplifier made with
>>> three MAX 44245 op amps.  >My problem is that I have
>>> too much noise in the 50 to 100 kHz bandwidth ...
>> 
>> If you look at the datasheet, that op-amp has a strong
>> peak of noise around 40kHz (130nV/rtHz) vs. less than
>> 50nV/rtHz at 1kHz (still pretty noisy).  ...
> 
> 50 nV/rtHz, Yuck!  And they call it a low noise opamp.

 Did y'all notice it's an auto-zero amplifier, to get
 its 7.5uV Vos max spec?  That means it has input
 switches running, with charge injection, etc.


-- 
 Thanks,
    - Win

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#433743

On Thursday, November 3, 2016 at 1:58:58 PM UTC-4, Winfield Hill wrote:
> George Herold wrote...
> > Spehro Pefhany wrote:
> >> On Thu, 03 Nov 16, the renowned val <a@aa> wrote:
> >> 
> >>> I have a basic instrumentation amplifier made with
> >>> three MAX 44245 op amps.  >My problem is that I have
> >>> too much noise in the 50 to 100 kHz bandwidth ...
> >> 
> >> If you look at the datasheet, that op-amp has a strong
> >> peak of noise around 40kHz (130nV/rtHz) vs. less than
> >> 50nV/rtHz at 1kHz (still pretty noisy).  ...
> > 
> > 50 nV/rtHz, Yuck!  And they call it a low noise opamp.
> 
>  Did y'all notice it's an auto-zero amplifier, to get
>  its 7.5uV Vos max spec?  That means it has input
>  switches running, with charge injection, etc.
Nope, but that perhaps explains the 30kHz noise peak.  

To the OP; The few times I played around with making my own 
three opamp Instrument Amp. (IA) I found that getting the 
CMRR respectable was a matter of resistor/impedance tweaking... 
expensive except for a one of. 

Better to buy someone's else's laser trimmed IA. 

Do you have a copy of AoE3? There's a wonderful
section on IA's.  I wish I'd seen the 
CMRR graph vs frequency before a previous project.  
That alone is worth the price of admission, IMHO. 

Why is there so much EMI?  
Can you shield it more?  
What 'cha doing?  
 
George H.  
> 
> 
> -- 
>  Thanks,
>     - Win

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#433824

Le 03/11/2016 à 23:36, George Herold a écrit :

> Nope, but that perhaps explains the 30kHz noise peak.  
> 
> To the OP; The few times I played around with making my own 
> three opamp Instrument Amp. (IA) I found that getting the 
> CMRR respectable was a matter of resistor/impedance tweaking... 
> expensive except for a one of. 
> 
> Better to buy someone's else's laser trimmed IA. 
> 
> Do you have a copy of AoE3? There's a wonderful
> section on IA's.  I wish I'd seen the 
> CMRR graph vs frequency before a previous project.  
> That alone is worth the price of admission, IMHO. 
> 
> Why is there so much EMI?  
> Can you shield it more?  
> What 'cha doing?  
>  
> George H.  
>> 
>> 
>> -- 
>>  Thanks,
>>     - Win
> 

 I'm impressed by how much support you can get here. Thank you all !

I didn't knew that AoE3 was available now, I'm still on AoE2, but not for
long ;)

What I'm trying to do is an Hall effect probe. Maybe I should try another
IC like LMC660 (CMOS input) as suggested by George. I remember once it did
the job in replacing a LT1014 for better EMI protection in a similar
product.

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#433832

On Friday, November 4, 2016 at 6:36:12 AM UTC-4, val wrote:
> Le 03/11/2016 à 23:36, George Herold a écrit :
> 
> > Nope, but that perhaps explains the 30kHz noise peak.  
> > 
> > To the OP; The few times I played around with making my own 
> > three opamp Instrument Amp. (IA) I found that getting the 
> > CMRR respectable was a matter of resistor/impedance tweaking... 
> > expensive except for a one of. 
> > 
> > Better to buy someone's else's laser trimmed IA. 
> > 
> > Do you have a copy of AoE3? There's a wonderful
> > section on IA's.  I wish I'd seen the 
> > CMRR graph vs frequency before a previous project.  
> > That alone is worth the price of admission, IMHO. 
> > 
> > Why is there so much EMI?  
> > Can you shield it more?  
> > What 'cha doing?  
> >  
> > George H.  
> >> 
> >> 
> >> -- 
> >>  Thanks,
> >>     - Win
> > 
> 
>  I'm impressed by how much support you can get here. Thank you all !

Grin.. Well Welcome to SED.  It's a pretty good bunch of guys (and a few nice gals.)  

George H. 
> 
> I didn't knew that AoE3 was available now, I'm still on AoE2, but not for
> long ;)
> 
> What I'm trying to do is an Hall effect probe. Maybe I should try another
> IC like LMC660 (CMOS input) as suggested by George. I remember once it did
> the job in replacing a LT1014 for better EMI protection in a similar
> product.

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#433943

On Friday, November 4, 2016 at 9:36:12 PM UTC+11, val wrote:
> Le 03/11/2016 à 23:36, George Herold a écrit :
> 
> > Nope, but that perhaps explains the 30kHz noise peak.  
> > 
> > To the OP; The few times I played around with making my own 
> > three opamp Instrument Amp. (IA) I found that getting the 
> > CMRR respectable was a matter of resistor/impedance tweaking... 
> > expensive except for a one of. 
> > 
> > Better to buy someone's else's laser trimmed IA. 
> > 
> > Do you have a copy of AoE3? There's a wonderful
> > section on IA's.  I wish I'd seen the 
> > CMRR graph vs frequency before a previous project.  
> > That alone is worth the price of admission, IMHO. 
> > 
> > Why is there so much EMI?  
> > Can you shield it more?  
> > What 'cha doing?  
> >  
> > George H.  
> >> 
> >> 
> >> -- 
> >>  Thanks,
> >>     - Win
> > 
> 
>  I'm impressed by how much support you can get here. Thank you all !
> 
> I didn't knew that AoE3 was available now, I'm still on AoE2, but not for
> long ;)
> 
> What I'm trying to do is an Hall effect probe. Maybe I should try another
> IC like LMC660 (CMOS input) as suggested by George. I remember once it did
> the job in replacing a LT1014 for better EMI protection in a similar
> product.

The classic way of doing well with a Hall probe is to AC excite it and use phase sensitive detection to get your output. 

There's a lot of literature on AC-excited bridges, lock-in amplifiers and the like. The Art of Electronics mentions them in passing quite often.

You can generate your signal at a frequency where your amplifiers are relatively quiet - above the 1/f noise corner - and amplify it as AC before demodulation and low pass filtering.

You can band-pass filter the amplified signal before you demodulate it, but you do have to be careful to keep the phase shift in the bandpass filter around the modulation frequency both predictable and frequency-insensitive - you will have to allow for it.

-- 
Bill Sloman, Sydney

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#434070

> Le 03/11/2016 à 23:36, George Herold a écrit :
> 
>> Nope, but that perhaps explains the 30kHz noise peak.  
>> 

I have tested with OP462 10nV/(Hz)1/2 and the noise in now down to 0.3mV on
the scope, in agreement with the calculation.

>> To the OP; The few times I played around with making my own 
>> three opamp Instrument Amp. (IA) I found that getting the 
>> CMRR respectable was a matter of resistor/impedance tweaking... 
>> expensive except for a one of. 
>> 
>> Better to buy someone's else's laser trimmed IA. 
>> 
>> Do you have a copy of AoE3? 

I now have one since Friday. Bedtime reading for weeks !

>>There's a wonderful
>> section on IA's.  I wish I'd seen the 
>> CMRR graph vs frequency before a previous project.  
>> That alone is worth the price of admission, IMHO. 
>> 
>> Why is there so much EMI?  

There is not, I just want to make it less sensitive to EMI perturbations.
So I use a small colpitts oscillator tuned to 150 MHz to test. It seems
that often it is at this frequency that you have a lot of dc rectification
from the op amps.

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#433774

On 3 Nov 2016 10:58:37 -0700, the renowned Winfield Hill
<hill@rowland.harvard.edu> wrote:

>
> Did y'all notice it's an auto-zero amplifier, to get
> its 7.5uV Vos max spec?  That means it has input
> switches running, with charge injection, etc.

Didn't notice that- I guess that accounts for the noise hump. EMI
seems to be an issue with some of the chopper amps, though this is one
of the ones that claims to have filters: 

"These devices also feature integrated EMI filters
to reduce high-frequency signal demodulation on the
output" 

There are actually some out there which auto-zero ONLY on power-on.
Once. Scary stuff. 

--sp 


-- 
Best regards, 
Spehro Pefhany
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8

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#433872

On Thu, 03 Nov 2016 09:21:52 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Thu, 03 Nov 16 12:26:32 +0000, the renowned val <a@aa> wrote:
>
>>Hello all,
>>
>>I have a basic instrumentation amplifier made with three MAX 44245 op amps.
>>My problem is that I have too much noise in the 50 to 100 kHz bandwidth and
>>I don't know how to reduce it. Maybe I don't have enough phase margin
>>around 100 kHz (the GBW product of the 44245 is 1 MHz and the gain of the
>>first stage of the instr. amplifier is about 10). However, the data sheet
>>says that the MAX 44245 is unity gain stable so I wonder what I did wrong
>>to get these small oscillations.
>>
>>Do you have any idea on how this noise could be reduced without altering
>>the 3 dB bandwidth of 50 kHz ?
>>
>>Thank you for any help !
>>
>>val
>>
>>PS. I put a schematics here : http://www.smacc.fr/download/IA.pdf
>
>
>If you look at the datasheet, that op-amp has a strong peak of noise
>around 40kHz (130nV/rtHz) vs. less than 50nV/rtHz at 1kHz (still
>pretty noisy). You have a gain of what, 20, or something like that and
>don't forget noise is RMS, so P-P will be many times higher (maybe 6
>or 8x) 
>
I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
1.414 * RMS.

>Maybe try another op-amp. Your resistor values do seem a bit low for a
>90uA op-amp. 
>
>--sp 

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#433875

On Friday, November 4, 2016 at 1:23:46 PM UTC-4, Kevin Glover wrote:
> On Thu, 03 Nov 2016 09:21:52 -0400, Spehro Pefhany
> <speffSNIP@interlogDOTyou.knowwhat> wrote:
> 
> >On Thu, 03 Nov 16 12:26:32 +0000, the renowned val <a@aa> wrote:
> >
> >>Hello all,
> >>
> >>I have a basic instrumentation amplifier made with three MAX 44245 op amps.
> >>My problem is that I have too much noise in the 50 to 100 kHz bandwidth and
> >>I don't know how to reduce it. Maybe I don't have enough phase margin
> >>around 100 kHz (the GBW product of the 44245 is 1 MHz and the gain of the
> >>first stage of the instr. amplifier is about 10). However, the data sheet
> >>says that the MAX 44245 is unity gain stable so I wonder what I did wrong
> >>to get these small oscillations.
> >>
> >>Do you have any idea on how this noise could be reduced without altering
> >>the 3 dB bandwidth of 50 kHz ?
> >>
> >>Thank you for any help !
> >>
> >>val
> >>
> >>PS. I put a schematics here : http://www.smacc.fr/download/IA.pdf
> >
> >
> >If you look at the datasheet, that op-amp has a strong peak of noise
> >around 40kHz (130nV/rtHz) vs. less than 50nV/rtHz at 1kHz (still
> >pretty noisy). You have a gain of what, 20, or something like that and
> >don't forget noise is RMS, so P-P will be many times higher (maybe 6
> >or 8x) 
> >
> I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
> 1.414 * RMS.

0.707?  But that is for sine waves.  With noise it's different.  
In theory if you wait long enough you can get an arbitrarily large peak
voltage.  

George H.  
> 
> >Maybe try another op-amp. Your resistor values do seem a bit low for a
> >90uA op-amp. 
> >
> >--sp

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#433911

On Fri, 04 Nov 2016 10:23:41 -0700, the renowned Kevin Glover
<kevinfglover@yahoo.com> wrote:

>On Thu, 03 Nov 2016 09:21:52 -0400, Spehro Pefhany
><speffSNIP@interlogDOTyou.knowwhat> wrote:

>>
>I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
>1.414 * RMS.

Yes, p-p is 2.8x RMS for a pure sine wave. 

For Gaussian noise it should be less than 8x RMS p-p 98% of the time,
but in theory it's unbounded. 

--sp 

-- 
Best regards, 
Spehro Pefhany
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8

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#433919

On Friday, November 4, 2016 at 6:17:02 PM UTC-4, Spehro Pefhany wrote:
> On Fri, 04 Nov 2016 10:23:41 -0700, the renowned Kevin Glover
> <kevinfglover@yahoo.com> wrote:
> 
> >On Thu, 03 Nov 2016 09:21:52 -0400, Spehro Pefhany
> ><speffSNIP@interlogDOTyou.knowwhat> wrote:
> 
> >>
> >I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
> >1.414 * RMS.
> 
> Yes, p-p is 2.8x RMS for a pure sine wave. 
> 
> For Gaussian noise it should be less than 8x RMS p-p 98% of the time,
> but in theory it's unbounded. 

Grin, It's not all that hard to see... Gain up a noise source to 
some reasonable ~1V rms value and watch how often it rails your 
+/- 15V opamp. 

George H.


> 
> --sp 
> 
> -- 
> Best regards, 
> Spehro Pefhany
> Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8

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#433944

Kevin Glover wrote:


>
> I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
> 1.414 * RMS.
> 

** That ratio is only for steady amplitude sine waves. 

Various ratios apply for other defined and steady waveforms, like triangle, half sine etc. But for a random noise signals, it must be measured to find the average rectified or RMS value that applies to the particular noise. The measurement bandwidth of the test instrument will affect the result in many cases too. 

If the noise has low frequency components, the reading on a meter will vary so one takes a visual average.

....   Phil 

 

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#433959

Le 04/11/2016 à 18:23, Kevin Glover a écrit :
>>
> I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
> 1.414 * RMS.
> 

There is a good video on conversion from rms noise to peak-to-peak values
here :

https://www.youtube.com/watch?v=-KcODSYXiZA

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#433969

val wrote:

>
> >>
> > I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
> > 1.414 * RMS.
> > 
> 
> There is a good video on conversion from rms noise to peak-to-peak values
> here :
> 
> https://www.youtube.com/watch?v=-KcODSYXiZA
>


** As if a novice like you knows what a "good" vid on the topic was. 

 That is not an informative vid, it is way misleading. 

 

....  Phil 

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#433982

On 11/5/2016 7:37 AM, Phil Allison wrote:
> val wrote:
>
>>
>>>>
>>> I thought RMS was 70.7 of peak. Converting RMS to peak is the inverse,
>>> 1.414 * RMS.
>>>
>>
>> There is a good video on conversion from rms noise to peak-to-peak values
>> here :
>>
>> https://www.youtube.com/watch?v=-KcODSYXiZA
>>
>
>
> ** As if a novice like you knows what a "good" vid on the topic was.
>
>  That is not an informative vid, it is way misleading.
>
>
>
> ....  Phil
>

It seemed to make sense to me. What did you find misleading about it?

John

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