Forums

noise in instrumentation amplifier

Started by val November 3, 2016
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
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
On Thursday, November 3, 2016 at 7:27:59 AM UTC-5, val 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
Are you displaying the noise performance for the In-Amp itself or with it connected to the sensor ? How "clean/stable" are the In-Amp power supplies ? If the In-Amp is connected to the sensor, you need to consider the CMRR is affected greatly by the resistor tolerances of the diff amp.
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
On Thu, 3 Nov 2016 06:26:38 -0700 (PDT), gravpoet
<tciacciarelli@gmail.com> wrote:

>On Thursday, November 3, 2016 at 7:27:59 AM UTC-5, val 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 > >Are you displaying the noise performance for the In-Amp itself or with it connected to the sensor ? > >How "clean/stable" are the In-Amp power supplies ? > >If the In-Amp is connected to the sensor, you need to consider the CMRR is affected greatly by the resistor tolerances of the diff amp. > > >
C6 also hurts the CMRR at high frequencies, above roughly 100 KHz. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Le 03/11/2016 &agrave; 15:21, George Herold a &eacute;crit :
> 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.
On Thursday, November 3, 2016 at 12:27:10 PM UTC-4, val wrote:
> Le 03/11/2016 &agrave; 15:21, George Herold a &eacute;crit : > > 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.
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
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
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