Noise of an Instrumentation Amplifier Circuit

[@technics6215]

I really like all these educational materials from Analog Devices and Matt :)

[@analogdevicesinc]

You are welcome and thanks for the positive feedback!

[@MattDuffADI]

Another example of how to calculate noise is on page 18 of the AD8429 datasheet: "Calculating the Noise of the Input Stage". Note that the AD8429 example also includes the Rg noise. To keep it simple, in our AD8226 video, we neglected the Rg noise, since it is typically fairly small compared to the other noise sources. However to have the most accurate estimate, add in the Rg noise as we did in the AD8429 example.

[@damny0utoobe]

Thanks

[@MattDuffADI]

It depends on what you want to do. If you want to compare the noise to your input signal, you want to refer to input. If you want to see how the noise compares to the that of your ADC, you want to refer it to the output. The key thing when doing your calculations is to make sure you are consistent - i.e. don’t calculate using the resistor noise referred to input, but the amplifier voltage noise referred to output.

[@jjcorroon]

Actually thinking about it, referring to the input is more appropriate. We would normally want to refer the noise to the input signal, expressing it as a percentage of full scale deflection. If you put all the gain in the first stage then you don't need to worry about what comes after.

[@MattDuffADI]

Unlike op amps, in amps have this funny property that some of the noise is dependent on gain and some of isn't. We denote these two terms in the datasheet as "eni" and "eno". These terms can be referred to input or output. For example with AD8226 at G=10: Referred to input: eni=22 nV/rt(Hz), eno =12 nV/rt(Hz) Referred to output: eni=220nV/rt(Hz), eno=120 nV/rt(Hz)

[@shaggygoooxide]

Why is the gain of the amplifier not applied to the input noise? I would have expected the input noise contributions to add in quadrature, then gain by 10x which would then add in quadrature with the output noise. What am I missing?

[@XxfieryfirexX]

Hello Sir. How do you limit the bandwidth of an in-amp

[@kishorekumars]

why do u have both input referred noise density as well as output referred noise density for the amplifier?

[@vasilrezachev4517]

Hi Matt! Great series of videos! I have a question regarding really low bandwidth application - our frequency of interest is DC to 10 Hz. However, the 1/f noise is specified from 0.1 Hz to 10 Hz. In this case, for noise calculation do we use only the 1/f noise value or we include the spectral density calculated over the bandwidth (10 Hz) and added to the 1/f noise in RSS fashion? We intend to use AD8422 for our application. Thanks in advance!

[@MattDuffADI]

Hi Vasil, Apologies for the delay in response. The 0.1 Hz to 10Hz number includes both the 1/f noise and white noise. Because the 1/f corner of the AD8422's voltage noise is quite low, its 0.1 Hz to 10 Hz number is caused primarily by the part's white noise, rather than 1/f noise, which is fairly unusual in a non-autozero amplifier. Its 0.1 Hz to 10 Hz current noise number is dominated by the 1/f noise as is typically the case. Matt

[@assadij]

Hi Matt, How would you bias the inputs of an in-amp for a capacitive source while trying to keep the noise down. I have a source with 2nF and frequency response requirement from 5Hz to 2kHz! Resulting in two 31Meg resistors for biasing the inputs. That's 713nV/rtHz per resistor! Thanks in advance.

[@MattDuffADI]

Hi James, While you will indeed get this kind of noise from each resistor, when you have your sensor attached across the in amp inputs, very little of that noise will actually reach in the in amp, especially at higher frequencies. Model the resistor noise as a noise source connected to the following impedance string: 31Meg, 2 nF, 31 Meg. Compute the signal across the 2 nF created by that impedance divider. That should tell you how much noise actually goes into the input from each resistor.

[@assadij]

Hi Matt, Should we multiply the Resistor-Noise (not the eni) by the Gain before calculating the total noise? Thanks.

[@MattDuffADI]

Hi James, all noise sources in this video were calculated referred to the input. What we are saying is that all the noise sources are equivalent to one 38 nV/rt(Hz) source at the input. You can also calculate referred to output, in which case all noise sources would have been 10x bigger and our answer would have been 380 nV/rt(Hz).

[@assadij]

MattDuffADI Thank you Matt.

[@srudeeppatil]

Thanks for your videos...Great work, keep it coming...

[@JoeL-wk5ut]

As for my Britain Master Degree courses, it merelu introduced to us the Voltage Noises,, I even could know what is a current noise and resistor noise up to now, when the couses are nealy about to ends... How a simple courses I learnt but it must be useless when I take a job in market... Sadly I am.

[@jjcorroon]

I didn't get something. Why isn't the eni amplified by the gain and why is the eno divided by the gain. Don't we want to refer all the noise to the output where it will be sampled?

[@ngnews24-tv13]

Please check the 100 fA to nA calculation again. I think its way less than 0.1nA or am I getting something wrong?. I love analog devices. Segun from Nigeria

[@MattDuffADI]

My voiceover could have been clearer. "I want to convert femto to nano" => "I want to change the femtoamps to picoamps so that when I multiply by kiloohms, I get nanovolts." But the math is correct. 100 fA * 25kohm = 2.5 nV. sqrt(2.5nV^2 + 2.5nV^2) = 3.5 nV.

[@Blue.star1]

that's a lot of noise with instrumentation amplifier when compared with an opamp

[@tomcatwild7197]

god bless you master