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INA118 The input noise issues

Mickey Zhang
Genius 12080 points
Other Parts Discussed in Thread: INA118, TINA-TI

Hi Team,

The customer is using INA118.  He needs to assess SNR of his system when the frequency is less than 5HZ.

Q1: Look the attachment.  Is the parameter information the input voltage noise density and the current noise density for the 6.5 Electrical Characteristics of the data sheet?

Q2: If it is the input voltage noise density and the current noise density,  how much is the 1/fcorner for the input voltage noise  and the current noise ?

Q3: How to calculate the totla input voltage noise? 

If VN is the input voltage noise density, Iin is  the current noise density, RS is equivalent source resistance , fci is the frequency for the the current noise and fcv is  the frequency for the input voltage noise,  the total input voltage noise= root {[Vin*(1+fcv/f)]^2+0.5*[Iin(1+fci/f)*RS]^2}?

Thanks,
Best Regards,
  • 0
    TI__Mastermind 25825 points

    Hello Mickey,

    The specifications whose units are 'nV/rtHz' Or 'pA/rtHz' are in fact noise spectral densities.    What is given in the data sheet is also found in Figure 9 of the INA118 data sheet, which is shown below.  You will see both the 1/f and broadband noise in Figure 9.

    In order to calculate the noise, please watch the TIPL series on Noise.  Here is the link:  http://www.ti.com/precisionlabs

    There is also an excellent presentation on instrumentation amplifier noise located here:

    https://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/3514.instrumentation-amplifier-noise

  • 0 in reply to Pete%20Semig
    TI__Genius 12080 points
    Hi Pete,
    Thank you for your reply.
    Q1: From Figure 9, is the 1/fcorner for the input voltage noise 600HZ? Is the 1/fcorner for the input current noise 50HZ?

    Q2: Is the formula correct, that is the total input voltage noise= root {[Vin*(1+fcv/f)]^2+0.5*[Iin(1+fci/f)*RS]^2}?
  • 0 in reply to Pete%20Semig
    TI__Genius 12080 points
    Hi Pete,
    Can you see my reply? The customer is waiting for my reply.
  • 0 in reply to Mickey Zhang
    TI__Mastermind 25825 points

    Hello Mickey,

    I'm not quite sure where you obtained that equation or the corner frequencies. At first glance it looks like the units of your equation do not work out. The units for fcv/f and fci/f will cancel. Then you will be left with root((nV/rtHz)^2+0.5*(pA/rtHz)^2). In some way you then end up with something like (nV)/rtHz. This is discussed in the TIPL series I linked to (Lecture #2, slide 6).  

    Have you had a chance to look at the INA noise presentation I linked to? You need to take into account both the broadband and 1/f noise in the calculations in addition to external resistor noise, noise from a reference buffer (if included), etc. This is shown by slide 22 in the presentation.  So, to help you perform a proper noise analysis please provide me with the customer's schematic.  You can look me up in the TI directory and e-mail me directly. The schematic will tell us the gain and help us understand what resistors are around the device. We can then try performing a noise analysis in TINA-TI and compare it with the simulated results.