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# OPA627: Can we use Input voltage noise to calculate the total noise?

Part Number: OPA627

Hi,

In TI Precision Lab Training about OP AMP noise part3,

voltage noise is divided into 1/f noise and broadband noise.

1/f noise is calculated with a complex equation.

Can we use the parameter offered in the datasheet "input voltage noise BW = 0.1 Hz to 10 Hz" to replace 1/f noise to calculate the total noise since 0.1Hz to 10Hz should be low frequency?

And for the resistor noise, I don't understand why we use R1//RF to calculate the noise.

What's wrong for calculating R1 noise, Rf noise independently and add them together?

• Hi Justifice,

1. voltage noise is divided into 1/f noise and broadband noise.

Correct- that's why for high bandwidth applications 1/f noise contribution may be neglected.

2. 1/f noise is calculated with a complex equation.

The 1/f noise equation is a square-root function as shown below.

3. Can we use the parameter offered in the datasheet "input voltage noise BW = 0.1 Hz to 10 Hz" to replace 1/f noise to calculate the total noise since 0.1Hz to 10Hz should be low frequency?

1/f may be neglected for high bandwidth application BUT otherwise cannot be replace with number given in datasheet UNLESS the actual bandwidth is from 0.1Hz to 10Hz.

4. And for the resistor noise, I don't understand why we use R1//RF to calculate the noise.

In order to find the input referred noise, you need to first find input equivalent resistor value (Rin_eq). Then you find output equivalent by multiplying it by noise gain.

5. What's wrong for calculating R1 noise, Rf noise independently and add them together?

R1 noise gets gained up by G=(RF/R1) but RF is NOT gained up.  That is why you must first find the equivalent input resistor noise and only then multiple it by the noise gain.

• Justifice,

I agree with everything that Marek is saying.  Here is some extra detail on why the noise RTI is the parallel combination of the feedback resistors.  The analysis below shows why noise RTI is the parallel combination of the feedback resistors.  The derivation shows how you can use superposition to analyze each noise source independently.  The final result of the derivation is that the noise RTI is the parallel combination of Rf and Rg.

noise RTI for non-inverting amp.pptx

Best regards,

Art