# How to calculate input referred noise of lossy integrator?

1. ## How to calculate input referred noise of lossy integrator?

Inserted picture is the lossy-integrator in my circuit. To precisely run Simulink, I would like to calculate input referred noise for this lossy integrator, but I just don't know how to calculate.

Can anyone help me to calculate the input referred noise please?

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2. ## Re: How to calculate input referred noise of lossy integrator?

WHy do you have positive feedback? and why do you want a lossy integrator? How do you dump the charge to initialize your integrator?

Look forward to seeing a well thought written Functional Requirement of your circuit before you re-design it.

3. ## Re: How to calculate input referred noise of lossy integrator?

There's no positive feedback. It's a true differential amplifier with regular negative feedback. Just the same as a "doubled" single ended OP circuit.

To calulate input referred noise, you need the amplifier noise parameters and gain.

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4. ## Re: How to calculate input referred noise of lossy integrator?

SunnySkyguy// I don't know what you are talking about. It DOES NOT have positive feedback, and lossy integrator is used to to implement a first-order low-pass filter. It is just a fully-diff amp.

FvM// Yes, I know the amplifier noise parameter (spot noise) and gain. Could you give me any formula or at least starting point of input-referred noise calculation please?

5. ## Re: How to calculate input referred noise of lossy integrator?

I missed the tiny "-" on fuzzy image .. Sorry differential output. I was wrong I was looking at R1 + input. oops (obvious now)

Refer to Friis formula on wiki

Note: TI has Fully-differential op-amps with an extra input pin (Vocm).
The purpose of this pin is to provide a place to input a potentially noisy signal that will appear simultaneously on both inputs
– i.e. common mode noise. The fully-differential op-amp can then reject the common mode noise.

The Guard pin (new). http://www.ti.com/lit/an/sloa064/sloa064.pdf
The VOCM pin can be connected to a data converter reference voltage pin to achieve tight tracking
between the op-amp common mode voltage and the data converter common mode voltage. In
this application, the data converter also provides a free dc level conversion for single supply
circuits. The common mode voltage of the data converter is also the dc operating point of the
single-supply circuit. The designer should take care, however, that the dc operating point of the
circuit is within the common mode range of the op-amp + and – inputs. This can most easily be
achieved by summing a dc level into the inputs equal or close to the common mode voltage.

Is this for biomedical? If so, body has very high CM AC hum. or similar issue form SMPS nearby.

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6. ## Re: How to calculate input referred noise of lossy integrator?

Fully differentially amplifiers are mainly used for wideband applications, possibly as differential drivers at lower frequencies, e.g. for high performance audio or instrumentation ADCs with differential input.

The usual starting point for noise calculation is an equivalent circuit with noise sources. For an estimation, you can assume an ideal amplifier (infinite gain). The calulation is presented in most analog design text books.

7. ## Re: How to calculate input referred noise of lossy integrator?

My textbook answer who's name I did not know but looked up in post #5 is the Friis formula.

But there are design formulae. If you dont need fully differential output, dont use it.

In terms of Noise Temperature which I remember using for a low noise VLF 14~21KHz receiver in 1975 used this.

click>> I defer to the analog experts at TI for the optimization methods with source impedance and the math to go with it.

Cutting to the chase... I extract their conclusions

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