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# Correlated current noise

Category: Datasheet/Specs

Hello!

From my observations, almost all modern low voltage noise bipolar opamps have some sort of bias current compensation.

If the opamp input sees an impedance greater than zero, the product of the impedance and the noise current adds an additional noise voltage to the circuit.

Because bias current compensated opamps have additional transistors connected to the input pins (those of the compensation current sources) those current noise adds to the current noise of the input transistors.

This can clearly be seen in the datasheet. So far so so good.

Some literature states, that the noise current of bias compensated opamps is at least partly correlated and this leads to a noise penalty, if the impedances seen by the opamp inputs are not equal.

Only a few datasheets (e.g. AD8597) differentiate between corraleted and uncorrelated current noise.

I have the following questions:

Is it correct, that current noise of bias compensated opamps is (at least partly) correlated?

If yes, is there a rule of thumb, how much is correlated?

Does this depend on the implementation of the compensation?

A standard high level noise model of an opamp uses two uncorrelated current noise sources. Is the potential current noise correlation usually considered in the spice models?

Best regards

## Top Replies

• Good day. Often, bias-compensated op-amps have their current noise correlated. From a design perspective, the amount of correlation depends on how the compensation was implemented. Datasheets…

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• Hi  ,

Good day. Often, bias-compensated op-amps have their current noise correlated. From a design perspective, the amount of correlation depends on how the compensation was implemented. Datasheets like AD8597 differentiate between the two. But, most datasheets, do not show the correlated noise value. However, these values are very small to be a problem and it is safe to assume that op-amp noise currents are uncorrelated.

The op-amp current noise model is designed to show the device's behavior as the datasheet dictates. Hence, regardless of the correlation, the model will show you the expected performance of the device as shown on its TPC. This does not show how much is the correlation but this gives you an overview of the current noise performance of the part. Hope this helps.

Regards,
Gilbeys

• Hi  ,

Good day. Often, bias-compensated op-amps have their current noise correlated. From a design perspective, the amount of correlation depends on how the compensation was implemented. Datasheets like AD8597 differentiate between the two. But, most datasheets, do not show the correlated noise value. However, these values are very small to be a problem and it is safe to assume that op-amp noise currents are uncorrelated.

The op-amp current noise model is designed to show the device's behavior as the datasheet dictates. Hence, regardless of the correlation, the model will show you the expected performance of the device as shown on its TPC. This does not show how much is the correlation but this gives you an overview of the current noise performance of the part. Hope this helps.

Regards,
Gilbeys

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