G<1 for ADA4930

Hi bartok,

When using amplifiers at very low gain (G<=1), stability becomes an issue. Stability is determined by the amplifier of Noise Gain (NG), where NG=1+Rf/Rg. We need enough phase margin of 45 degrees or more at a NG=1 in order for the amplifier to be stable. Looking at Figure 31 of the ADA4930 datasheet, the ADA4930 doesn't have enough phase margin at NG=1. I'm afraid that for your particular application of G<1, ADA4930 is not the best part.

May I know what other specs you are looking for your amplifier? Perhaps ADA4932,which is stable at gains less than 1, can be used instead. ADI also offers funnel amps of fixed attenuation such as AD8275 (G=0.2) and AD8475 (G=0.4,0.8) which are designed for attenuating signals.

A similar question was already posted on EZ. Jonathan Pearson has done a detailed explanation on noise gain and phase margin.

Also, you might be interested to read on this FAQ for further information.

Hope this helps. Good luck!

Regards,

Neil

Thank you for your answer.

But if i add two capacitor (5pf) in parallel of the feedback resistor, it should decrease the bandwith and resolve the problem of the phase margin.

Am i wrong ?

Hi Bartok,

The purpose of adding a feedback cap in stabelizing an amplifier is dependent on the input capacitance.

the RC formed from the output to the input by the feedback resistor and the input capacitance gets

counteracted by the RC formed by the feedback resistor in parallel with the feedback capacitor.

So if you use a large resistor in the feedback that cause d the amplifier to oscillate, by adding a capacitor

in parallel you could make it stable again.

Now if the cause of the instability was something else, like phase margin issues, sometimes the feedback

cap alone can not solve the problem. It will have to be done by making sure the noise gain of the amplifier

is at the stable level.

I hope this helps

Charly

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