[time-nuts] Opamp datasheet noise specs and their relation to phase noise

Charles Steinmetz csteinmetz at yandex.com
Fri Mar 6 16:01:01 EST 2015


Stephan wrote:

>Opamp noise is usually specified in its datasheets as input voltage noise,
>V/sqrt(Hz), and input current noise, A/sqrt(Hz) versus frequency, Hz.

Actually, those are the voltage and current noise *densities*.  I'm 
not trying to be gratuitously picky, it's just that casual designers' 
understanding of noise, and their noise calculations, often come to 
grief because of just this kind of confusion.

>Is it possible to estimate the opamp's phase noise at a specific frequency
>(say a sinusoid at 10MHz or 100MHz) from these curves?

No, not really, because:

>I'm assuming it can be seen as amplitude noise that is converted to phase
>noise.

Correct.  Some of the output noise of the amplifier is converted to 
phase noise "directly" -- meaning, any instantaneous measurement of 
the sine wave has an uncertainty in both amplitude and time due to 
the added noise.  BUT this is not the main AM to PM conversion problem.

The input noise (typically, but not exclusively at baseband rather 
than at the RF frequency) also modulates the operating parameters of 
the amplifier itself (typically, but not exclusively by modulating 
internal device capacitances), causing the amplifier's frequency 
response at RF to be modulated in synchronism with the noise -- which 
modulates the phase of the RF being amplified or processed by the 
amplifier.  This process is different for every amplifier topology, 
so there is no way to calculate phase noise from the amplifier noise 
specifications without much, much more information (specifically, a 
very detailed model of the amplifier based on very detailed models of 
its component parts).  It needs to be measured.  [There may be 
nonlinear modeling software capable of making a first approximation, 
but I'm not aware of any.]

Since the AM to PM conversion is most troublesome at baseband, PN is 
reduced by using devices with low noise at low frequencies 
(especially flicker or 1/f noise) and by making the internal 
amplifier gain low at low frequencies, for example by shunting low 
frequencies to ground with inductors where possible.

>Which in turn make it seem to me that it is dependent on the slope
>of the zero-crossing. Meaning it is dependent on signal amplitude and
>frequency?

Not relevant.

Best regards,

Charles





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