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

Stephan Sandenbergh ssandenbergh at gmail.com
Thu Mar 12 16:31:55 EDT 2015


Thank you Charles for the detailed answer.

Any good texts on the subject that you can recommend?

At the moment I have the books written by Motchenbacher and Henry Ott. NIST
papers are another obvious source, but sometimes it is a little hard to
fill in, or understand, the details without adequate
pre-knowledge/experience.

Regards,

Stephan.

On 6 March 2015 at 23:01, Charles Steinmetz <csteinmetz at yandex.com> wrote:

> 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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