[time-nuts] SMD TADD-1 distribution amplifier

Charles Steinmetz csteinmetz at yandex.com
Fri Dec 18 20:18:24 EST 2015


Anders wrote:

>Far-out PN/AM is still 7dB short of the 6502!
>Looks like an SNR issue to me, rather than some issue with the linear
>regulator noise feeding through?!
>AD8055 in non-inverting circuit with 1+2k7/2k7 gain has 9.6 nV/sqrt(Hz)
>input-referred voltage noise PSD (if I calculated correctly..)
>With an ADA4899-1 and lower value resistors I get about -4.5 dB improvement
>to 3.4 nV/sqrt(Hz) input-referred

You're the victim of a very unfortunate choice of op-amp.

The op-amp that the TADD-1 was designed around (MAX477) is specified 
with 5 nV/sqrtHz (typical) of input voltage noise at 10MHz.  No 
details are given about its noise performance at lower frequencies, 
but the fact that the noise is specified at 10MHz suggests that the 
1/f corner frequency is probably high, very likely 10kHz or 
higher.  This further implies that its 10Hz input voltage noise is 
more than 1,000 nV/sqrt/Hz.

The AD8055 is specified at 6 nV/sqrtHz at 10kHz, rising to ~150 
nV/sqrtHz at 100Hz below a corner frequency of ~1kHz.  Extrapolating 
the curve suggests that the 10Hz voltage input noise is > 1,000 nV/sqrtHz.

The AM and PM noise you are measuring is caused primarily by noise at 
baseband, *not* by the in-band noise of the op-amp.  Baseband noise 
AM modulates the signal, and it is also converted to PN because the 
fluctuating voltage modulates the bandwidth of the op-amp (by 
modulating the locations of the second and subsequent amplifier poles 
with signal-dependent bias changes).  So these egregiously noisy (at 
baseband) op-amps cause high AM and PM noise.

Compare those with the following op-amp specs (like the specs above, 
these are all "typical"):

ADA4899:       1nV/sqrtHz at 100kHz       10nV/sqrtHz at 10Hz
AD8010:          2nV/sqrtHz at 10kHz     ~12nv/sqrtHz at 10hz  [note 
specific bypassing instructions]
LME49713:      1.9nV/sqrtHz at 10kHz      11.5nV/sqrtHz at 10Hz

So, at 10Hz, each these three possible choices is more than 100x 
quieter than the MAX477 or AD8055.  (They are also quieter in the 
signal band, but not by as much.)

Best regards.

Charles




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