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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