[time-nuts] SMD TADD-1 distribution amplifier
Bob Camp
kb8tq at n1k.org
Sat Dec 19 13:52:35 UTC 2015
Hi
A biassed CMOS gate (NC7SZ125 is one) is quieter than anything you will feed through the
amplifier below about 10 KHz. Above that it’s as quiet as all but a very few sources. At less
than 10 cents each, they are hard to beat.
Bob
> On Dec 18, 2015, at 9:46 PM, Li Ang <379998 at qq.com> wrote:
>
> Hi Charles,
> I'm making a 1-to-4 distribution amplifier for 10MHz. Can you give any suggestion? The schematic is attached. The opamps I'm considering are LMH6609 LMH6624 LMH6702.
> Does the piezoelectric effect of capacitors need to be considered here?
>
>
>
>
>
>
> Thanks
> BI7LNQ
>
>
> ------------------ Original ------------------
> From: "Charles Steinmetz";<csteinmetz at yandex.com>;
> Date: Sat, Dec 19, 2015 09:18 AM
> To: "Discussion of precise time and frequency measurement"<time-nuts at febo.com>;
>
> Subject: Re: [time-nuts] SMD TADD-1 distribution amplifier
>
>
>
> 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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