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 <379...@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";<csteinm...@yandex.com>; > Date: Sat, Dec 19, 2015 09:18 AM > To: "Discussion of precise time and frequency > measurement"<time-nuts@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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions > there.<d7041...@4e865554.f7c47456.gif><41cc1...@4e865554.f7c47456.gif><distribution_amp.gif><distribution_amp2.gif>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.