The LMH6624 has too much distortion to be particularly useful for 10MHz with outputs of 3dBm or more.
Bruce On Saturday, 19 December 2015 10:01 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. _______________________________________________ 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.