[time-nuts] new op amp for distribution amplifiers

[Anders Wallin]

Hello Gerhard, everyone,

I made a test using the THS3491 on a slightly modified PCB from my "FDA"
design [1].

At first with 200 Ohm feedback/gain resistors all I got was (pulsed, in
bursts) self-oscillation (around 150MHz or so).
The datasheet [2] does however warn about this and suggests 976 Ohms for a
gain of 2V/V, so I went with 1k.
That got rid of the self-oscillation - but now I am left with severe (16 dB
@ 230 MHz) gain-peaking! (see attached figure)
The datasheet shows a 2V/V gain-plot with only minor peaking at >700MHz.

>From past simulations [3] the gain-resistors do contribute at some point to
the noise, so increasing them further for more stability is not great for
AM or PN.
I did not bother with the phase-noise probe yet - better to get rid of that
gain-peaking first (?).
Is there anything else that can be done, besides increasing Rf/Rg for more
stability?
Anything obviously wrong with the layout? It does work fine with LMH6702
and LMH6609.

Thanks for any suggestions!

Anders

[1] https://www.ohwr.org/projects/pda-8ch-fda-8ch/wiki/wiki
[2] http://www.ti.com/lit/ds/symlink/ths3491.pdf
[3]
http://www.anderswallin.net/2015/12/frequency-distribution-amplifier-v2-simulations/
[image: ths.jpg]


On Sat, Nov 17, 2018 at 1:00 AM Gerhard Hoffmann <dk4xp at arcor.de> wrote:

>
> Have you seen the new THS3491 ? LMH6702 on steroids.
>
>
> This one begs to be designed into new distribution amplifiers:
>
> Everything from DC / 1pps to a few 100 MHz into 50 Ohms, without any
> changes.
>
> (the usual gain = 2,    50R/50R out design)
>
> rise / fall 1.3 ns for 10V step
>
> slew rate 8V / ns
>
> BW 320 MHz for 10Vpp at gain of 5
>
> Volt. noise 1.7 nV/rt Hz
>
> supply max +/- 16V,  drives up to 420 mA
>
> SO-8 with thermal pad  or leadless
>
> <     http://www.ti.com/lit/ds/symlink/ths3491.pdf    >
>
>
> regards,
>
> Gerhard
>
>
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