[time-nuts] new op amp for distribution amplifiers

[Gerhard Hoffmann]

Am 17.12.18 um 16:22 schrieb Anders Wallin:
> Stylish dead-bug! ;)
> On my board I increased Rf/Rg to 4k7, to get rid of the gain-peaking.
> With 4k7 resistors I see +20 dBc/Hz worse phase-noise with the THS3491
> compared to LMH6702 :(  (image attached)
> If noise power scales with the resistors then it explains about 4k7/200R =
> 14 dB, for one amp, and perhaps 17 dB for two amps. Still missing 3dB
> somewhere in that reasoning...
>
> John: would you say if one can use the suggested 976R Rf/Rg resistors then
> below -160dBc/Hz for a 10MHz carrier is possible for a gain of +2?
>
> At least we can say THS3491 is much more tricky to work with than LMH6702
> so far... (?)

I can't claim any personal experience with the THS3491. They are on

my next DigiKey order, but the data sheet calls for Rf=976 Ohms

(chipscale package)  and for 2K1 in the SO-8 package. And the SO-8

still has at least 2 dB peaking in the data sheet. The huge difference

of 976 Ohms versus 2100  Ohms just for the package (and still close

to fail) makes it clear that the SO-8 is not made for low-gain 
applications,

and that with a board that's made for it. (GND removal under the input)


Already at 5V/V it seems to be well-behaved to 600 MHz and then

dropping without a peak, and that without outrageous resistors.

Maybe it's possible to augment Rfdbk with ferrite beads @>200 MHz

but they are pretty good Hi-Q inductors at low frequencies.


BTW already the LMH6702 can be somewhat picky  with regard to

decoupling; a cap directly from VCC to VEE is much more

important than anything to GND, esp. for distortion.

Op amps don't care much about the concept of GND; they are happy

when their inputs are equal.


Has anybody succeeded to massage the Spice model so that it fits

LTspice?


A distribution amplifier that can be used today for 5V 1PPS into 50 Ohms

and tomorrow for 18 dBm at 100 MHz should be quite usable if one gets

the noise down.


regards, Gerhard